Depending on your body composition and overall health, it turns out there is a best time of day to exercise. People living with obesity reap the greatest health benefits from being physically active in the evening, according to a new Australian study.
Researchers at the University of Sydney’s Charles Perkins Centre found that participants who did the bulk of their moderate-to-vigorous aerobics from 6–11:59 p.m. had the lowest risk of premature death, cardiovascular disease, and microvascular disease. The results were published last month in the journal Diabetes Care.
The World Health Organization (WHO) has deemed obesity a global epidemic, calling it “one of today’s most blatantly visible—yet most neglected—public health problems.” Globally, one in eight people were living with obesity in 2022. That year, 2.5 billion adults 18 and older were overweight, 890 million of whom were living with obesity. More than 390 million children and teens ages 5–19 were overweight, as were 37 million children under 5.
WHO recognizes obesity as a chronic disease that can not only impair health but also impede quality of living. It comes with a heightened risk of myriad medical problems, from stroke and certain cancers to neurological disorders and chronic respiratory diseases.
“Exercise is by no means the only solution to the obesity crisis,” study coauthor Angelo Sabag, PhD, said in a news release. “But this research does suggest that people who can plan their activity into certain times of the day may best offset some of these health risks.”
Frequency of physical activity more important than total amount
The Australian study was observational, analyzing the data of nearly 30,000 people from the UK Biobank who had a body mass index of at least 30, which is considered obese. Of those, about 3,000 also had Type 2 diabetes. All were older than 40, with an average age of 62. The cohort was 53% female.
Each study participant wore an accelerometer on their dominant wrist around the clock for a week. These devices allowed researchers to classify participants’ movements by type—sedentary, standing utilitarian, walking, or running/high-energy—and intensity—sedentary, light, moderate, or vigorous.
Researchers focused on periods of moderate-to-vigorous physical activity (MVPA) lasting at least three minutes. About half of participants didn’t have any such MVPA periods and formed the control group. The rest were divided into morning (6–11:59 a.m.), afternoon (noon to 5:59 p.m.), and evening (6–11:59 p.m.) groups based on when the majority of their MVPA bouts occurred.
“We didn’t discriminate on the kind of activity we tracked,” coauthor Matthew Ahmadi, PhD, said in the news release. “It could be anything from power walking to climbing the stairs, but could also include structured exercise such as running, occupational labor, or even vigorously cleaning the house.”
Because the data were collected between 2013–2015, researchers were able to track participants’ health over time. Over an average eight years, they measured incidence of cardiovascular disease, microvascular disease, and mortality among participants. People who already had cancer or cardiovascular disease were excluded.
Compared to the control group, the morning, afternoon, and evening MVPA groups all had a lower risk of death. However, the evening group showed the lowest risk. Researchers observed similar results for cardiovascular and microvascular disease, with the evening group showing the lowest risk.
Among the subset of participants who also had Type 2 diabetes, the evening MVPA group again showed the lowest risk of death, cardiovascular disease, and microvascular disease.
In addition, the frequency with which people completed their evening MVPA was more important than the total amount of their daily physical activity, researchers found.
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Why is evening exercise beneficial to health?
Insulin, the hormone that helps your body use glucose, or sugar, for energy, is the key to why physical activity in the evening is so beneficial, explains Loretta DiPietro, PhD, a professor in the Department of Exercise and Nutrition Sciences at the George Washington University Milken Institute School of Public Health. Insulin is produced in the pancreas, an organ that typically needs help at the end of the day.
“Especially as people get older, [the] beta cells of the pancreas get old, insulin secretion is blunted and it’s a little bit delayed. And this occurs more and more as the day goes on,” DiPietro tells Fortune. “Muscle contractions, all by themselves, will help clear glucose from the blood.”
People tend to eat their biggest meal of the day in the evening, a time when they’re also the most inactive, DiPietro says. That means they’re going to bed with high glucose levels (hyperglycemia), driving up their hemoglobin A1C, a measure of average blood sugar during the past three months.
“Hyperglycemia causes so much damage, first inflammation and then damage to blood vessels in the microvascular system,” DiPietro says. “Especially in these vulnerable people with obesity and diabetes, [evening exercise] could really lower the risk of mortality and microvascular disease. Makes perfect sense.”
While DiPietro wasn’t involved in the Australian study, she coauthored a 2013 Diabetes Care study showing that for older adults with obesity and at risk of impaired glucose tolerance, a 15-minute walk 30 minutes after each meal was as effective as a 45-minute morning walk in improving glycemic control. What’s more, her findings suggested after-dinner walks were the most beneficial.
You needn’t worry about hitting the gym after a heavy dinner. A light walk around the neighborhood will do, DiPietro says: “Europeans have been doing this for centuries.” If you do prefer vigorous exercise at night, however, try not to do so close to bedtime. Previous research has shown nighttime exercise to upset the body’s internal clock, making it harder to fall asleep.
The University of Sydney researchers tried to eliminate bias by controlling for variables including age, biological sex, medication use, smoking and alcohol intake, and fruit and vegetable consumption. Even so, they couldn’t rule out reverse causation—the possibility that some people engaged in minimal physical activity because they had an underlying disease.
The bottom line, according to DiPetro, is there’s no bad time of day to better your cardiometabolic health.
“Exercise or get sufficient physical activity whenever you can,” she says. “But for those of you [with obesity], you probably get more bang for your buck to do it after the evening meal.”
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