Afternoon exercise may reduce the risks of premature death more than morning or evening workouts, according to a new large-scale study of more 90,000 men and women.
And other new research indicates that there can be unique benefits to slotting exercise into the morning hours, suggesting that the best time to exercise depends on what we want from a workout.
Why worry about exercise timing?
There is already mounting evidence that the health effects of exercise depend, to some extent, on when we are active. In past research, people at risk for diabetes improved their blood-sugar control more if they worked out in the evening than in the morning, while in other studies, people lost more body fat if they exercised early rather than later in the day.
But most of these studies were small and their results narrow and inconsistent, so it’s been difficult to draw conclusions about when we should exercise.
One of the new studies helpfully steamrolls any concerns about study size, though. Published this month in Nature Communications, the new study marshals data about 92,139 men and women who had joined the UK Biobank, a health study of adults in the United Kingdom, and worn an activity tracker for a week.
Using the trackers’ readouts, the researchers divvied up the volunteers according to how often and when they moved around, checked mortality records for up to seven years after people joined the Biobank, and compared movement patterns and deaths.
The strongest and least-surprising correlation they found was that the men and women who most frequently engaged in moderate or vigorous physical activity (the equivalent of a brisk walk), lived longer than people who rarely worked out, no matter what time of day people got up and moved.
But the researchers also uncovered subtle links between midday exercise and even better odds of a long life.
People who clustered their physical activity between 11 a.m. and 5 p.m. or spread it out throughout the entire day were less likely to die prematurely from heart disease or other causes (except cancer) than people who mainly exercised before 11 a.m. or after 5 p.m.
This afternoon time frame, the researchers point out, neatly coincides with the time of day when, statistically, people are least likely to experience a heart attack.
The benefits of afternoon exercise for longevity were most pronounced for men and the elderly. But, overall, the findings suggest exercise timing “may have the potential to maximize the health benefits of daily [physical activity],” the researchers conclude.
Morning exercise targets body fat
But for many of us, afternoon exercise in the middle of the workday is logistically challenging or temperamentally undesirable.
For those people, a new study featuring healthy mice on tiny treadmills, offers some hope.
The research builds on a separate, ambitious study from last year in which researchers at the Karolinska Institute in Stockholm and other institutions, catalogued almost every molecule that changed in the body tissues of mice, depending on when they ran.
To their surprise, they noticed the changes were especially pronounced in the animals’ fat tissue. “We hadn’t expected fat to be so affected,” said Juleen Zierath, a professor of clinical integrative physiology at the Karolinska Institute and one of the senior authors of the study. She and her co-authors had anticipated that the animals’ muscles and livers, which help fuel exercise, would show the most molecular alterations.
So, for the new study, they decided to focus on fat and how exercise timing changed it. They had male mice run for an hour on tiny treadmills or stand on the same treadmills as a control. Some ran a few hours after they had awaked, equivalent to midmorning for us.
Others ran a few hours into the period when the animals would normally slow and rest.
Some of the animals skipped eating before their workout; others nibbled beforehand on kibble at will.
The researchers drew blood and fat tissue from the animals repeatedly in the hours after they exercised. And they found far more going on in the fat of the animals who ran within a few hours of waking up. Their fat released substantially more fatty acids, the building blocks of fat, into the animals’ bloodstream, ready for use as muscle fuel. And the remaining fat tissue showed larger increases in biochemical markers of heat production and mitochondrial activity than the fat from the evening (in mouse terms) runners, as well as in the activity of certain genes related to fat metabolism.
In effect, a single session of early-day exercise had set up conditions in the animals’ fat tissue that presumably would lead to greater fat burning and fat loss over time than the same exercise in the evening, Zierath said.
How should you decide the best time to exercise?
“This study identified some fascinating effects,” said Jeffrey Horowitz, a professor at the University of Michigan, who studies exercise and metabolism. He was not involved with the new study.
But it also complicates the question of when to work out. Is afternoon exercise preferable if our goal is longevity, while morning exertions are better to narrow our waistline?
In reality, any differences “will be marginal,” Zierath said.
The biggest gains in the longevity study, after all, came from frequent activity, whatever the time of day.
And any increases in fat burning after morning workouts would likely be small, Zierath said.
“We’re talking fine-tuning,” she said. But, over months, years, or a lifetime, “those small changes can be meaningful.”
So, if you’d like to burn a little more fat with each workout and slowly lower your body’s fat stores, there may be advantages to exercising before noon. But if your primary aim is more life, then get up and move around more during the afternoon, if possible.
But most important is to just get up and move.
Do you have a fitness question? Email YourMove@washpost.com and we may answer your question in a future column.
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