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The criticism from an anonymous peer reviewer caught Matthew Heath by surprise.

The University of Western Ontario kinesiology professor had submitted a study on the cognitive benefits of exercise, involving seven men and five women. But the inclusion of women, the reviewer argued, was a mistake, “due to cognitive and physiological differences in the menstrual cycle.” To avoid this complication, women should have been excluded from the study.

Heath disagreed – so he decided to investigate this claim. In a study published last month, Heath, undergraduate research student Kennedy Dirk and kinesiology professor Glen Belfry tested the effects of exercise on cognition in women at different stages of their menstrual cycles.

The results, which appear in the journal Medicine & Science in Sports & Exercise, found no differences linked to hormonal fluctuations. That’s good news for Heath (whose original study was eventually published despite the reviewer’s objections), but it highlights a continuing challenge in exercise research: An overwhelming number of studies either omit women completely or make the mistake of assuming that women are, as physiologist Stacy Sims puts it, simply “small men.”

The new study involved 15 female subjects who did 20 minutes of moderate cycling, preceded and followed by a test measuring executive function, which involves cognitive processes such as working memory and attentional control. They repeated this process once during the early follicular phase of their menstrual cycle, when estrogen and progesterone levels are at their lowest, and once during the midluteal phase, when they’re elevated. Performance on the cognitive test increased after exercise by the same amount in both tests.

The idea that hormonal changes might influence cognitive function isn’t totally unfounded, Heath points out. A review of the relevant literature by Swedish researchers in 2014 suggested that emotional processing may change across the menstrual cycle, but concluded that such differences were “small and difficult to replicate” – hardly a good reason to exclude women from studies of this type.

That doesn’t, however, mean that men and women are interchangeable in all exercise studies. On average, men tend to be bigger and heavier than women, have different distributions of muscle-fibre type and patterns of fat storage, and respond to physical stresses in slightly different ways.

For example, a study published this month in Sports Medicine by University of Calgary researchers Candela Diaz-Canestro and David Montero analyzed previous research comparing how men and women respond to endurance training. For a given level of training, they found that men seem to get a slightly bigger boost in VO2max, a measure of aerobic fitness. On the other hand, women seemed to get a greater boost in lifespan from increasing their VO2max by a given amount.

These differences are subtle, but they do exist. And the solution, Heath and others argue, isn’t to exclude women from studies – it’s to include them, and where relevant analyze the results separately to look for differences.

The U.S. National Institutes of Health, the world’s largest funder of biomedical research, has mandated the inclusion of both men and women in clinical trials since 1994, points out Brock University doctoral researcher Kate Wickham. But attitudes such as those of Heath’s anonymous reviewer remain surprisingly common.

When Wickham set out to explore the performance-boosting effects of nitrate-rich beet juice during her master’s degree at the University of Guelph, she found more than 100 studies on the topic that features all-male subject populations. In comparison, there were just seven all-female studies.

Based on the extremely limited data available, it seems that women may actually get a bigger endurance boost from beet juice than men. But it’s not clear whether that reflects some subtle difference in physiology or whether it’s simply a result of women typically being smaller than men (and thus getting a higher nitrate dose from a bottle of beet juice), or the fact that women tend to eat more nitrate-rich foods such as spinach and arugula.

The bottom line is that we don’t know the answer to these and many other questions – and we won’t until research that includes both men and women is not just accepted but expected.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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