Is it better for our bodies to work out at certain times of the day?
A new study of exercise timing and metabolic health suggests that, at least for some people, the answer is yes.
The study, which looked at men at high risk of Type 2 diabetes, found that those who completed afternoon workouts upped their metabolic health far more than those who did 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 in both animals and people 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 daily 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 biological work.
Tuned by our lifestyles, these clocks create multiple circadian rhythms inside of us that prompt our bodies' temperatures, hormone levels, blood sugar, blood pressure, muscular strength and other biological systems to dip and crest throughout the day.
Circadian science shows that disrupting normal, 24-hour circadian patterns can impair our health.
People working overnight shifts, for instance, whose sleep habits are upended, tend to be at high risk for metabolic problems such as obesity and Type 2 diabetes. The same is true for people who eat late at night, outside usual dinner hours.
More encouraging research suggests, though, that manipulating the timing of sleep and meals can improve metabolic health.
But much of this research focused 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, for instance, 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 without metabolic problems.
A much-discussed 2019 study, on the other hand, found that men with Type 2 diabetes who completed a few minutes of high-intensity interval training 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 spiked in an unhealthy fashion.
Dr 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 effects of the intense workouts, he wondered if the timing of moderate workouts might affect how the workouts changed people's metabolisms.
Fortuitously, he and his colleagues had a ready-made source of data.
Several years earlier, they had asked adult men at high risk for Type 2 diabetes to ride stationary bicycles three times a week for 12 weeks while the researchers tracked their metabolic health.
The scientists also, incidentally, had noted when the riders showed up for their workouts.
Now, Prof Schrauwen and his colleagues pulled data for the 12 men who had consistently worked out between 8am and 10am and compared them with another 20 who always exercised between 3pm and 6pm. They found that the benefits of afternoon workouts decisively trumped those of morning exercise.
After 12 weeks, the men 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 dropped somewhat more fat from around their middles than the morning riders, although their exercise routines had been identical.
The study, 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 than earlier ones.
But Prof Schrauwen says he believes that moderate afternoon exercise may have an effect on the foods we consume later in the evening and "help to faster metabolise people's last meals" before they sleep. This could leave our bodies in a fasted state overnight, which may better synchronise body clocks and metabolisms and fine-tune health.
He and his colleagues hope to explore whether the timing of lunch and dinner alters those results. The team also hopes to look into whether evening workouts might amplify the benefits of afternoon exertion, or perhaps undercut them, by worsening sleep.
Ultimately, Prof 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 the day - but only if we opt to keep doing it.
NYTIMES
