August 13, 2019

There's always great debate over the optimal time to exercise for weight loss. While some recent studies have found that exercising in the morning could lead to better weight loss than exercising later in the day, Aaron Carroll, a health care economist and a professor of pediatrics at Indiana University, warns that you should think twice before overhauling your workout routine.

Cheat sheets: Learn more about why study design matters

The original study

The piece of research that gained attention a few weeks ago is a sub-analysis of a broader study, called the Midwest Exercise Trial 2.

For the study, researchers assigned about 100 overweight young men and women to work out five times a week at a physiology lab until they had burned up to 600 calories per session. A control group was assigned not to workout. Researchers studied the participants for 10 months, after which nearly every participant who was assigned to work out had lost weight. However, the amount of weight lost fluctuated significantly.

To investigate why weight loss amounts were so different, Willis conducted a subanalysis, in which he and his colleagues looked through the trial data to see if there was a relationship between the time of day when people worked out and the amount of weight they lost.

In the trial, participants were able to visit the gym whenever they wanted to between 7 a.m. and 7 p.m., signing in each time. When tracking weight change against exercise schedules, Willis and colleagues found that people who worked out before noon had lost more weight on average than those who worked out after 3 p.m.

Willis said the statistical associations were strong but also noted uncertainty. "Based on this data, I would say that the timing of exercise might—just might—play a role" in whether people lose weight and how much weight they lose with exercise, Willis said.

But despite Willis' caveat, the analysis findings gained widespread attention, making headlines in several outlets.

The New York Times ran the story under the headline, "Morning Exercise May Offer the Most Weight Loss Benefits." The men's fitness magazine Men's Health took a similar approach, headlining the coverage, "People Who Work Out in the Morning May Lose More Weight."

The critique of the news coverage

In a video posted on the Incidental Economist, Carroll argues that news outlets have been overstating the researchers' findings. Namely, he notes that this subanalysis did not prove a causal relationship between what time someone exercises and how much weight they lose.

"This study was a subanalysis of [Midwest Exercise Trial 2]" and should not be regarded as similarly rigorous to the study itself, Carroll argues. He explains that the subanalysis was "not a randomized controlled trial, people were not randomized to time of day exercise." He adds, "It's totally possible that there are other differences in people who choose to exercise at different times of the day that matter."

He points to the fact that the researchers had noticed two differences between the early exercise groups and the late exercise groups, with no explanation.

"People in the early exercise group ate more, more than 200 calories each day, than late exercisers did at 10 months. Why? We don't know," he said. "People who exercised later in the day tended to be less active overall than those who exercised earlier in the morning. Why? We don't know."

He also noted that in each exercise time group, "the differences from person to person are huge. … If you randomly picked a person in any group you'd still see big differences" in weight loss, Carroll said.

This was also not a very large study, Carroll adds. "We're talking about like 20 people in each" workout group. "These are not concrete randomized controlled trial data that should convince you to change your mind on anything," Carroll contends (Doherty, Incidental Economist, 8/9; Reynolds, New York Times, 7/31; Adebowale, Men's Health, 8/1).

Cheat sheets: Evidence-based medicine 101

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