This study is looking at whether eating all meals within a 6-hour window each day (called Time-Restricted Eating, or TRE) can improve body composition, metabolism, heart health, and blood markers over 12 weeks. The study also wants to find out whether allowing participants to eat normally on weekends makes the diet easier to follow long-term.

Adults aged 18-65 with a BMI of 26 or higher who do not have heart disease or diabetes may be eligible. Participants will be randomly placed into one of two groups: (1) TRE every day for 12 weeks, or (2) TRE on weekdays with normal eating on weekends. All participants will visit the lab twice - once at the start and once at the end - for measurements of body fat, resting calorie burn, heart function, and blood tests. Between visits, participants will log their meals and steps at home and check in with the research team every two weeks.

This randomized, two-arm parallel study examines the effects of Timed Restricted Eating (TRE) with or without Intermittent Energy Restriction (IER) on body composition, resting energy expenditure (REE), cardiometabolic blood markers, cardiovascular modulation, psychosocial well-being, dietary adherence, and spontaneous physical activity over a 12-week intervention in adults with overweight or obesity.

Study Design and Participants Sixty adults (30 male, 30 female) aged 18-65 years with a BMI ≥26 kg/m² will be enrolled. The target sample accounts for an anticipated one-third attrition rate, yielding a final expected sample of 40 participants (20 per group), based on a power analysis using an effect size of f=0.25 and power of 0.80. Eligible participants must be weight-stable (no more than 5 kg change in the prior 3 months), physically inactive (≤150 min/week of moderate-intensity activity), free of cardiovascular disease and diabetes, and not taking medications that could influence study outcomes. Individuals with implanted electronic devices (e.g., pacemakers, ICDs) are excluded due to the use of bioimpedance spectroscopy.

Randomization Following baseline assessments, participants will be randomly assigned in a 1:1 ratio to one of two groups using sex-stratified permuted block randomization (block sizes of 2 and 4), generated a priori using the blockrand package in R.

Intervention Groups Group 1 (TRE Only): Participants restrict all calorie-containing food and beverages to a 6-hour afternoon eating window (e.g., 2-8 pm) every day for 12 weeks.

Group 2 (TRE + IER): Participants follow the same 6-hour eating window Monday-Friday, but return to unrestricted eating on weekends (Saturday-Sunday) to test whether periodic diet breaks improve long-term adherence.

Laboratory Visits (Pre- and Post-Intervention) Both visits (~3-3.5 hours each) follow identical protocols. Participants arrive after an overnight fast of at least 8 hours, having consumed ~500 mL of water in the 1-2 hours prior, and having abstained from vigorous exercise, alcohol, and caffeine for at least 24 hours.

Body composition is assessed using the 4-compartment (4C) model - the gold standard for body composition - combining: (1) DXA for bone mineral content; (2) air displacement plethysmography (BodPod) for body volume; (3) bioimpedance spectroscopy (BIS) for total body water; and (4) a calibrated digital scale for body mass. These are combined using the Wang equation to derive fat mass, fat-free mass, and body fat percentage.

Cardiovascular modulation is assessed with the ATCOR SphygmoCor® system, capturing peripheral and central hemodynamics, arterial stiffness via carotid-femoral pulse wave velocity (applanation tonometry), and pulse wave reflection variables (AIx, AIx75, AP, SEVR, reflection magnitude).

REE is measured via indirect calorimetry using a ventilated canopy (metabolic cart) over a 30-minute supine rest period. The 5-minute window with the lowest coefficient of variation (≤5%) is used for analysis.

Venous blood is drawn from a forearm vein and sent to LabCorp for a standard metabolic panel including fasting glucose, lipid panel (total cholesterol, LDL, HDL, triglycerides), liver enzymes, and kidney function markers.

Psychosocial well-being is assessed using the validated WHOQOL-BREF questionnaire (31 items across four domains: physical health, psychological health, social relationships, and environment).

12-Week Intervention Monitoring Participants self-document the time of their first and last calorie-containing meal daily and record daily step counts from a research-grade pedometer. The research team conducts check-ins every two weeks for compliance assessment and ease-of-adherence surveys. Adverse events are monitored continuously and reported per IRB requirements.

Post-Intervention Only Relative energy balance (average daily calorie surplus or deficit over 12 weeks) will be estimated post-hoc from DXA-derived changes in fat mass and fat-free mass using previously validated equations, minimizing participant burden by eliminating the need for daily dietary recall.

Inclusion Criteria:

• Between 18 and 65 years of age
• Body mass index (BMI) of 26 kg/m² or higher (overweight or obese range)
• Physically inactive, defined as 150 minutes or less of moderate-intensity physical activity per week for the 3 months prior to enrollment
• No previous history of cardiovascular disease or Type 1 or Type 2 Diabetes Mellitus
• Not currently taking medications that may affect blood sugar, cholesterol, or body weight (e.g., glucose-lowering drugs, statins, or weight loss medications)
• Weight stable, defined as no more than 5 kg (about 11 lbs.) of body weight change in the 3 months prior to enrollment
• Willing and able to follow the study requirements, provide voluntary consent, and complete all study procedures

Exclusion Criteria:

• Younger than 18 or older than 65 years of age
• BMI below 26 kg/m²
• More than 150 minutes per week of moderate-intensity physical activity in the past 3 months
• History of cardiovascular disease or Type 1 or Type 2 Diabetes Mellitus
• Currently taking medications that may impact study outcomes
• Weight change of more than 5 kg in the past 3 months
• Pregnant, breastfeeding, or planning to become pregnant during the study
• History of an eating disorder
• Previous weight loss surgery (e.g., gastric bypass or sleeve gastrectomy)
• Unable to fast for approximately 18 hours at a time due to a medical condition
• Currently working night shifts
• Has a pacemaker or any implanted electronic device (due to bioimpedance spectroscopy testing)
• Any health condition the research team determines would put the participant at risk