Exercising within one time window is best for weight loss, study finds

By Kristen Rogers, CNN

(CNN) — If your main fitness goal is weight loss, you may want to consider the time of day you’re exercising. A new study offers clues that could help maximize your efforts.

Exercising between 7 a.m. and 9 a.m. has been associated with having a lower waist circumference and body mass index than people who work out during midday or evening, according to a study published Tuesday in the journal Obesity.

“This is exciting new research that is consistent with a common tip for meeting exercise goals — that is, schedule exercise in the morning before emails, phone calls or meetings that might distract you,” said Rebecca Krukowski, a clinical psychologist with expertise in behavioral weight management who wasn’t involved in the study, in a news release.

Positive links between moderate-to-vigorous physical activity and weight loss have been previously reported by other researchers. However, findings regarding the best time to exercise to shed pounds have been mixed, so the authors of the latest study looked into what influence doing activity at different hours might have on the relationship between exercise and obesity.

The authors studied health and activity data from 5,285 people who had participated in the 2003 to 2006 cycles of the US Center for Disease Control and Prevention’s National Health and Nutrition Examination Survey. (The researchers chose those specific years because that was when accelerometers, or activity trackers, were first used in the survey.)

After participants had their BMI and waist circumference recorded, they wore activity trackers on their right hip during waking hours for 10 hours or more each day for four to seven days.

Those who exercised in the morning — between 7 a.m. and 9 a.m. — had an average BMI of 27.5, compared with midday and evening exercisers, who had a 28.3 BMI on average. Midday was defined as 9 a.m. to 4 p.m., and evening 5 p.m. to 8 p.m. The average waist circumference, adjusted for diet quality and calorie intake, was 96 centimeters (37.7 inches), 97.8 centimeters (38.5 inches) and 97.5 centimeters (38.4 inches), respectively.

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These findings held true regardless of sex, ethnicity, education, tobacco use, alcohol consumption or sedentary behavior. Additionally, even among people who all met the physical activity guidelines — at least 150 minutes per week — achieving this goal in the morning was associated with the lowest BMI and waist size.

“This research added valuable evidence that is based on a national sample of US (participants), which has not been done before on the topic of timing of exercise and weight loss,” said the study’s lead author Dr. Tongyu Ma, a research assistant professor of rehabilitation sciences at the Hong Kong Polytechnic University, via email.

However, participants’ BMI and waist size were measured before the activity tracking period, and they weren’t measured again afterward — so the authors weren’t able to prove that exercising in the morning directly impacted either measurement.

Ma is planning on doing more studies to confirm the findings and whether there is a causal relationship between exercising in the morning and having a smaller BMI and waist size, he said.

Why the time might matter

The reasons behind the findings may have to do with both physiology and lifestyle habits, experts said.

Because of the study design, “it is not known whether people who exercise consistently in the morning may be systematically different from those who exercise at other times, in ways that were not measured in this study,” said Krukowski, a professor of public health sciences and codirector of the Community-Based Health Equity Center at the University of Virginia’s School of Medicine.

“People who exercise regularly in the morning could have more predictable schedules, such as being less likely to be shift workers or less likely to have caregiving responsibilities that impede morning exercise,” she added. “Predictable schedules could have other advantageous effects on weight that were not measured in this study, such as with sleep length (or) quality and stress levels.”

Additionally, morning people may be biologically different from night owls, experts said.

Based on previous studies, the authors noted, morning exercisers have been more likely to have a lower daily caloric intake and passively expend more energy when they’re not exercising. That may sound contradictory, but this tendency likely occurs because working out in a fasted state in the morning means your body is relying on stored fat for energy rather than stored glucose from food. This may mean early birds’ bodies are better equipped for increasing fat oxidation, or burning, both during the exercise and over the following day, even if they’re sedentary post-workout.

In the latest study, morning exercisers were the most sedentary even though they had the lowest BMI and waist circumference.

“I personally like this because it tells me that as long as I work out in the morning, it’s okay to sit in my office and focus on work for the rest of the day without worrying about weight gain,” Ma said via email.

Greater weight loss can also result from doing exercise that’s more concentrated or structured, the authors said — another pattern they found among the morning group.

If you can fit it in, “early morning aerobic exercise — such as biking, running or even brisk walking to start with — is a promising tool for weight loss,” Ma said. “Most people actually are more likely to stick to a morning workout routine than that in midday or evening.”

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