Current Perspectives on Protein Supplementation in Athletes: General Guidance and Special Considerations for Diabetes—A Narrative Review

A systematic search was performed across PubMed/MEDLINE, Embase, Web of Science, Scopus, SPORTDiscus, Cochrane CENTRAL, and UNF OneSearch up to August 2025. Search terms combined keywords and controlled vocabulary related to protein supplementation, athletes, exercise modalities (resistance, endurance, mixed-modality, team sports), and diabetes (type 1 and type 2). Boolean operators and truncation were applied (e.g., “protein supplement* AND “athlete*” OR “exercise” AND “diabetes” OR “glycemic control”). Reference lists of included studies and relevant reviews were also screened to capture additional eligible studies.

Two reviewers independently screened titles, abstracts, and full texts against inclusion criteria. Eligible studies met the following criteria: Population included trained or physically active individuals ≥ 16 years old undergoing resistance, endurance, mixed-modality, or team-based training. Interventions involved isolated or blended protein supplementation, with or without carbohydrate, and outcomes of interest included performance, recovery, body composition, and mechanistic endpoints such as muscle protein synthesis. For diabetes, glycemic outcomes (e.g., postprandial excursions, insulin use, hypoglycemia, HbA1c) were also considered. Animal studies, case reports, and non-peer-reviewed materials were excluded. Comparators were placebo, carbohydrate, or different protein types/doses. Outcomes were performance, recovery, body composition, mechanistic endpoints (e.g., muscle protein synthesis), and, for diabetic athletes, glycemic measures (HbA1c, postprandial glucose, insulin, hypoglycemia). Animal studies, case reports, and non-peer-reviewed materials were excluded.

A PRISMA-style flow diagram was generated to illustrate the screening process. From 1237 records identified, duplicates were removed, leaving 1004 titles/abstracts screened. Of these, 451 full texts were assessed for eligibility, with 129 excluded (reasons: non-athlete populations, insufficient protein intervention details, or non-relevant outcomes). Finally, 322 studies were included in the narrative synthesis (Figure 1).

The methodological rigor of included studies was assessed using Cochrane RoB 2 for randomized trials, ROBINS-I for non-randomized interventions, and AMSTAR-2 for systematic reviews and meta-analyses.

Findings showed that most RCTs had low to moderate risk of bias, with common issues in allocation concealment and blinding; non-randomized trials often had serious risk of bias due to confounding and small sample sizes; and systematic reviews were generally of moderate quality, though some lacked protocol registration.

Overall, evidence was judged as moderate-quality, supporting cautious but meaningful conclusions for both general and diabetic athlete populations.

Athletes require substantially higher protein intakes than sedentary individuals due to greater amino acid oxidation, accelerated protein turnover, and the demands of muscle repair and adaptation to training. While the adult RDA of 0.8 g/kg/day is adequate to prevent deficiency, it is insufficient for supporting performance adaptations [1,5]. Witard et al. reported that habitual protein intake in both male and female endurance athletes is around 1.5 g/kg of body weight/day [42]. Sports nutrition consensus statements, including those from the ISSN and ACSM, recommend 1.2–2.0 g/kg/day for most athletes under energy balance [2]. Within this range, resistance-trained athletes typically benefit from intakes of 1.6–2.2 g/kg/day to maximize hypertrophy and strength, with requirements reaching ~3.1 g/kg/day (based on fat-free mass) during periods of caloric restriction [3,6]. Endurance athletes generally require 1.4–1.8 g/kg/day to offset amino acid oxidation during prolonged activity and to support mitochondrial protein synthesis.

However, during intensified training or carbohydrate restriction, protein requirements may increase further [7]. Differences across studies often reflect variations in methodology, training context, and participant characteristics. Morton et al., pooling resistance-trained adults in energy balance, found that muscle hypertrophy plateaued at ~1.6 g/kg/day, consistent with the “muscle-full” effect, where MPS is maximized once essential amino acid thresholds are met [3]. By contrast, Helms et al. reported benefits of higher intakes in lean athletes undergoing caloric restriction, where increased protein preserved muscle despite energy deficits [6]. Similarly, Aagaard et al. showed that endurance athletes under carbohydrate restriction required higher protein intakes to compensate for elevated amino acid oxidation [7]. Collectively, these findings emphasize that optimal protein requirements are context-dependent, shaped by training modality, energy availability, and nutritional background.

Energy balance is a critical determining factor. During deficits, as in weight-class sports, physique preparation, or injury recovery, protein requirements rise to protect lean mass. Areta et al. found that caloric restriction increased protein turnover [5], while Longland et al. showed that 2.3–3.1 g/kg fat-free mass/day best preserved lean tissue under hypocaloric conditions [43]. In contrast, in energy surplus, additional dietary energy supports anabolism, reducing protein needs. Roberts et al. reported that ~1.6 g/kg/day was sufficient for optimizing muscle growth during hypercaloric feeding, with no added benefit from higher intakes [44]. Similarly, Morton et al., in a meta-analysis of 49 studies, concluded that supplementation enhances fat-free mass and strength, with maximal benefit near 1.6 g/kg/day, though higher intakes may be advantageous during deficits or high training loads [3].

Protein quality is another critical variable. Factors, including the concentrations of amino acids, indispensable amino acids, and branched-chain amino acids (BCAAs), may influence the anabolic efficiency of a protein source. Some other factors, such as protein digestibility, digestion rate, and absorption kinetics, may also influence their metabolic effect [45]. High-quality proteins such as dairy, eggs, and lean meats, with PDCAAS or DIAAS values approaching 1.0, deliver adequate essential amino acids, especially leucine, to effectively stimulate MPS [46]. Plant-derived proteins, often lower in digestibility or amino acid completeness, may require higher intake or blending with complementary sources to achieve comparable effects [47]. However, more recently, a meta-analysis of 30 RCTs by Reid-McCann showed no significant difference between plant or animal protein for muscle strength (n = 14 RCTs) or physical performance (n = 5 RCTs) [48]. Individual characteristics also influence protein needs. Body composition and lean mass indexing also influence protein prescriptions. Helms et al. emphasized that recommendations relative to fat-free mass (FFM) rather than total body weight better reflect anabolic needs, particularly in athletes with high muscularity or elevated body fat [3]. For instance, two athletes of equal body weight but different FFM will have distinct protein requirements [47]. Age is also a determining factor. Required protein intake in master athletes is often at the higher end (≥1.6 g/kg/day) to achieve similar adaptations as younger athletes [49]. Vegetarian and vegan athletes may also need around 2.0 g/kg/day, or use strategic protein blending, to ensure adequate leucine and essential amino acid intake [50].

Overall, while guidelines recommend 1.2–2.0 g/kg/day, various factors may influence the optimal intake, including training type, energy balance, body composition, protein quality, age, and dietary background. Intakes around 1.6 g/kg/day seem adequate in energy balance, while 2.3–3.1 g/kg/day (relative to FFM) may be necessary during caloric deficits or in heavy training phases [3]. Balancing intake to these relative factors optimizes adaptation and preserves lean mass across diverse athlete populations.

There is limited evidence comparing protein requirements between healthy and diabetic athletes. However, insights from clinical and exercise studies provide guidance. In type 1 diabetes (T1D), insulin deprivation drives increased skeletal muscle catabolism, underscoring the importance of adequate protein to offset losses [8]. In type 2 diabetes (T2D), effects on protein metabolism are more variable. Kouw et al. observed accelerated age-related muscle loss, while Bell et al. showed that despite elevated postabsorptive protein turnover in poorly controlled T2D, the anabolic response to insulin and feeding was preserved [9]. Similarly, Bassil and Gougeon reported that resistance to insulin’s anabolic actions can be overcome with adequate amino acid intake, particularly BCAAs, or through physical activity, highlighting the value of both nutrition and exercise interventions [51].

Nutritional strategies for diabetic athletes must balance performance goals with metabolic consequences. Although protein is critical for recovery and adaptation, intake must align with glycemic control and, where relevant, renal health. Hornsby and Chetlin recommended caution with very high protein diets in diabetic athletes [52], while Raj et al. noted that high protein intake is safe for healthy kidneys but requires monitoring in those at risk for nephropathy [53]. Emerging approaches such as chia seed supplementation have been utilized and preliminary results have shown potential for lessening postprandial glucose excursions [54].

Compared with healthy athletes, where intakes of 1.4–2.0 g/kg/day generally support optimal adaptation [2], diabetic athletes face additional complexity. Protein can influence postprandial glucose regulation [55], delay glycemic rise at higher doses in T1D [31], and improve satiety and glycemic control in T2D when consumed as pre-meal whey (~15 g) or as part of preload strategies [33,34,39]. Exercise itself can enhance protein utilization in T2D, helping to overcome insulin resistance and augment anabolic responses [51]. As Millward emphasized, these nuances highlight the need for personalized recommendations based on diabetes type, glycemic control, renal function, and training demands [56].

Healthy athletes generally require 1.4–2.0 g/kg/day of protein to optimize recovery and adaptation [2]. While diabetic athletes have similar needs but with added considerations, in T1D, adequate leucine-rich protein helps offset muscle catabolism, and in T2D, combining protein intake (1.4–2.0 g/kg/day) with exercise enhances anabolic sensitivity and supports glycemic control [8,51]. Pre-meal whey protein (10–20 g) can also reduce postprandial glucose in T2D [33,34]. Therefore, while overall targets are comparable, diabetic athletes benefit from tailoring protein type, timing, and distribution to metabolic health.

In summary, while diabetic and non-diabetic athletes share broadly similar protein needs, well above the RDA of 0.8 g/kg/day, diabetic athletes require additional tailoring. Intakes should be personalized not only to training mode and energy balance but also to metabolic health. High-quality protein sources rich in leucine, strategic distribution across meals, and exercise integration can help counteract insulin resistance and preserve lean mass. At the same time, monitoring renal function and glycemic responses remains essential to ensure safety and maximize performance outcomes (Table 2).

While meeting total daily protein targets is the foundation of training adaptation, growing evidence shows that the distribution and timing of intake significantly influence muscle protein synthesis (MPS) and recovery. Morton et al. showed that evenly spaced protein feedings throughout the day promoted greater gains in muscle mass and strength than skewed patterns, where most protein was consumed at dinner [3]. Similarly, Mamerow et al. found that distributing ~0.3 g/kg protein across 3–4 meals enhanced 24 h MPS compared with a single high-protein evening meal [4]. Areta et al. further reported that repeated moderate feedings (~20 g every 3 h) produced higher cumulative MPS than fewer large boluses or very frequent small doses, underscoring the benefits of protein pacing [5].

The timing of protein intake around exercise is also critical. Pre-exercise ingestion (1–3 h before training) increases amino acid availability during and after activity, potentially supporting both acute performance and recovery [58]. Esmarck et al. observed that consuming protein within 0–2 h post-exercise maximizes the synergy between exercise-induced sensitivity and amino acid availability [59]. While Phillips et al. argued that the “anabolic window” may extend up to 24 h, earlier ingestion ensures overlap with peak MPS signaling [60]. For athletes with multiple daily sessions, immediate post-exercise intake becomes particularly important to restore recovery capacity before subsequent bouts [61]. Kerksick et al. emphasized that nutrient timing, strategically placing both whole foods and supplements around exercise, optimizes recovery, tissue repair, and even mood states [62]. More recently, Stokes et al. highlighted that protein pacing (~0.3 g/kg every 3–4 h) may be more important than a single large bolus, shifting the focus away from a narrow post-exercise window [63].

The energy balance status and protein distribution interplay with each other. Hector et al. showed that in energy balance status, the protein requirement in adults is around ∼0.24 g protein/kg at each meal to maximize the stimulation rates of MPS [64]. Consuming 4 × 20 g/3 h protein by healthy, trained males following a bout of resistance exercise was superior to consuming 8 × 10 g/1.5 h or 2 × 40 g/8 h on MPS during the day. Authors suggested that a similar pattern of protein intake may be useful for athletes during dietary energy restriction. Murphy et al. reported that in older men who were on calorie-restricted diets, a balanced pattern of dietary protein distribution and intake throughout the day supported superior MPS than skewed, particularly when combined with resistance training [65].

Acute dose–response studies consistently show that MPS follows a saturable pattern after protein ingestion. Phillips et al. showed that ~20 g of high-quality protein (~0.25 g/kg) is sufficient to maximize MPS in young adults after resistance exercise, with larger doses contributing primarily to oxidation [66]. Witard et al. refined this range to ~0.25–0.4 g/kg per meal (~20–40 g depending on body size) [42]. Still, optimal doses can be varied by context: resistance-trained athletes achieve maximal stimulation at ~0.3 g/kg when protein quality is high, whereas endurance athletes may require ~0.4–0.5 g/kg to offset exercise-induced amino acid oxidation [67]. In older athletes, Moore et al. found that anabolic resistance elevates the requirement, requiring ~0.4–0.5 g/kg per meal to achieve equivalent MPS responses [68].

Strategic timing beyond the training day can further enhance adaptations. Pre-sleep intake of ~30–40 g casein or blended protein provides a sustained amino acid release, supporting overnight muscle protein accretion and lessening nocturnal muscle protein breakdown [28,69]. This approach is particularly beneficial in resistance-trained and master athletes, who may experience higher anabolic resistance.

Protein digestion kinetics also influence optimal timing strategies. Fast-digesting proteins, such as whey or hydrolysates, rapidly elevate plasma amino acids and are well suited for post-exercise recovery [70]. Casein, a slow-digesting protein, extends amino acid release and is advantageous before sleep or during long feeding intervals to reduce protein breakdown. Moderate-digesting proteins like soy and pea can be consumed throughout the day or blended with fast proteins to extend the anabolic response (Figure 2). Wilkinson et al. showed that such blends extend positive net protein balance compared with single-source feedings [71]. Overall, these findings suggest that aligning protein digestion kinetics with training and recovery windows, fast proteins for rapid repair, slow proteins for sustained overnight support, and blends for balanced coverage, maximizes dietary protein efficiency.

Collectively, the evidence highlights that while daily intake is the most important determinant of adaptation, distributing protein evenly, timing intake strategically around exercise and sleep, and matching digestion kinetics to recovery demands provide meaningful advantages for optimizing performance and recovery.

Meeting total daily protein requirements is the foundation for all athletes. However, distribution of protein intake across meals is equally important. Kerksick et al. and Phillips recommend ~0.25 g/kg protein per meal (≈20–40 g of high-quality protein) consumed every 3–4 h to maximize MPS, improve body composition, and enhance performance [72,73]. Vliet similarly reported that ~30 g per meal effectively supports anabolic processes in resistance-trained athletes [74]. For individuals with type 2 diabetes, these strategies may provide additional benefits. Campbell and Rains showed that deriving 20–30% of total energy from protein improved glycemic control, enhanced satiety, and preserved lean mass during weight loss [75]. More recently, Smith et al. reported that consuming a low-dose whey protein preload (15 g before main meals) reduced daily hyperglycemia by 8%, increased time in euglycemia by nearly two hours, and lowered mean 24 h glucose by 0.6 mmol/L in individuals with T2D [11]. These findings suggest that diabetic athletes may benefit from similar per-meal targets as their healthy peers, with the added advantage of improved glycemic regulation. However, renal health must be monitored, particularly in athletes at risk of nephropathy [37,75].

Beyond the daily distribution, the timing of protein intake plays a crucial role in both adaptation and metabolic control. Pre-exercise ingestion (1–3 h before training) increases amino acid availability and supports acute performance [58]. Esmarck et al. showed that immediate post-exercise protein enhances the synergy between exercise-induced insulin sensitivity and amino acid delivery [59], while Cribb and Hayes emphasized its importance in athletes with multiple daily sessions [61]. More recently, Stokes et al. and Kerksick et al. proposed that protein pacing (~0.3 g/kg every 3–4 h) may be more influential than a single post-exercise bolus [62,63]. Circadian factors are also influential: Guntoju and Pramod reported greater postprandial glucose and lipid elevations in the evening compared to the morning, suggesting that aligning protein intake with periods of optimal glycemic control may further benefit diabetic athletes [76]. Pre-sleep protein ingestion, especially casein, provides sustained amino acid release overnight and has been shown to improve nitrogen balance and training adaptations [69,77].

In athletes with diabetes, timing strategies must also account for glycemic variability. Robertson et al. recommended a balanced meal of carbohydrate, protein, and fat 3–4 h before competition to maximize energy availability, as well as bedtime snacks to reduce nocturnal hypoglycemia [10]. Muntis et al. reported that pre-exercise protein reduced hypoglycemia risk in adolescents with T1D [78], while Zisser et al. suggested the integration of protein intake with an insulin regimen to minimize glycemic excursions [79]. Although co-ingestion of protein and carbohydrate supports recovery and body composition [62,80], Breen et al. found that in insulin-sensitive individuals, adding protein to post-exercise glucose did not further enhance glycemic control, highlighting that benefits may be more relevant for those with impaired insulin sensitivity [81]. Continuous glucose monitoring (CGM) provides valuable insights into individual responses, particularly in younger athletes managing T1D [79].

Overall, while diabetic athletes share similar total protein targets with their non-diabetic counterparts (≈1.2–2.0 g/kg/day), protein strategies must also support glycemic stability. Even per-meal distribution (~0.25–0.4 g/kg every 3–4 h), strategic pre- and post exercise intake, and pre-sleep casein ingestion all enhance recovery and adaptation. For diabetic athletes, these strategies carry additional benefits for glycemic regulation but require integration with insulin therapy and careful monitoring of renal function. A personalized approach that aligns protein intake with both exercise demands and metabolic health is therefore essential.

In summary, meeting total daily protein needs is the basis of training adaptation; per-meal distribution and strategic timing provide additional advantages for muscle protein synthesis, recovery, and performance. Evidence suggests that ~0.25–0.4 g/kg of high-quality protein every 3–4 h maximizes MPS, with endurance athletes, older athletes, and those in heavy training often requiring the upper end of this range. Even distribution across 3–6 meals consistently outperforms skewed patterns, while aligning intake with periods of heightened anabolic sensitivity, particularly pre- and post-exercise, further enhances adaptation. Complementary strategies, such as pre-sleep casein to sustain overnight accretion and tailoring protein type to digestion kinetics, help extend the anabolic window throughout the day. For athletes with diabetes, timing is particularly important: beyond supporting training adaptations, it also stabilizes glycemia and reduces hypoglycemia risk. Practical approaches include small pre-meal whey doses, post-exercise protein–carbohydrate combinations, and bedtime snacks with protein and complex carbohydrates. When combined with continuous glucose monitoring and individualized protocols, these strategies allow diabetic athletes to integrate protein timing into both performance optimization and metabolic safety. Ultimately, a personalized approach that considers total intake, per-meal dosing, exercise timing, and metabolic context offers the most effective pathway to recovery, adaptation, and long-term performance (Table 3).

Protein requirements vary not only across sports but also with training status, macrocycle phase, and energy balance. Novice athletes often exhibit robust gains in muscle size and strength from training alone, while additional protein above the RDA provides only modest benefits unless baseline protein intake is inadequate [82,83]. This shows greater untrained potential for hypertrophy, heightened muscle sensitivity to training-induced MPS, and relatively lower training stress [18]. In contrast, experienced or elite athletes face adaptation plateaus, higher training volumes, and elevated amino acid oxidation, which increase their dependency on dietary protein. MacInnis et al. reported that advanced athletes benefit from more precise nutrient timing and recovery strategies [84], while Morton et al. found that protein supplementation significantly improved lean mass and strength in this group when daily intake was below ~1.6 g/kg/day [3]. Collectively, these findings suggest that novices may progress with minimal supplementation, whereas elite athletes derive more consistent performance benefits from targeted protein intake strategies.

Protein requirements also fluctuate across a training macrocycle. During hypertrophy and strength phases, maximize growth is achievable when the protein intake is at the higher end of the range (1.8–2.2 g/kg/day) [85]. Maintenance intakes (~1.6 g/kg/day) may suit in competition phases when energy balance is neutral, while in taper or deload periods, lower turnover reduces needs [44]. However, adequate protein remains important to support recovery and mitigate muscle loss [86].

Energy balance is another major determinant. Energy deficits, as in weight-class sports, physique preparation, or injury recovery, reduce anabolic signaling and increase protein breakdown, elevating reliance on dietary protein. Hector et al. confirmed that restriction heightens protein needs [64]. Longland et al. demonstrated that 2.3–3.1 g/kg FFM/day combined with resistance training best preserved lean tissue in hypocaloric states. In contrast, energy surpluses naturally support anabolism, lowering relative protein needs [43]. Pasiakos et al. noted that protein still contributes to recovery and accretion under surplus conditions [67], though Roberts et al. found that ~1.6 g/kg/day was sufficient for hypertrophy, with little added benefit from higher intakes [44]. These findings suggest that protein requirements peak during deficits and are moderate during surpluses.

Athletes training multiple times per day or with minimal recovery between sessions also have elevated turnover rates. Moore et al. reported that higher daily intakes (~2.0–2.4 g/kg/day) better support repair and adaptation under such conditions [87], while Cribb and Hayes suggested the importance of precise timing between sessions to maximize limited recovery windows [61]. Individual variability may also influence needs. Aagaard et al. found that although female athletes oxidize less protein during exercise than males, their per-meal requirements for MPS are similar when adjusted for body mass [7]. Helms et al. reported that athletes habitually consuming >2.0 g/kg/day gain little additional performance benefit from further supplementation, though higher intakes may aid satiety and lean mass preservation during energy deficits [6].

Overall, protein requirements are dynamic, elevating with training advancement, high-volume or multi-session training, and energy deficits, while moderating during maintenance or surplus phases. Personalizing recommendations based on training phase, energy balance, and individual characteristics such as sex, body composition, and habitual intake ensures that protein strategies are both effective and specific.

There is limited evidence on protein requirements in diabetic athletes. However, available evidence suggests their needs to be broadly like non-diabetic athletes, with additional considerations for glycemic regulation and catabolism. For endurance-trained athletes with diabetes, protein targets approximate 1.5 g/kg/day, rising further during carbohydrate-restricted training or rest days when amino acids contribute more heavily to energy metabolism [42]. Strength-trained diabetic athletes similarly require 1.4–2.0 g/kg/day, consistent with ISSN recommendations. However, higher intakes may be justified during caloric restriction to preserve lean tissue [2].

Energy balance strongly modifies requirements. In energy-deficient states, diabetic athletes face a heightened risk of muscle loss due to the combined stress of caloric restriction and glycemic fluctuations. Catabolic pressures may be greater in type 1 diabetes or poorly controlled type 2 diabetes, where insulin dosing variability and appetite disturbances destabilize energy availability [88]. Under such conditions, protein intakes of 2.0–2.4 g/kg/day might be necessary to preserve lean mass [89,90]. In contrast, adequate energy availability not only supports protein utilization but also enhances insulin sensitivity and contributes to improved glucose regulation, a key factor in diabetes management. Therefore, guidelines for diabetic athletes must remain highly individualized. Monitoring training load, glycemic control, and energy intake enables adjustment of protein targets to balance performance and metabolic safety. Kerksick and Kulovitz emphasized that prioritizing energy balance enhances protein utilization and adaptation [91]. Campbell and Rains and Riddell et al. further noted that, while broad protein targets for athletes apply, diabetic athletes require more precise timing strategies and, in some cases, higher intakes to counteract greater catabolic risk [37,75]

Chronotype, or an individual’s preference for morningness or eveningness, also influences circadian regulation of glucose and protein metabolism. Morning types typically have higher insulin sensitivity earlier in the day, while evening types show impaired glucose handling and greater risk of type 2 diabetes [92,93]. Muscle protein synthesis is also more efficient earlier in the day [94], suggesting that protein supplementation may be most beneficial when timed with periods of optimal insulin action. In diabetes, evening chronotypes often consume more energy and protein late at night, which can worsen glycemic control [95]. Whey supplementation improves postprandial glucose through insulin and incretin release [96], and these effects appear stronger when consumed in the morning or midday. Aligning protein supplementation with chronotype may therefore optimize both glycemic control and anabolic outcomes in individuals with diabetes.

In summary, the same principles that determine protein requirements in athletes, training status, cycle phase, and energy balance, apply equally to those with diabetes. However, the added challenges of glycemic variability, insulin management, and risk of catabolism demand modified approaches. Intakes of 1.4–2.0 g/kg/day generally support training adaptation, but during energy deficits or intensive training, requirements may rise to 2.3–3.1 g/kg FFM/day. For diabetic athletes, aligning intake with exercise demands, insulin regimens, and energy availability is essential to preserve lean mass, maintain glycemic stability, and optimize performance.

Protein hydrolysates have attracted interest in sports nutrition due to their rapid absorption and potential to accelerate recovery after intense exercise. Protein hydrolysates are absorbed more rapidly compared with intact proteins. This rapid digestion and absorption lead to faster plasma amino acid elevation, especially leucine, which may trigger muscle protein synthesis (MPS) more quickly [221,222]. Some studies suggest hydrolysates can enhance early post-exercise recovery and reduce muscle soreness, as seen with whey hydrolysate after eccentric exercise [223]. However, longer training studies often show no difference in strength or hypertrophy outcomes compared with intact proteins when total intake is sufficient [223].

Overall, while hydrolysates provide faster amino acid delivery and may benefit athletes needing rapid recovery or those with GI issues, intact high-quality proteins achieve comparable long-term performance adaptations when consumed in adequate amounts [224]. The main determinants of performance remain total protein intake, quality, and timing, with hydrolysates offering situational rather than universal advantages.

Resistance training has been studied extensively for protein supplementation, as hypertrophy and strength gains are directly dependent on amino acid availability and training-induced stimulation of MPS [225,226]. Mechanical overload, muscle damage, and metabolic stress collectively trigger MPS, and without sufficient amino acid supply, the response is transient and insufficient to prevent muscle protein breakdown (MPB) [161,227]. Protein supplementation, particularly fast protein such as whey, facilitates rapid aminoacidemia, increases leucine availability to activate mTORC1 signaling, and supports satellite cell activation, thus amplifying the anabolic response to training [228]. Morton et al. analyzed 49 randomized controlled trials and reported that protein supplementation significantly increased fat-free mass (FFM) and one-repetition maximum (1RM) strength in resistance-trained individuals [3]. Maximal gains were observed at ~1.6 g/kg/day in energy balance, with diminishing returns beyond this intake, although higher intakes (2.3–3.1 g/kg FFM/day) may be beneficial during caloric restriction or high-volume training [3]. In novice athletes, resistance training alone results in substantial hypertrophy, with supplementation adding only modest benefits unless baseline protein intake is insufficient. In contrast, experienced athletes exhibit smaller absolute gains due to adaptation plateaus, but supplementation may yield proportionally greater improvements when dietary protein intake is otherwise suboptimal.

While strength outcomes improve with protein supplementation, they are generally less pronounced than hypertrophy gains, as neural adaptations, tendon stiffness, and motor learning contribute significantly to strength development [3]. Whey protein, particularly during post-exercise, enhances 1RM strength in bench press and squat compared to carbohydrate or placebo [3]. Plant proteins such as soy and pea, when matched for protein and leucine content, produce comparable hypertrophy and strength outcomes to whey, although 10–20% higher doses or leucine fortification may be required to match anabolic potential due to differences in digestibility and amino acid profile [147,228].

Protein supplementation also plays a role in recovery. Resistance exercise induces microtrauma and delayed-onset muscle soreness (DOMS), which temporarily reduces muscle function [229]. Supplemental protein accelerates recovery by enhancing repair processes, attenuating oxidative damage, and supporting glycogen replenishment when combined with carbohydrates [229]. Whey’s rapid absorption makes it ideal for post-exercise recovery, while casein provides prolonged amino acid delivery that is advantageous overnight or during long intervals between meals [230]. Long-term hypertrophy and strength outcomes are similar when total intake is matched, regardless of protein type. However, blends of dairy and/or plant proteins may extend the anabolic window by combining rapid and sustained amino acid delivery [3].

As practical recommendations, in resistance-trained athletes, a daily intake of 1.6–2.2 g/kg/day is appropriate in energy balance, with 2.3–3.1 g/kg FFM/day warranted during caloric restriction [3]. Protein should be evenly distributed across 3–6 meals (~0.3–0.4 g/kg/meal) with at least one serving timed within 2 h post-exercise [230]. Whey protein is optimal for rapid post-workout recovery, while casein or blended proteins are suited for sustained coverage, particularly pre-sleep or in high-frequency training. Plant-based athletes can effectively utilize soy or pea proteins but should consider fortification or blending strategies to warrant adequate leucine intake. In multi-session training days, rapid-digesting protein immediately after the first session is particularly important to maximize recovery before subsequent exercise bouts [230].

Resistance training (RT) is particularly important for individuals with diabetes, as it improves insulin sensitivity and lowers HbA1c in adults with type 2 diabetes (T2D) [231]. Protein supplementation can complement these effects by stimulating MPS; however, altered insulin action in diabetes may influence protein metabolism. The anabolic response to protein and insulin remains largely preserved in T2D, meaning diabetic athletes can benefit from protein similarly to their non-diabetic peers, while also leveraging its glycemic effects [57,232,233].

In men with longstanding T2D treated with oral agents, adding a protein hydrolysate to carbohydrate doubled postprandial MPS compared to carbohydrate alone, with responses similar to matched controls [232]. Despite elevated postabsorptive protein turnover in poorly controlled T2D, the anabolic response to insulin and amino acids is intact [205,206]. Consequently, with adequate amino acid availability and insulin (endogenous or exogenous), robust anabolic responses can be achieved with RT.

Several RCTs in older adults diagnosed with T2D indicate that RT itself drives most improvements in strength, body composition, and glycemic control, while protein supplementation provides little additional benefit when dietary intake is already satisfactory. In a 24-week trial (n = 198), combining RT with whey protein (2 × 20 g/day) and vitamin D did not result in additional improvement in HbA1c, HOMA-IR, body composition, or strength compared with RT alone [234]. Similar findings were reported in other RCTs using whey protein (20–33 g/day) or leucine-rich amino acid blends during RT, which did not enhance outcomes beyond exercise [235,236,237]. Importantly, renal markers remained within safe ranges, with only small increases in urea noted [235,236].

Nevertheless, protein can provide glycemic benefits outside of direct training adaptations. A 7-day crossover trial showed that consuming 15 g whey protein before main meals reduced daily hyperglycemia and increased time in range in individuals with T2D [11]. Acute studies with protein hydrolysates have shown reduced postprandial glucose and hyperglycemia prevalence [238,239], although longer-term interventions showed inconsistent effects on day-long glycemia [240].

As practical recommendations, small pre-meal whey doses (10–20 g) may blunt postprandial glucose levels, particularly before carbohydrate-rich meals or recovery feedings [232,238,239,240]. Daily protein requirements for diabetic athletes align with general recommendations (~1.4–2.0 g/kg/day), with per-meal intakes of 0.3–0.4 g/kg (~25–40 g) from high-quality sources to achieve the leucine threshold of ~2–3 g per meal [2,62]. Postexercise protein supports recovery, and blends combining fast- and slow-digesting proteins may be more useful when full meals are delayed. Continuous glucose monitoring (CGM), self-monitoring of blood glucose (SMBG), and insulin adjustments must be incorporated in general plans [37].

Safety remains an important consideration. Moderate whey protein supplementation has shown no negative impact on kidneys in older adults diagnosed with T2D. However, regular monitoring is recommended, particularly in those with nephropathy [235]. While BCAAs and leucine are potent insulinotropic and anabolic agents [241], chronically elevated circulating BCAAs have been linked to insulin resistance in observational studies, underscoring the need to balance total protein intake and overall diet [55,242].

In summary, resistance training is the primary driver of improved strength, body composition, and glycemic control in diabetic athletes, with protein supplementation as an aid to optimize adaptation and metabolic management. RCTs in older T2D populations show limited additive effects of supplementation when protein intake is already sufficient. However, small pre-meal doses of whey may help with glycemic control. Total daily intakes of 1.4–2.0 g/kg/day distributed evenly across meals are recommended, with adjustments upward during caloric deficits or heavy training. Personalized strategies such as incorporating CGM data, renal monitoring, and insulin regimen are vital. More studies in younger and competitive diabetic athletes are needed to clarify the role of protein supplementation in long-term performance and glycemic outcomes.

Endurance performance is primarily driven by oxidative energy production, high mitochondrial density, and effective substrate utilization. However, protein plays a pivotal role in recovery, adaptation, and lean mass protection in athletes exposed to prolonged or high-intensity aerobic exercise [42]. Endurance training accelerates amino acid turnover and raises protein needs compared with sedentary individuals, with benefits for recovery, mitochondrial remodeling, and muscle maintenance when intake is adequate [2,243].

Unlike resistance training, where hypertrophy is the main goal, protein in endurance athletes is critical for repairing exercise-induced muscle damage, supporting metabolic adaptations, and preserving muscle tissue [42]. Protein ingestion helps repair muscle fibers damaged by eccentric contractions and enhances glycogen repletion. Co-ingestion of protein with carbohydrate accelerates resynthesis of glycogen, particularly when carbohydrate intake is suboptimal (<1.0 g/kg/h post-exercise) [68]. Certain amino acids, particularly leucine, also promote mitochondrial biogenesis via the activation of signaling path-ways such as mTORC1 and PGC-1α. Protein intake during the post-exercise period suppresses proteolysis, further supporting recovery and lean mass retention during heavy training blocks or competitive seasons [243].

The short-term recovery effects of proteins are well-studied. Several studies showed that adding ~20–40 g high-quality protein to post-exercise carbohydrate intake reduces markers of muscle damage such as plasma CK and myoglobin and lowers delayed-onset muscle soreness (DOMS). Co-ingestion of protein and carbohydrate can improve time-to-exhaustion or time-trial performance in sessions performed within 24 h, particularly when carbohydrate intake is limited [30].

In chronic long-term adaptation, a combination of endurance training and protein supplementation results in various favorable outcomes. During high-volume training, protein supplementation reduces lean mass loss, particularly in energy-deficit conditions such as training camps or stage races. Intakes ≥ 1.8 g/kg/day may support mitochondrial protein synthesis and oxidative enzyme activity [42]. However, results on improvements in VO₂max and lactate threshold are inconsistent, underscoring carbohydrate availability and training intensity as primary drivers of aerobic performance [145,244].

The interaction of carbohydrate and protein in endurance sports has shown favorable effects. Carbohydrates are the main fuel for endurance exercise while protein is most impactful when carbohydrate availability is restricted. Low glycogen increases amino acid oxidation, compromising protein balance; supplemental protein offsets these losses and reduces muscle damage [42]. In multi-session days, protein intake between bouts enhances recovery when glycogen replenishment is incomplete [68].

The type of protein is also a determining factor in endurance training. Whey protein supports rapid post-exercise recovery and glycogen resynthesis when co-ingested with carbohydrate. As previously described, casein helps overnight recovery, particularly useful in multi-day events. Soy and pea proteins are effective alternatives. However, leucine content must be adjusted. Moreover, soy’s isoflavones provide additional antioxidant benefits in high-oxidative-stress contexts (e.g., ultramarathons) [18,243,245,246]. Protein blends combining fast and slow proteins may synergistically extend amino acid delivery, supporting both acute and prolonged recovery phases.

In athletes with diabetes, the anabolic response to protein supplementation remains largely intact. Compared to healthy controls, men with longstanding type 2 diabetes (T2D) experience similar increases in muscle protein synthesis (MPS) following protein–carbohydrate ingestion [232]. However, clamp studies showed that the anabolic effects of insulin and energy are preserved in poorly controlled T2D when amino acid availability is adequate [57]. Protein supplementation in these athletes supports the same adaptations as in non-diabetic peers, but its role in glycemic management provides additional benefits and challenges. In T2D, small pre-meal whey protein doses of ~15 g have been shown to blunt postprandial glucose spikes, improve time-in-range, and reduce daily hyperglycemia [11,96]. In contrast, in type 1 diabetes (T1D), high-protein and high-fat meals often irritate delayed hyperglycemia three to five hours post-ingestion that requires extended or adjusted insulin dosing strategies [31,247]. These differences emphasize the importance of balancing protein timing to carbohydrate intake and insulin management.

For endurance athletes with diabetes, practical approaches include maintaining daily intakes of ~1.2–1.8 g/kg/day (often framed as 1.4–2.0 g/kg/day), with higher intakes recommended during heavy training blocks, energy restriction, or carbohydrate-restricted phases [2,62,127,243]. Per-meal dosing of 0.3–0.4 g/kg (20–40 g) every three to four hours ensures leucine thresholds are met and MPS is maximized. Post-exercise meals should pair 20–40 g of protein with ~1.0–1.2 g/kg/h carbohydrate across early recovery. When carbohydrate intake is limited, protein can partially compensate by supporting glycogen repletion and muscle repair [21,30]. Pre-meal whey doses of 10–20 g consumed before large carbohydrate-rich meals help moderate postprandial glucose levels in T2D [11,96]. Before sleep, ~30–40 g of casein provides sustained aminoacidemia overnight, though T1D athletes must manage the risk of delayed hyperglycemia with careful insulin strategies [247].

Safety considerations are vitally important. Moderate whey supplementation has not shown any negative impact on kidneys in older adults with T2D conducting resistance or endurance training, although those with nephropathy should individualize protein intake under medical guidance [235]. In athletes with T1D, high-protein meals can raise late insulin needs, which may interfere with overnight recovery if not properly adjusted [31,247]. Despite promising findings, only a few trials (with multi-week intervention) have evaluated protein supplementation in competitive diabetic endurance athletes using performance or continuous glucose monitoring (CGM) outcomes. Future research needs to compare whey, casein, and plant or blended proteins with various levels of carbohydrate intake, with integration of CGM-guided nutrition strategies.

In summary, protein supplementation in endurance athletes primarily supports recovery, preserves lean mass, and enhances adaptation. It is more important when carbohydrate availability is limited. Whey protein is effective post-exercise due to rapid absorption; casein supports prolonged recovery overnight; soy and pea proteins provide plant-based alternatives with added antioxidant and metabolic benefits; and blends extend the anabolic window by combining fast and slow proteins. For diabetic endurance athletes, the anabolic response to protein remains preserved, but insulin management and nutrient timing are critical. Small pre-meal whey doses can stabilize postprandial glycemia in T2D, while T1D athletes must carefully adjust insulin regimens when consuming protein-rich meals. Ultimately, individualized approaches integrating carbohydrate intake, training load, and glycemic control are essential for optimizing both performance and metabolic health.

HIFT incorporates mixed-modality exercise regimens such as CrossFit^®, tactical/military conditioning, and sport-specific training programs. It integrates resistance exercise, Olympic lifting, gymnastics, sprint intervals, and aerobic conditioning within the same training cycle or even a single session. This unique training model challenges the musculoskeletal, neuromuscular, and metabolic systems simultaneously, combining the eccentric muscle-damaging loads of resistance training with oxidative stress and substrate depletion typical of endurance exercise. These overlapping demands highlight the potential importance of protein supplementation for optimizing adaptation and supporting recovery in HIFT athletes.

Protein supplementation can serve multiple roles in this context. By facilitating muscle repair and remodeling, protein is particularly important for the resistance-based elements of HIFT, such as Olympic lifts and heavy squats [248]. It also supports mitochondrial and oxidative adaptations elicited by the endurance aspects of training [249] while attenuating muscle protein breakdown caused by repeated high-intensity bouts under fatigue [250]. When co-ingested with carbohydrate, protein can also accelerate glycogen resynthesis during high-training-volume periods [251]. Because HIFT targets diverse adaptations, protein needs may fluctuate more than in single-modality sports, depending on whether training blocks emphasize strength, metabolic conditioning, or both [20].

The research on HIFT athletes is limited. However, emerging trials provide some insights. A 2025 randomized crossover study found that increasing daily protein intake from ~1.0 g/kg to 1.6 g/kg using whey or egg-white protein over six weeks did not significantly enhance VO₂ max, maximal strength, or core endurance compared to an isocaloric carbohydrate control group. Improvements were observed across all groups due to training alone. Authors suggest that recreational HIFT participants may meet most adaptation needs with protein intakes slightly above 1.0 g/kg/day [24]. In contrast, a case series of competitive CrossFit^® athletes showed that post-workout whey protein (~0.3 g/kg) combined with carbohydrate accelerates recovery between same-day sessions. It also reduces delayed-onset muscle soreness (DOMS) and preserves performance in repeated workouts [252]. Similarly, studies in military personnel undergoing HIFT-style conditioning show that protein intakes around 1.8–2.0 g/kg/day better preserve lean mass. This was more significant under caloric-deficit conditions common in field training [64].

Performance outcomes in HIFT vary depending on baseline intake and energy availability. Strength gains appear most responsive when total protein intake is below ~1.4 g/kg/day, with supplementation helping to optimize hypertrophy and performance [20]. Above this threshold, benefits are less consistent unless athletes are in caloric-deficit or high-volume phases. For aerobic and anaerobic capacity, supplementation generally does not improve performance beyond training alone. However, adequate protein may improve training consistency during demanding weeks by enhancing recovery from muscle microdamage [47]. Recovery is a particular focus in HIFT, given the high training frequency. Whey protein consumed post-session reduces soreness and accelerates neuromuscular recovery, especially when combined with carbohydrate [253,254]. In multi-day competition formats, such as the CrossFit Games^®, pre-sleep casein has been shown to support overnight recovery and performance readiness [255].

Protein type also plays an important role in recovery plans for HIFT. Whey protein, due to its rapid digestion, is the best option for immediate post-exercise recovery [256]. Casein, with slower absorption, provides sustained amino acid availability that is useful overnight or during extended gaps between meals [255]. Plant-based proteins such as soy and pea are effective alternatives when doses are adjusted to meet leucine thresholds, although slightly higher intakes may be required [164]. Blends of fast- and slow-digesting proteins provide both rapid and sustained amino acid release, making them especially valuable for heavy training days with long intervals between meals [20].

Practical guidelines suggest that HIFT athletes should aim for daily protein intakes of 1.6–2.0 g/kg/day under normal conditions, increasing to 2.4 g/kg/day during caloric deficits or periods of very high training loads [20]. A post-exercise dose of 0.3–0.4 g/kg within one hour of training, evenly distributed across 3–6 meals per day, is recommended. On competition or multi-session days, rapid-digesting protein are suitable options after the first workout, with casein supplementation before sleep to maximize overnight recovery. Ultimately, supplement choice should be guided by individual dietary preferences, tolerance, and training schedule. Moreover, protein blends provide a flexible and effective option for sustaining recovery in the demanding context of HIFT.

HIFT is becoming increasingly popular among individuals with diabetes. In type 2 diabetes (T2D), short HIFT interventions have been shown to improve insulin sensitivity, β-cell function, body composition, and broader cardiometabolic risk markers [257,258]. Evidence from high-intensity interval training (HIIT) shows similar benefits for glucose regulation in people with or at risk for T2D [259]. In type 1 diabetes (T1D), outcomes are more nuanced: a 12-week HIIT trial did not lower HbA1c overall, but participants who adhered to more than half the sessions did experience greater HbA1c reductions. Authors suggested that carefully programmed and monitored conditioning can be safe and beneficial [260].

With HIFT imposing both heavy mechanical and metabolic stress, higher protein intake levels are vital to support adaptation, recovery, and lean mass preservation [2]. Timing, such as post-exercise protein intake or distributing protein evenly across the day, is also crucial to further enhance muscle protein synthesis [62]. However, in trained adults, adding protein beyond an already adequate diet does not consistently improve HIFT performance. An eight-week trial comparing whey and pea proteins found similar adaptations [164], while a randomized triple-crossover study reported no advantage of whey or egg-white protein over placebo during six weeks of HIFT [24]. Similarly, an acute trial showed that a carbohydrate–protein drink before standardized CrossFit workouts did not improve repetitions [261]. Although these findings are not diabetes-specific, they may suggest that the performance impact of supplementation depends more on meeting baseline protein needs than on exceeding them.

For athletes with T2D, HIFT or HIIT blocks typically improve insulin sensitivity and cardiometabolic profiles, though HbA1c responses vary depending on duration and adherence [258,262]. Protein, particularly whey, consumed before meals can blunt postprandial glycemic response. It may be due to stimulation of incretin hormones, enhancement of insulin secretion, and lower gastric emptying rate [11,96,263]. Therefore, small pre-meal whey doses (10–15 g) may be a practical strategy on high-carbohydrate training days. In T1D, individualized planning is critical, as protein-rich or mixed meals can delay and prolong hyperglycemia. Dual or extended insulin bolus strategies, continuous glucose monitoring (CGM), and access to rapid-acting glucose are vitally important [37,247,264].

General protein recommendations for HIFT athletes with diabetes align with those for non-diabetic populations, at around 1.4–2.0 g/kg/day from high-quality sources, distributed across 3–5 meals or snacks [2,62]. The upper end of this range may be warranted during energy deficit or heavy training blocks. Post-exercise recovery meals should ideally include 20–40 g of high-quality protein (0.3–0.4 g/kg) along with carbohydrate to promote muscle remodeling and glycogen repletion [62]. For T1D athletes, delayed glycemia following higher-fat or high-protein meals should be expected and managed with appropriate insulin regimens [37,247].

If total daily protein intake is inadequate, for example, when appetite is suppressed post-workout, a 20–30 g whey or milk-protein supplement can serve as a practical bridge. In T2D, small pre-meal whey “preloads” is one of the most evidence-based strategies to control glycemic spikes [11,96,263]. However, when daily protein needs are adequate, extra supplementation shows little additional benefit for HIFT performance in trained adults [24,164,261]. Renal function must also be monitored, especially in athletes with albuminuria or reduced eGFR.

No randomized trials directly test protein supplementation in diabetic HIFT athletes at present. Therefore, guidance relies on extrapolating from diabetes-specific HIIT/HIFT studies, pre-meal whey trials in T2D, and protein supplementation research in general HIFT populations. Overall, protein supplementation is vital for recovery, lean mass preservation, and glycemic management. However, individualized strategies, directed by CGM, insulin adjustments, and medical oversight, are necessary. Well-designed clinical trials in diabetic HIFT populations are still needed to refine these recommendations (Table 5).

For master athletes, tracer studies and intervention trials showed that a per-meal target of ~0.4–0.5 g/kg of high-quality protein (roughly 30–45 g per meal) is necessary to maximize MPS, which is greater compared with ~0.25–0.3 g/kg in younger adults [267]. Daily requirements of 1.6–1.9 g/kg/day seem adequate in energy balance status [265]. Higher intakes of 2.0–2.4 g/kg/day are more suitable in calorie restriction status, injury recovery, or heavy training phases [64]. Because the peak MPS response is blunted with age, distributing protein across meals evenly becomes more important. The recommendation is around three to four meals providing ~0.4 g/kg protein each [268,269]. Post-exercise protein feeding is equally important since exercise transiently restores muscle sensitivity to amino acids. Pre-sleep protein (30–40 g of casein or a slow-release blend) also helps counteract overnight catabolic effects on muscle breakdown [269].

Leucine plays a crucial role in overcoming anabolic resistance. It is suggested that master athletes should prioritize sources providing at least 2.5 g of leucine per serving [111]. Whey protein, with its rapid digestion and high leucine density, is a superior choice during post-exercise, while casein protein, which sustains amino acid availability during overnight fasting, can be a better choice before sleep [255]. For plant-based athletes, soy and pea proteins can be effective. However, they often require slightly larger doses or fortification with free leucine or complementary blends such as pea and rice [111]. Notably, resistance training itself strongly mitigates anabolic resistance. Combining high-quality protein with regular strength training preserves lean mass and muscle quality. It also supports bone density, tendon health, and functional capacity in older adults [270,271,272]. For endurance-dominant master athletes, conducting two to three resistance training sessions per week optimizes the benefits of protein intake [271].

Preservation of muscle and bone quality is an inevitable heath concern as female master athletes near the menopausal transition. Hormonal shifts in estrogen, progesterone, and testosterone can progress skeletal muscle atrophy and accelerate bone decay [273,274,275]. Combined, female athletes are often at greater risk for skeletal injuries such as stress fractures and bone breaks as they age, with heightened risk surrounding and following menopause. It has been demonstrated that in older adults with osteoporosis, high protein intake (0.8 g/kg/day) leads to higher bone density, slower rate of bone loss, and reduction in skeletal injury (i.e., hip fractures) [276]. Coupling strength training with high-quality protein consumption supports protective pathways involved in preserving skeletal muscle mass and bone integrity. Enhancing mTORC1 activation and insulin growth factor-1 levels in particular contributes to enhancing muscle protein synthesis and reducing protein breakdown [276,277]. Thus, adequate training and protein intake during middle age is likely to support the preservation of skeletal muscle mass and delay the inevitable decay in bone density that is accompanied by aging and accelerated through menopause [278].

Training condition is also a determining factor for protein needs. After endurance sessions, protein primarily supports repair. It also helps maintain net protein balance, with glycogen resynthesis benefits particularly when carbohydrate intake is suboptimal [21,30]. After strength-based sessions, protein drives myofibrillar remodeling. Although the distribution across meals is more important than exact timing, post-exercise feeding is still a practical strategy. In general, most masters benefit from ~1.4–2.0 g/kg/day, with intakes toward the higher end during energy deficits or heavy training blocks [2,3,62]. As previously mentioned, pre-sleep casein (30–40 g) enhances both myofibrillar and mitochondrial protein synthesis in older adults, further supporting its role in master athlete nutrition [125,279,280].

Master athletes with diabetes face a dual challenge: the age-related decline in anabolic responsiveness and the metabolic limitations imposed by diabetes. It has been suggested that the anabolic response to protein is mainly preserved in type 2 diabetes (T2D) when insulin and amino acid availability are adequate [57,232]. However, as previously mentioned, older athletes typically require higher per-meal protein and leucine doses to optimize MPS, with pre-sleep protein emerging as an effective strategy [125,279]. Together, these findings show the significance of protein supplementation in supporting diabetic master athletes to meet daily targets, assist with training adaptations, and improve glycemic control.

Diabetes and aging both increase the risk of sarcopenia and functional decline and are likely exacerbated in women surrounding menopause. T2D, in particular, is consistently linked to a greater prevalence of sarcopenia and accelerated loss of lean mass and strength [281,282,283]. Although poorly controlled T2D is associated with elevated protein turnover, the net anabolic response to carbohydrate and protein feeding remains intact. It may suggest that master athletes with diabetes can still adapt to training if protein intake is adequate [57,232]. Several randomized trials reported no additional benefits of whey supplementation beyond well-structured resistance training when baseline protein intake was already sufficient [234,235,236]. These results highlight that supplementation is most effective when it helps close intake gaps, ensures per-meal leucine thresholds are met, or assists with strategic timing (e.g., post-session or pre-sleep).

Glycemic status can also be influenced by protein supplements. Small pre-meal whey “shots” (~15 g) have consistently lowered postprandial glucose and improved time-in-range in individuals with T2D. Meta-analytic evidence links these effects to slower gastric emptying and incretin stimulation [11,107,263]. This strategy can be particularly beneficial on training days when carbohydrate intake is higher. In contrast, protein- and fat-rich meals can cause delayed postprandial hyperglycemia in T1D, which requires a balanced insulin regimen such as split or extended boluses guided by CGM [31,247]. Thus, evening recovery meals must be managed to avoid nocturnal hypo- or hyperglycemia.

In terms of safety, studies in older adults with T2D have shown that moderate whey supplementation during resistance training did not have any negative impact on kidneys over 12 weeks. However, minor changes in urea levels were observed [235,236]. In broader T2D populations, higher-protein diets tend to improve insulin resistance and lipid profiles. Moreover, plant-forward strategies resulted in modest improvements in glycemic control [177]. For master athletes, both dairy- and plant-based proteins can be suitable. Blend proteins may be particularly advantageous for those with reduced appetite or who eat infrequently to ensure adequate leucine and essential amino acid intake.

Practical recommendations for diabetic master athletes include aiming for 1.4–2.0 g/kg/day, split across 3–5 meals or snacks. Each feeding should provide ~0.3–0.4 g/kg of protein (25–40 g), delivering at least 2–3 g of leucine. Pre-sleep casein (30–40 g) is a useful strategy on heavy training days, and 10–15 g whey before carbohydrate-rich meals can help blunt postprandial glycemia in T2D. Renal health should always be monitored, particularly in athletes with albuminuria or reduced eGFR.

There are no long-term trials directly testing different protein types or timing strategies in diabetic master athletes at present. It has been suggested that protein supplementation provides little additional benefit beyond well-programmed resistance training and adequate total intake. However, it is still valuable for closing nutritional gaps, managing glycemia, and supporting recovery. Future research should focus on CGM-guided strategies, plant versus dairy comparisons, and pre-sleep interventions to optimize performance and long-term metabolic health in this unique population.

In summary, master athletes, typically over 35–40 years of age, face unique nutritional challenges due to anabolic resistance, reduced recovery capacity, and increased risk of sarcopenia, particularly in those with diabetes. Evidence indicates that higher per-meal protein doses (~0.4–0.5 g/kg, or 30–45 g high-quality protein providing ≥2.5 g leucine) are required to maximize muscle protein synthesis, with daily intakes of 1.6–2.0 g/kg/day sufficient for most, and up to 2.4 g/kg/day beneficial during energy deficit or injury recovery. Even distribution across three to four meals, post-exercise feeding, and pre-sleep casein supplementation are especially valuable for maintaining muscle mass, function, and recovery. Whey protein offers rapid leucine delivery for immediate recovery, while casein, soy, pea, or blends can provide sustained or plant-based alternatives, though higher doses or fortification may be needed for plant proteins. In diabetic master athletes, the anabolic response to protein is largely preserved, but the combination of aging and diabetes heightens sarcopenia risk and complicates glycemic control. Clinical trials in older adults with T2D show little additive benefit from whey supplementation when baseline protein intake is already sufficient, though small pre-meal whey “shots” (10–15 g) consistently lower postprandial glucose and improve time-in-range. For T1D, protein-rich meals often cause delayed hyperglycemia, requiring tailored insulin strategies and CGM oversight. Overall, adequate and well-distributed protein intake, paired with resistance training, remains central to preserving lean mass, functional capacity, and metabolic health in aging athletes, while protein supplementation is most useful to close intake gaps, optimize timing, and provide glycemic benefits in diabetic populations (Table 6).