Exercising in the morning or evening can influence results, study suggests

Morning exercise has very different effects on metabolism than the same workout performed later in the day, according to an ambitious new animal study of exercise timing.

The study, which involved healthy laboratory mice running on tiny treadmills, mapped hundreds of disparities in the numbers and activities of molecules and genes throughout the rodents’ bodies, depending on whether they ran in the morning or at night.

Many of these changes are related to fat burning and other aspects of animal metabolism. Over time, these changes can substantially influence your risk of illness and well-being.

While the study featured rodents, its findings are likely relevant to anyone wondering whether it’s better to work out before work, or whether we can get as many — or more — health benefits from exercising after work.

As anyone knows, our internal operations and those of nearly all living creatures follow a well-orchestrated and comprehensive 24-hour circadian rhythm.

Recent animal and human studies show that nearly every cell in our body contains a version of a molecular clock that coordinates with a whole-body timing system to drive most biological operations.

Thanks to these internal clocks, our body temperature, blood sugar, blood pressure, hunger, heart rate, hormone levels, sleepiness, cell division, energy expenditure, and many other processes rise and fall in repeated patterns throughout the day.

These internal rhythms, while predictable, are also flexible. Our internal clocks can recalibrate themselves, research has shown, based on complex clues inside and outside of us. In particular, they respond to light and dark, but they are also affected by our sleeping habits and eating schedules.

Recent research suggests that the time of day we exercise also sets our internal clocks. In previous studies with mice, running at different times affected the animals’ body temperature, heart function and energy expenditure throughout the day and altered the activity of genes related to circadian rhythm and aging.

Results in people, however, have been inconsistent.

In a small 2019 study of men who adhered to an exercise program to lose weight, for example, those who worked out in the morning lost more pounds than those who worked out later in the day, even if they all completed the same exercise routine. .

But in a 2020 study of men at high risk of type 2 diabetes who started exercising three times a week developed greater insulin sensitivity and blood sugar control when they trained in the afternoon than in the morning.

These results echoed similar findings from 2019, in which men with type 2 diabetes who exercised intensely first thing in the morning had unexpected and undesirable spikes in blood sugar levels after exercise, while the same afternoon workouts improved their blood sugar control. .

Few of these studies have ventured further, however, to analyze the molecular changes that promote health and circadian outcomes, which may in part explain the discrepancies between the studies. Experiments that examined the effects of exercise at a microscopic level, usually in mice, tended to focus on a single tissue, such as blood or muscle.

But scientists who study physical activity, metabolism and chronobiology suspect that the impacts of exercise timing would extend to many other parts of the body and involve a complex interaction between various cells and organs.

So for the new study, published this month as the cover article of the journal Cell Metabolism, an international consortium of researchers set out to try to quantify almost all of the metabolism-related molecular changes that occur during exercise at different times of day.

Using healthy male mice, they had some run moderately on wheels for an hour early in the day and others run for the same amount of time in the evening. An additional group of mice was on locked wheels for an hour at these same times and served as a sedentary control group.

Beginning about an hour after the workouts, the researchers took repeated samples of the muscle, liver, heart, hypothalamus, white fat, brown fat and blood from each animal and used sophisticated machines to identify and enumerate nearly every molecule in these tissues related to use. power. They also looked at markers of activity in genes related to metabolism. They then tabulated the totals between tissues and between groups of mice.

Interesting patterns emerged. Since rats are nocturnal, they wake up and are active at night and get ready for bed in the morning, a time opposite to ours (unless we are vampires or teenagers).

When the mice ran at the start of their active time — the morning equivalent for us — the researchers counted hundreds of molecules whose number increased or decreased after exercise, and that differed from the levels seen in mice that ran closer to their wake time. sleep or not exercise.

Furthermore, some of these changes occurred almost identically in different parts of the body, suggesting to researchers that various organs and tissues were, in fact, communicating. The rodents’ muscles and livers, for example, shared many molecular changes when the animals ran in the morning, but less so when they ran just before bed.

“It was remarkable” to see how exercise timing affected the levels and activities of so many molecules throughout the animals’ bodies, said Juleen Zierath, professor of clinical integrative physiology at the Karolinska Institute in Stockholm, Sweden, and executive director of the Research Center. of the Novo Nordisk Foundation at the University of Copenhagen (Denmark), which oversaw the new study.

In general, differences in molecular profiles between morning workouts (in terms of rats) and later workouts tended to signal greater use of fat than blood sugar to fuel the initial exercise. The opposite occurred when the mice ran at night. If these patterns hold true for humans, it might suggest that morning exercise contributes more to fat loss, while later in the day exercise may be better for blood sugar control.

But mice are not people, and we still don’t know whether the same molecular patterns hold for us. Study researchers are working on a comparable experiment involving people, Zierath said.

Translated by Luiz Roberto M. Gonçalves