Late Afternoon Sprint

When Is the Best Time for Exercise?

Writing for the New York Times, Gretchen Reynolds purports to tell us the best time of day for exercise. Except that she doesn’t. Instead, she describes a study that is too small and too flawed to support the claims its authors make in Physiological Reports. And finally, buried at the end of her article, she makes an essential observation:

“Exercise is good for us at any time of day – but only if we opt to keep doing it.”

Superior Effects in the Afternoon?

Honestly, we wonder how this study was published. The title itself makes a claim that the study cannot verify:

“Exercise training elicits superior metabolic effects when performed in the afternoon compared to morning in metabolically compromised humans”

This was a study of 32 men. No women. The morning exercise group included only 12 subjects. Just scanning the paper, we learn that the authors took their data from another study that “was not designed to investigate the effect of the timing of exercise.”

Cause and Effect?

To find “superior effects” for exercise in the afternoon, we would expect a randomized assignment of subjects to morning or afternoon training groups. But instead, researchers assigned subjects to these different times based on “scheduling possibilities and personal preferences.” Thus, any number of things – apart from time of day – could be driving the difference. Were fewer morning slots available? Were those 12 subjects in the morning group different because they preferred or needed a morning schedule?

Quite literally, we do not know. All we have is a retrospective analysis of a small sample with an inadequate design to say anything about causality.

Readers Smarter Than Publishers

Sad to say, readers are smarter than the publishers on this subject. RL from Washington writes:

“‘The Best Time of Day to Exercise’ is a clickbait nonsensical headline for an article about a study that establishes nothing of the sort. The underlying study wasn’t even designed to gather data that would lead to that answer – the data were repurposed from another study designed to determine something else. The scientists are only drawing conclusions based on the data they already have.”

Other commenters point out the obvious gender data gap. These researchers study only men, but make broad conclusions.

Properly establishing cause and effect is important in health science. We suggest that Reynolds and the Times pay closer attention to this and stop publishing misinformation about exercise.

Click here for the study and here for the report by Reynolds. For a systematic review of exercise timing and glycemic control, click here.

Late Afternoon Sprint, photograph © David Rosen / flickr

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January 29, 2021