Comparison of morning vs evening exercise on weight loss and related health behaviors in individuals with overweight or obesity: study protocol for a 56-week randomized controlled trial (TIMEX).

The TIMEX study is designed as a two-arm, 56-week, block randomized (1:1) aerobic exercise intervention comparing morning versus evening exercise.

• One hundred twenty-eight adults with overweight and obesity will be randomized in a 1:1 ratio.
• Participants are randomized to morning exercise (AM-Ex, 06:00–10:00) or evening exercise (PM-Ex, 15:00–19:00).
• Exercise is moderate- to vigorous-intensity aerobic exercise progressing to 2000 kcal/week of energy expenditure.
• The study is registered at ClinicalTrials.gov (NCT02153252), registered on 12/10/2021.

The primary outcome of the TIMEX study is change in body weight at 7 months.

• Secondary outcomes include changes in body composition, energy intake, hunger and eating behaviors, energy expenditure, and physical activity behaviors.
• Secondary outcomes are assessed at both 7 and 13 months.
• The study aims to capture the full spectrum of energy balance components to understand mechanisms linking exercise timing to weight loss.

Aerobic exercise elicits modest weight loss but with substantial inter-individual variability, including some individuals gaining weight.

• The authors note that 'there is substantial inter-individual variability in the weight loss response with some individuals losing significant amounts of weight and others gaining weight.'
• The time of day exercise is performed is proposed as a potential contributor to this weight loss variability.
• Understanding exercise timing effects may lead to tailored interventions that enhance weight loss.

Recent evidence suggests time-dependent differences exist in how exercise affects body weight and energy balance regulation.

• The study rationale is based on emerging evidence that the timing of exercise may differentially affect body weight and energy balance.
• The authors propose that morning versus evening exercise may have distinct effects on energy intake, hunger, eating behaviors, and energy expenditure.
• The TIMEX study is designed to test whether a guidelines-based dose of aerobic exercise completed at different times of day produces different weight loss outcomes.

The exercise intervention targets a dose consistent with guidelines, progressing to 2000 kcal/week of energy expenditure from moderate- to vigorous-intensity aerobic exercise.

• The exercise prescription is described as 'guidelines-based dose of aerobic exercise.'
• Both AM-Ex and PM-Ex groups follow the same progressive energy expenditure target of 2000 kcal/week.
• The intervention duration is 56 weeks, with key assessment points at 7 months and 13 months.