Is it better for our bodies to work out at certain times of day? A new study of exercise timing and metabolic health suggests that, at least for some people, the answer is a qualified yes. The study, which looked at men at high risk for type 2 diabetes, found that those who completed afternoon workouts upped their metabolic health far more than those who performed the same exercise earlier in the day. The results add to growing evidence that when we exercise may alter how we benefit from that exercise.

Scientists have known for some time that the chronology of our days influences the quality of our health. Studies indicate that every tissue in our bodies contains a kind of molecular clock that chimes, in part, in response to biological messages related to our exposure to light, food and sleep.

These cellular clocks help to calibrate when our cells divide, fuel up, express genes and otherwise go about their normal work. Tuned by our lifestyles, these clocks create multiple circadian rhythms that prompt our temperatures, hormone levels, blood sugar, blood pressure, muscular strength and other biological systems to dip and crest throughout the day.

Circadian science also shows that disrupting normal, 24-hour circadian patterns can impair our health. People working overnight shifts, whose sleep habits are upended, tend to be at high risk for problems such as obesity and type 2 diabetes. Same for people who eat late, outside usual dinner hours.

But much of this research focussed on when we eat or go to bed. Whether, and how, exercise timing might influence metabolic health has been less clear, and the results of past experiments have not always agreed. Some suggest that morning workouts amplify fat burning and weight loss. But those experiments often manipulated the timing of breakfast and other meals, as well as exercise, making it difficult to tease out the particular circadian effects of workouts. They also typically involved healthy volunteers.

A 2019 study, on the other hand, found that men with type 2 diabetes who completed a few minutes of high-intensity interval sessions in the afternoon substantially improved their blood sugar control after two weeks. If they did the same intense workouts in the morning, however, their blood-sugar levels actually spiked in an unhealthy fashion.

Patrick Schrauwen, a professor of nutrition and movement sciences at Maastricht University Medical Centre in the Netherlands, read that 2019 study with interest. He and his colleagues had been studying moderate exercise in people with type 2 diabetes, but in their research, they had not considered the possible role of timing. Now, seeing the varying impacts of the intense workouts, he wondered if the timing of moderate workouts might likewise affect how the workouts changed people’s metabolisms.

Fortuitously, he and his colleagues had a source of data, in their own prior experiment. Several years earlier, they had asked adult men at high risk for type 2 diabetes to ride stationary bicycles at the lab three times a week for 12 weeks, while the researchers tracked their metabolic health. The scientists had also noted when the riders showed up for their workouts. They pulled out data for the 12 men who had worked out between 8 and 10am and compared them with another 20 who always exercised between 3 and 6pm. They found that the benefits of afternoon workouts decisively trumped those of morning exercise.
After 12 weeks, those who had pedalled in the afternoon displayed significantly better average insulin sensitivity than the morning exercisers, resulting in a greater ability to control blood sugar. They also had dropped somewhat more fat from around their middles, even though everyone’s exercise routines had been identical.

“I believe doing exercise is better than not doing exercise, irrespective of timing,” Schrauwen says.

“However, this study does suggest that afternoon exercise may be more beneficial” for people with disrupted metabolisms than the same exercise done earlier.

The study, published in Physiological Reports, involved only men, though. Women’s metabolisms might respond differently.

The researchers also did not delve into why the later workouts might affect metabolism differently. But Schrauwen believes moderate afternoon exercise may have an impact on the foods we consume later in the evening and “help to faster metabolise people’s last meals”. This effect could leave our bodies in a fasted state overnight, which may better synchronise body clocks and metabolism. He and his colleagues hope to explore the underlying molecular effects in future studies, and whether the timing of lunch and dinner alters results. The team also hopes to look into whether evening workouts might amplify the benefits of afternoon exertion, or undercut them, by worsening sleep.

Ultimately, Schrauwen says, the particular, most effective exercise regimen for each of us will align “with our daily routines” and exercise inclinations. Because exercise is good for us at any time of day — but only if we opt to keep doing it.

NYTNS