The effect of time-restricted eating on arterial stiffness indices in men with metabolic syndrome: study protocol for a randomized controlled trial

Metabolic syndrome (MetS) is a set of simultaneous systemic abnormalities associated with increased risk for cardiovascular diseases (CVD) that was first introduced and discussed in the literature about three decades ago. Based on the International Diabetes Federation (IDF), MetS is defined as the presence of central obesity plus at least two other risk factors, including hyperglycemia, hypertriglyceridemia, low high-density lipoprotein (HDL), and hypertension [1]. Insulin resistance, which is believed to be the main mechanism driving MetS, increases the risk of type II diabetes mellitus (T2DM), CVD, and other diseases [2].

The global increase in the prevalence of MetS due to inactivity and obesity and its relationship with T2DM and CVD has become a subject of interest for researchers aiming to reduce the burden of MetS in both clinical and social settings [1, 2]. Scuteri et al. concluded that the pattern of MetS components differed between countries and cultures, and thus treatment approaches may need to also vary depending on the population [3].

The global prevalence of MetS is 20–25% [4, 5], while a meta-analysis of 69 Iranian studies over a period of 17 years reported that one third of Iranian adults had MetS. Prevalence varied by age and sex such that, in people under 40, males were prominently affected, whereas MetS prevalence was higher among postmenopausal women compared to age-matched men [6]. Furthermore, the patterns of MetS components differ between men and women [7]. Notably, MetS is more strongly associated with an increased risk for coronary heart disease (CHD) and CVD in women [8].

Nutrition, in terms of both quantity, quality, and timing of food intake, plays an important role in MetS development [9]. The suggested mechanism linking metabolic disease with meal irregularity is through effects on the circadian rhythm since food acts as a “zeitgeber,” i.e., “time keeper” [10]. The desynchronization of circadian rhythm due to irregular eating patterns causes lipid metabolism impairment, glucose intolerance, insulin insensitivity, and activation of inflammatory cascades [11]. Moreover, the insulin resistance resulting from meal irregularity activates several mechanisms associated with atherosclerosis. Insulin resistance leads to dyslipidemia, inflammation, and endothelial dysfunction, thereby increasing the risk of atherosclerosis [12]. In this regard, a link can be established between both meal irregularity and metabolic syndrome with arterial stiffness.

The effect of time-restricted eating (TRE) on MetS has been documented in several animal and human studies. It has been proposed that TRE can improve insulin insensitivity [13]. Furthermore, research has reported that TRE may lead to a reduction in body weight and blood pressure, as well as improvements in body composition, and lipid and glucose metabolic profiles [11]. TRE appears to be a safe intervention, with minimal adverse events reported in studies [14].

The cardiometabolic effects of TRE have also been investigated. A trial showed that a 12-week 10-h TRE significantly improved cardiometabolic indices in 12 individuals with MetS [15]. Zuo et al. conducted a study investigating the effects of a high-protein, intermittent fasting low-calorie (HP-IF-LC) diet on obese individuals. During the 12-week weight loss phase (first phase) of the study, they found a significant decrease in pulse wave velocity (PWV). In the 1-year weight maintenance phase (second phase) of the study, they compared a high-protein, intermittent fasting (HP-IF) diet with a heart healthy (HH) diet and observed a significant decrease in PWV [16]. A systematic review also found that Ramadan fasting, which involves restricting food and drink intake to a specific window between sunrise and sunset, improved lipid profile and anthropometric features in healthy individuals but not in diabetic individuals [17]. In another study, participants followed early time-restricted feeding (eTRF) with an eating window of 6 h compared to a 12-h eating period for 5 weeks. Eight overweight prediabetic males were assigned to intervention and control groups. The eTRF led to significant reductions in systolic and diastolic blood pressure, fasting insulin level, mean insulin level, and peak insulin value. They observed an improvement in PWV and augmentation index (AIx), although not statistically significant [13].

However, to the best of our knowledge, studies did not evaluate the effect of TRE alone on arterial stiffness, and some assessed the effects of TRE in populations other than individuals with MetS. A meta-analysis in 2020 suggested that despite the observed benefits of TRE in literature, randomized controlled trials (RCT) with larger sample sizes are necessary to further support the usefulness of TRE in prevention and treatment of MetS [11].

The proposed study aims to recruit a larger number of participants to assess the effect of a 6-week 10-h TRE protocol on PWV in individuals with MetS. Other cardiometabolic effects of TRE will also be investigated. It is hypothesized that TRE will reduce PWV, a marker of arterial stiffness, and improve the cardiometabolic profile in individuals with MetS. Considering the sex-related differences in MetS components and the potential effect of the menstrual cycle on metabolism, this study limits recruitment to men. With this approach, we will have a homogeneous group, which will reduce variability and minimize the confounding factors, leading to more robust results. Also, taking into account that the relationship between unhealthy lifestyles and MetS is stronger in men than in women [18], focusing this study on men may help develop gender-specific prevention and intervention programs.

The primary objective is to determine the effect of a 10-h TRE compared to standard dietary recommendations on indices of arterial stiffness among men with MetS.

The secondary objective is to determine the effect of 10-h TRE compared to standard dietary recommendations on:

This study will be conducted at the center of the PERSIAN Cohort Study at Mashhad University of Medical Sciences, at Imam Reza Academic Hospital, located in the North-East of Iran.

The principal researcher will inform eligible individuals about the study details, providing both written and verbal information in clear and comprehensible language. Subsequently, the interested individuals will be asked to provide written consent, in accordance with the standard procedures of the trial. Those who decline to give written consent or demonstrate significant distress will not be included in the study. In the case of illiterate individuals, since our study includes tests requiring literacy, it is not possible to recruit them.

The consent form outlines the terms for data usage in case of participant withdrawal from the study and grants permission for sharing relevant data with specific academic experts or other regulatory authorities (ethics number: IR.MUMS.MEDICAL.REC.1401.213).

This study aims to evaluate the pure effects of TRE intervention on arterial stiffness indices. The comparison will be between the TRE group and the control group.

To mitigate the risk of contamination in TRE trials, where control group participants may adjust their eating times to mimic the intervention group, we emphasize the importance of maintaining their usual eating window and closely monitor their adherence to align with their designated group. Also, we planned to employ the appropriate statistical methods to assess and adjust the potential impact of contamination, as explained in the “” section. Statistical methods

No specific side effects are anticipated following the TRE intervention. The potential reasons for discontinuing the intervention include the occurrence of any adverse effects due to the intervention or measurements or the participant’s unwillingness to continue participation. The Ethics Committee of Mashhad University of Medical Sciences (MUMS) will make decisions regarding the referred cases.

Compliance is defined as adhering to the planned 10-h TRE in the intervention group and avoiding eating time restrictions in the control group. Participants will be asked to write down the start and end times of food consumption in the prepared forms, which will allow us to improve adherence and identify any deviations from the prescribed eating window. Also, the participants will be asked to keep a food and drink diary including the time of consumption for 3 consecutive days at weeks 1, 3, and 6. Data collected from the reported 3-day food and drink diary will also be used in analyses. To further increase study compliance, a daily message will be sent to the participants in the TRE group to remind them of their eating window. Those participants who do not respond to 2 messages per week will be followed up by phone call.

This trial allows all concomitant prescribed therapeutic interventions, in relevance to the measured parameters, without any restrictions. However, we will closely monitor for any changes in medications. If a participant needs to make a medication change, we will assess whether they should remain in the study on a case-by-case basis. Any change in medication dose will be considered a potential confounding factor. This approach will help to minimize confounding due to medication use and ensure the validity of our findings.

A general practitioner and a nurse will be present at each visit to monitor the participants’ well-being during and at the end of the session. Following blood sampling, the participants will be assessed for any potential adverse incidents. Signs and symptoms, such as dizziness, puncture site bleeding, shortness of breath, blood pressure, and imbalance, will be carefully evaluated for any necessary intervention.

The primary outcome of this trial is to determine the effect of a 10-h TRE compared to standard dietary recommendations on indices of arterial stiffness among men with MetS. The secondary outcomes consist of changes in anthropometric parameters, indirect calorimetry parameters, lipid profile (triglyceride, total cholesterol, LDL, and HDL), liver enzyme tests (AST, ALT, and ALP), FBG and insulin concentrations, insulin resistance (assessed by HOMA-IR) [19], appetite status, cognitive outcomes (assessed by GNG test and WCST), quality of life assessment (WHOQOL-BREF questionnaire), sleep quality (PSQI questionnaire), physical activity assessments (IPAQ and pedometer), and 3-day food and drink diary.

The schedule of enrolment, interventions, and assessments is shown in Table 1.

The sample size was calculated according to the percentage of PWV change in the study by Sutton et al. [13]. Considering an alpha error of 5%, beta error of 20%, and an effect size of 0.63, a sample size of 20 was estimated for each of the two study groups.

Taking into account a 20% loss to follow-up, 24 participants will be enrolled in each group.

The potential participants will be identified from the PERSIAN Cohort Study population at Mashhad University of Medical Sciences based on IDF criteria for MetS. They will be contacted via phone calls and invited to participate.

The recruited individuals will be allocated using a computer-generated sequence of random numbers, to the intervention and the control groups.

Each participant will receive a sealed envelope containing a random number which will assign them to either the intervention or control group.

An individual who is not part of the research team will be responsible for creating the allocation sequence, enrolling participants, and assigning them to the interventions.

The type of intervention does not allow blinding for either participants or field researchers who will provide advice on the type of regimen. The statistical analysis however will be conducted by a blinded statistician, to minimize potential bias in the data analysis.

Considering that only the statistician will be blinded, unblinding would not be needed in any circumstances.

Measurements will be conducted at baseline, week 3, and at the end of the intervention (week 6). All assessments will be conducted in the morning, following at least a 10-h fasting period. Before the visits, participants will be informed of the conditions required for the assessments and asked to comply with them.

It is expected that arterial compliance will improve in the TRE group compared to the control group. In relation to secondary outcome measurements, an improvement in lipid profile, liver enzyme tests, FBG, insulin concentration, and insulin resistance is expected. There might also be improvements in cognitive outcomes, a reduction in the hs-CRP inflammatory marker, and changes in body weight and composition, with the latter possibly leading to small changes in RMR.

Participants in the TRE group will receive daily messages to improve compliance, while all participants will be contacted and reminded of their scheduled visits.

In the current study, the database will be generated by entering data in the SPSS software version 20. To increase the accuracy of data collection, all data will be collected by the primary investigator. To ensure that the information collected during the study is kept securely and confidentially, each participant will be assigned a code that will be used to identify him throughout the study. Two separate, password-protected databases will be created, one containing the participants’ personal information (to which only the primary investigator will have access) and one containing the coded study data (to which the study team will have access). The written information, e.g., consent forms and any other data input forms will be kept in a locked cupboard in the primary investigator’s office. The final dataset will comprise all data collected on each participant identified using their unique code. Data sharing between investigators through networks will also be password-protected and access will be allowed only by investigators with a link sent by the primary investigator. All passwords will be changed every 3 months for security reasons.

As explained in the “Data management” section, to ensure the confidentiality of data, each participant will be assigned a code throughout the study, and the databases will be password-protected. Accessing data will be allowed only by other investigators with a link sent by the primary investigator. All passwords will be changed every 3 months for security reasons. No participants will be identified in any publication or other dissemination activities and participants will be free to withdraw from the study without providing any reason and without their care by our team or other healthcare professionals being affected in any way. Collected data, of any kind, will be destroyed 5 years after completion of the study.

Blood samples will be collected from the median cubital vein at each visit. The samples will be centrifuged, and the resulting serum and plasma will be stored at – 80 °C until analysis [38]. Also, the remaining samples will be stored at − 80 °C, at Mashhad University of Medical Sciences for future use.

The SPSS software version 20 will be used for data analysis based on the intention-to-treat method. Comparison between groups will be conducted by independent sample t-test and Mann-Whitney tests for quantitative data, while within-group comparisons will be assessed using paired sample t-test and Wilcoxon signed-rank tests. Comparison between groups will be conducted by chi-square test for qualitative data, and within-group comparisons will be assessed by McNemar’s test. Qualitative data (levels of physical activity based on IPAQ) will be presented using frequencies and percentages, while quantitative data (anthropometric parameters, arterial stiffness indices, indirect calorimetry parameters, laboratory tests, appetite VAS score, GNG score, WCST score, WHOQOL-BREF score, PSQI score, pedometer reports, 3-day food and drink diary reports) will be presented using mean and standard deviation or median and interquartile range depending on the data distribution. The relationship between outcomes will be assessed using Pearson’s correlation coefficient and Spearman’s rank correlation coefficient. A p-value of < 0.05 will be considered statistically significant.

To mitigate the risk of contamination in TRE trials, we will conduct sensitivity analyses to assess the impact of potential adjustments in the control group’s eating times. Furthermore, we will compare the results of the intention-to-treat analysis with per-protocol analyses, by checking the consistency of these results. Also, appropriate statistical methods will be employed to predict potential contamination and its impact on the study results.

This study will not include interim analyses. If frequent adverse effects are observed, the intervention will be halted, and a formal report will be submitted to the Ethics Committee of Mashhad University of Medical Sciences for their decision-making.

In this study, no additional analyses are intended to be conducted.

The trial will use an intention-to-treat analysis protocol, to minimize the impact of participant dropouts and non-adherence to the protocol by analyzing all participants according to their original assignment, regardless of whether they completed the study or not. However, missing data from participants who drop out or do not adhere to the protocol will be imputed using statistical methods to ensure that the analysis will not be biased by missing data.

We are committed to promptly publishing the complete study protocol and findings. Upon reasonable request, the corresponding author is able to present an anonymized dataset and statistical code.

The Ethics Committee and Vice Chancellor of Research and Technology at Mashhad University of Medical Sciences will oversee and coordinate all phases of the study. These are academic units, free from any conflicting interests.

The Ethics Committee of Mashhad University of Medical Sciences will be kept fully informed throughout the entire research process and will conduct at least two accuracy checks to ensure the integrity of the research.

TRE is a safe intervention, with only a few mild adverse events noted in some studies, according to a recent 2023 systematic review [14]. While severe adverse events are not expected in this study, minor events such as fatigue, dizziness, and headache may occur. These minor events will be monitored in terms of severity, time of occurrence, duration from the beginning of the intervention, and causality. This data will be reported to the Ethics Committee of Mashhad University of Medical Sciences for further decisions.

The study protocol will undergo an assessment by a team of research ethics professionals, and the study procedure will be reviewed by the assigned supervisor after 5% of the study participants are recruited, in accordance with university regulations. At the end of the study, the research procedure will be evaluated, by the same assessment team.

Any changes to the study’s protocol that may impact its conduct will be approved by the ethical committee of Mashhad University of Medical Sciences before being executed. Trial participants will be informed of the changes and their potential impact. The Iranian Registry of Clinical Trials will also be informed of any amendments conducted in the trial.

The study protocol has been approved by the Ethics Committee of Mashhad University of Medical Sciences (Code: IR.MUMS.MEDICAL.REC.1401.213). This trial will be conducted in accordance with the Declaration of Helsinki, and researchers are obligated to obtain written informed consent from all participating subjects.

Not applicable as no personal data was used in this article.

The authors declare that they have no competing interests.

Data sharing does not apply to this article, as no datasets are generated or analyzed yet so far, during the current study. But corresponding author may provide datasets upon reasonable request, after the collection and/or analysis of the datasets.