Undernutrition in adults increases mortality and impairs immunity and quality of life. Gut microbiota influences metabolism, suggesting potential for probiotic-based weight-gain strategies. This study aimed to assess the efficacy of a probiotic supplement combined with a weight-gain diet on anthropometric indices, appetite, and calorie intake in underweight adults. In this 8-week, double-blind, randomized, placebo-controlled trial, 95 underweight adults (aged 18-60 years) from Mashhad, Iran, were randomized to receive a probiotic supplement (Lacticaseibacillus rhamnosus GG, Lactobacillus acidophilus, Lacticaseibacillus casei; 10CFU/capsule) or placebo with a 500-kcal surplus diet. Primary outcomes included anthropometric measures, including body weight and BMI. Secondary outcomes included appetite (via visual analog scales), caloric intake, and stool consistency. The probiotic group (n = 47) showed greater increases than the placebo group (n = 48) in body weight (2.41 vs 0.44 kg, p < 0.001), BMI (0.83 vs 0.24 kg/m², p < 0.001), body fat percentage (3.23% vs 1.03%, p < 0.001), fat mass (1.90 vs 0.89 kg, p = 0.002), and caloric intake (263.55 vs 30.75 kcal/day, p < 0.001). The probiotic group showed significant increases in hunger (p < 0.001) and desire to eat (p < 0.001), with reduced satiety and fullness before meals (p < 0.001), as assessed by VAS. Stool consistency in the probiotic group shifted significantly toward softer, more regular stools, as evidenced by a one-unit increase in the Bristol Stool Score (p = 0.01), compared to no change in the control group (p = 0.97). Probiotic supplementation with a weight-gain diet significantly enhances anthropometric measures, appetite, and calorie intake in underweight adults, offering a novel strategy for managing undernutrition.Iranian Registry of Clinical Trials, IRCT20230310057667N1 (Issue date 30 March 2023), https://irct.behdasht.gov.ir/trial/69130. 9Clinical trial registration:
