Morning Or Evening? When You Exercise May Help Determine How Much Fat You Burn

Exercise is important for health, but does the time of the day affect its efficacy? Scientists say it does.

In a study, which was published Monday in the journal PNAS (The Proceedings of the National Academy of Sciences), researchers looked at the effects of the timing of exercise on fat (adipose tissue) metabolism.

"Nearly all cells in the body have an internal biological circadian clock that is synchronized by external cues," they explained. "A variety of factors including hormones, temperature, food-intake, and exercise can act on tissue-specific peripheral clocks to alter the expression of genes that influence metabolism, all in a time-of-day dependent manner."

To find out, the team studied the adipose tissue of mice, Karolinska Institutet noted in a news release. They looked at it "immediately" after high-intensity exercise at two points of the day: Early active phase, which is essentially late morning, and early rest phase, which corresponds to a late evening session. They also looked at the genes that were active in the fat tissues after the exercise.

Researchers found a timing-related impact of exercise on adipose tissue that appears to be in favor of the early active phase. The responses indicating a higher metabolic rate were "only" observed in mice that exercised in the early active phase. This was "independent of feeding status."

"Thus, the response of adipose tissue to exercise is time-of-day sensitive and may be partly driven by the circadian clock," the researchers wrote.

"Our results suggest that late morning exercise could be more effective than late evening exercise in terms of boosting the metabolism and the burning of fat, and if this is the case, they could prove of value to people who are overweight," study author Juleen R. Zierath, of Karolinska Institutet, said in the news release.

It's possible then that the time of day at which exercise is done may have an impact on the efficacy to burn fat. So, people can shift their exercise routines to the time of day wherein the effects of physical activity could, perhaps, be maximized.

However, further studies would be needed to see exactly how the findings translate to humans, said Zierath. Furthermore, whether it will help in adipose metabolism in cases of obesity or type 2 diabetes "warrants further investigation," the researchers said.