Metabolic

Various health benefits for morning and evening exercise

Gretchen Reynolds

CNA/THE NEW YORK TIMES — Exercising in the morning has very different effects on metabolism than doing the same exercise later in the day, according to an ambitious new animal study
training time.

The study, which involved healthy laboratory mice jogging on tiny treadmills, mapped hundreds of differences in the number and activity of molecules and genes in the rodents’ bodies depending on whether they ran lower first thing in the morning or later in the evening.

Many of these changes are related to fat burning and other aspects of the animal’s metabolism. Over time, such changes could significantly impact their disease risks and well-being. Although the study looked at rodents, its findings are likely relevant to any of us wondering if it’s better to exercise before work, or if we could reap just as much – or more – health benefits from exercising after work .

As anyone with a body knows, our internal workings, and those of almost all living things, follow a well-orchestrated and ubiquitous 24-hour daily rhythm. Recent studies in animals and humans show that almost every cell in our body contains some version of a molecular clock that is coordinated with a broader whole-body timing system to control most biological processes.

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 slow in repeated patterns throughout the day.

While these inner rhythms are predictable, they are also malleable. Our internal clocks can recalibrate themselves, research shows, based on complex cues from within and without. They react particularly to light and dark, but are also influenced by our sleeping habits and our diet.

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

However, results in humans have been inconsistent. For example, in a small 2019 study of men participating in an exercise program to lose weight, those who exercised in the morning lost more pounds than those who exercised later in the day, even though they all performed the same exercise routine.

But in a 2020 study, men at high risk for type 2 diabetes who began exercising three times a week developed better insulin sensitivity and blood sugar control when they exercised in the afternoon than in the morning.

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

Few of these studies, however, ventured deep below the surface to examine the molecular changes driving health and circadian outcomes, which may help explain some of the discrepancies from one study to the next. Those experiments that looked at the effects of exercise at the microscopic level, usually in mice, tended to focus on a single tissue, such as blood or muscle.

However, scientists studying physical activity, metabolism and chronobiology hypothesized that the effects of exercise timing would extend to many other parts of the body and involve intricate interactions between multiple cells and organs.

For the new study, published as the cover article in Cell Metabolism this month, an international consortium of researchers decided to quantify almost every molecular change related to metabolism that occurs during exercise at different times of the day.

With healthy, male mice, they jogged moderately on wheels for an hour early in the day and others walked the same amount in the evening. An additional group of mice sat on locked wheels for one hour during the same times, serving as a sedentary control group.

About an hour after the workout, the researchers repeatedly took samples of each animal’s muscle, liver, heart, hypothalamus, white fat, brown fat, and blood, and used sophisticated machines to identify and enumerate almost every molecule in those tissues associated with energy expenditure connected. They also checked activity markers of genes related to metabolism. Then they tabulated the totals between tissues and between groups of mice.

Interesting patterns emerged. Since mice are nocturnal, they wake up in the evening and become active and prepare for sleep in the morning, a schedule opposite to ours (unless we’re vampires or teenagers).

When the mice jogged at the start of their active period — which corresponds to morning for us — the researchers counted hundreds of molecules that increased or decreased in number after exercise, and that differed from the levels observed in mice that were closer ran at their bedtimes or didn’t exercise at all.

Furthermore, some of these changes occurred almost identically in different parts of the body, suggesting to the researchers that different organs and tissues were indeed communicating with each other. For example, the rodents’ muscles and livers showed many molecular changes when the animals ran in the morning, but less so when they jogged just before bedtime.

“It was quite remarkable” to see how much the timing of training affected the levels and activities of so many molecules in the animals’ bodies, said Professor Juleen Zierath of clinical integrative physiology at the Karolinska Institute in Stockholm, Sweden, and executive director of Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen, which oversaw the new study.

Overall, the differences in molecular profiles between morning exercise (from a mouse perspective) and later exercise indicated a greater reliance on fat than blood glucose to drive early exercise.

The opposite happened when the mice ran in the evening. If these patterns hold true in humans, it could indicate that morning exercise contributes more to fat loss, while late-day exercise may be better for glycemic control.

But mice aren’t humans, and we don’t yet know if the molecular patterns apply to us. The study researchers were working on a similar experiment in humans, said Dr. Zierath.

This study was also limited in scope, examining a single session of moderate aerobic exercise in male mice. It doesn’t show how other types of morning or evening exercise affect the inner workings of mice or humans.

Nor does it tell us what we eat or what time of day we eat, and whether chronotypes—whether we’re more morning or evening people—influence these effects, or whether being female is important.

But despite its limitations, “this is a very important study,” said a professor of medicine and endocrinology at the University of Minnesota, Dr. Lisa Chow, who was not involved in this research. It underlines the power of movement at any time of the day.

It also suggested that as additional studies build on these findings, we may be better able to plan our exercise to meet specific health goals.

Follow-up studies are likely to tell us, for example, whether an evening bike ride or jog is more effective at warding off diabetes than a morning walk or swim.

But for now, said Dr. Chow, “the best time for people to exercise would be whenever they have the opportunity to exercise.”

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