Scientific research is all about answering questions. You have a question about what works best to help someone do something better than before, so you conduct an experiment to compare your new way of doing that thing to the old way. By observing the results, you can make conclusions that inform your decisions moving forward. The “something” here can be anything—from building a car to be more fuel-efficient to running a marathon faster with fewer side effects. So long as the experiment is designed to answer a specific question, the scientific method works well across all fields of study. But as much as this method can be used to reveal all manners of truths, if the experiments are not well-designed they may provide inconsistent or even incorrect conclusions.
In order to avoid this, researchers are always looking to control every aspect of their experiments, such that the only variable is truly the one “something” being studied. For example, when studying the effects of a new disc wheel on the aerodynamics of a bike frame, the manufacturer will do everything in their power to ensure that the only thing that changes in the comparison is the wheel itself. They’ll use the same rider on the same stretch of road—or, they make put a rider-less bike (for no variation at all) with two different wheels in the same wind tunnel with the same wind speeds in order to measure the aerodynamic differences.
But here is where we can start to see how the scientific method begins to have limits. Is a study of a bike in a wind tunnel without a rider on it really giving results that are applicable to a bike that is ridden outdoors on real roads by a real person? It is this over-standardization of experiments, though well-intentioned, that is now starting to be viewed as a real limitation. For example, many medical trials use very homogeneous populations of often white males. We now know that the findings from these studies are often not applicable to women or to people who are not white.
This is where we come to our understandings of hydration in exercise. A recent review article in the Journal of Applied Physiology highlights this exact problem. Researchers from universities in Ontario, Canada reviewed much of the published evidence on the physiology of exercise-induced dehydration and found it to be lacking in its specificity to sex differences.
“An overarching theme for me is that females are so underrepresented in physiologic research, my goal is to bridge that gap.” said Kate Wickham, a PhD student in exercise science, currently doing research in Copenhagen, Denmark as part of her studies at Brock University. “One of the goals of this paper is to advocate for sex-specific research, so that we can develop guidelines for women for hydration.”
Wickham and her co-authors summarized the available evidence on this topic and noted that most of the guidelines on hydration in sport are based on studies that are largely made up of men, who have very different physiological needs than women in this regard. Wickham told me that “A lot of the sex differences in hydration needs often boil down to the differences in the absolute intensity of the work performed. Because of anthropomorphic differences, men have a greater capacity to perform work at higher intensity causing a greater demand for sweating and therefor a higher need for fluid.”
This means that, generally speaking, men have more muscle mass, so their intensity is higher and they generate more heat—and, in turn, need more fluid. So studies with fluid intake recommendations that were based on men are likely over stating the hydration need for women.
This is compounded when you consider that “even when men and women are matched for metabolic heat production, men still sweat more,” Wickham said.
However, it would be wrong to suggest that women should just hydrate less than men, as there are other physiologic differences between the sexes that are important to consider. In her paper, Wickham notes the potential effects of hormonal influences on core temperature as well as on sweat rates. During different phases of the menstrual cycle, the core body temperature may be higher than normal and there may be a propensity for lower sweat rates, although whether or not this has any role in overheating or adversely affecting performance is as yet unknown.
How post-menopausal women are impacted with respect to their hydration needs is another question that has not yet been answered. “There is some evidence that with aging, temperature regulation and sweat rates can change. As women age they have less sweat glands that work less efficiently and that may predispose them to earlier core temperature elevations,” she said. But this is still being evaluated.
There is another physiologic truth that Wickham points out in her paper that likely has a significant bearing on this question as well. Women have less total body water than men by virtue of the fact that they naturally have higher percentages of body fat. When women sweat, even if they sweat the same amount as a comparably sized male—which they don’t, as noted earlier—the percentage of body water lost will be greater than for the man because they are beginning at a lower starting point. Outside has more details on the physiological differences at play here.
Taken in sum, this means that women can tolerate less fluid losses than men can and they tend to be more at risk of overheating because of sweating less. “Women need to be more vigilant about fluid losses, because they have a narrower range of tolerance for fluid loss. Even as little as 0.5% (as compared to 1% or greater in men) can result in an impairment in core temperature that can adversely affect performance,” Wickham said. “Women have less fluid to lose, so they sweat less which results in their core temperature rising, so it’s a physiologic choice being made—maintain body hydration or dissipate heat.”
Wickham is continuing her own investigations in to best hydration practices for women, but for now—given the fact that there are currently no good guidelines for hydration designed for women specifically—what guidance does she have? “I’m going to use a bit of a pun here but it’s true; Don’t sweat it. If you have good hydration practices and habits in training already, then you don’t have to over think this. Athletes performing in any environment, but especially warmer ones, need to replace all of fluids, carbohydrates and electrolytes, and this should be something that they train. This isn’t something that you just throw together prior to an event.” And that answer holds true no matter your sex.