The ongoings inside of our bodies which are largely unbeknownst to us follow a rather busy, intricate, and mutable circadian schedule. These molecular clocks coordinate all of our biological systems throughout the day including blood sugar levels, hunger, heart rate, temperature, cell division, gene expression, awakeness, and energy expenditure among other processes.
The full scope of these internal clocks is not fully understood, but we do know that they recalibrate themselves based on cues from our bodies such as, most notably, synchronizing to light and sleep. But internal clocks are also set by meals which means that when we eat as well as what we eat can influence our health and metabolism.
Studies also suggest that exercise timing can also likewise tune these internal clocks, but in the past studies have been inconsistent in their findings with some suggesting morning workouts before breakfast burn more fat than evening exercising while others report the opposite; some recent results even indicate that early intense exercise may impair blood sugar control but the same exercises later in the day helped to smooth out blood sugar spikes and improved metabolic health. However, most of these studies focused on 1 type of exercise and rarely controlled meals during the studies making it difficult to determine the effects of exercise timing from what/when people eat.
This study aimed to control participant diet and workout timing, it began by recruiting 24 sedentary overweight men and checking their aerobic fitness, blood sugar control, and other aspects of health including eating habits. Then participants were set up with meal deliveries which consisted of about 65% fat, and they were instructed to consume these foods and nothing else for 5 days then return to the lab for more testing.
Participants were divided into 3 groups: one group exercising every day at 6:30 AM, another group exercising at 6:30 PM, and the final group remaining sedentary as controls. Exercise routines, with the exception of times, were identical, intermingling brief intense intervals on stationary bikes on one day with easier but longer workouts on the next day. Participants exercised for 5 consecutive days while consuming the delivered high-fat diet, after which the researchers repeated all the original testing to compare results.
Results revealed that after the first five days on the high-fat diet participant’s cholesterol levels increased, especially LDL levels, and their blood contained altered levels of certain molecules related to metabolic and cardiovascular problems suggesting greater risks for heart diseases. Surprisingly, the early morning exercise routine did very little to mitigate these effects, with those in the morning group showing the same heightened cholesterol and troubling molecular patterns in their blood as the control group. Evening exercise helped to lessen the worst impacts of the high-fat diet, with those in the evening group having lower cholesterol levels and improved patterns of molecules related to cardiovascular health in their bloodstreams after 5 workouts. Those in the evening group also developed better blood sugar control after their workouts while they slept than either of the other two groups.
According to study lead Trine Moholdt, the findings indicate that “the evening exercise reversed or lowered some of the changes” accompanying the high-fat diet while “morning exercise did not.”
While this study does not tell us why/how evening workouts were more effective in improving metabolic health, the researchers suspect that they may have greater impacts on molecular clocks and gene expression than morning workouts.
The researchers noted that this study is not in any way suggesting that morning exercise is not good for us, as those men did become more aerobically fit whatever the timing of the exercise was. According to Moholdt, any exercise is better than not exercising at all, but working out later in the day may offer unique benefits for helping to improve fat metabolism and blood sugar control, particularly for those eating a diet high in fat.
The study involved men only to avoid issues related to women’s menstrual cycles, the team hopes to conduct additional research to investigate the effects of exercise timing among women and older people as well as the interplay of exercise and timing of sleep.
