Is morning or afternoon exercise better for metabolic syndrome? Original paper

In this randomized controlled trial in participants with metabolic syndrome, high-intensity interval training was beneficial for several aspects of cardiometabolic health. Morning exercise appeared better than late afternoon exercise for some outcomes (e.g., insulin resistance).

The comparative effects of morning and afternoon aerobic exercise, specifically high-intensity interval training (HIIT), on metabolic syndrome.

The primary outcome was a metabolic syndrome z-score, which was based on the degree to which the participants exceeded each metabolic syndrome classification cutoff (see our page on metabolic syndrome for these metrics and their cutoffs). Other outcomes included individual metabolic syndrome components, body composition (e.g., weight, fat mass, fat-free mass), fasting insulin, HOMA-IR (a marker of insulin resistance), maximal fat oxidation rate, maximal power output, and VO₂max.

139 adults (90 men, 49 women) with metabolic syndrome.

The average age of the participants was 57, and all of the participants were engaging in less than 150 minutes of moderate-intensity physical activity per week before the start of the trial.

In this 16-week randomized controlled trial, the participants were assigned to 1 of 3 groups: morning exercise, afternoon exercise, or a waitlist control group.

The morning and afternoon exercise groups received the same exercise program. In a supervised setting, the participants cycled on a stationary bike, starting with a 10-minute warm-up at 70% of their maximum heart rate (HRmax; determined before the start of the trial). This was followed by 4 bouts of cycling at 90% HRmax for 4 minutes, interspersed with 3 minutes of cycling at 70% HRmax. Finally, the session ended with a 5-minute cooldown period. The participants were required to attend at least 90% of the 48 exercise sessions provided during the trial.

The morning exercise group sessions occurred between 8:00 a.m. and 9:00 a.m., and the afternoon group sessions occurred between 4:00 p.m. and 6:00 p.m. The participants were required to consume a meal at least 1-hour before all exercise sessions. The control group participants were asked to maintain their usual level of activity.

Compared to the control group, metabolic syndrome z-scores decreased (improved) in both exercise groups, and z-scores improved more in the morning compared to the afternoon exercise group (–52% vs. –19%).

Both exercise groups saw improvements in fat mass, waist circumference, diastolic blood pressure, maximal fat oxidation rate, maximal power output, and VO₂max compared to the control group. Only the morning exercise group saw improvements in systolic blood pressure, insulin, and HOMA-IR compared to the control group.

Based on food diaries that were collected monthly, all groups consumed similar amounts of calories and macronutrients during the trial.

This study (which was conducted in Spain) used waist circumference cutoffs for metabolic syndrome that are not recommended for use in Asian populations. It was not reported whether any participants were of Asian ethnicity.