The old saying goes that “men are from Mars, women are from Venus” — but are we so different? The age-old debate has become increasingly discussed in the sports realm, where researchers are trying to decide whether or not women and men should be training and fuelling differently.
Many have argued that women should be approaching their sport differently than men like exercise physiologist and nutrition scientist Dr. Stacy Sims, whose 2016 book ROAR: How to Match Your Food and Fitness to Your Unique Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life has garnered widespread acclaim. The book sets out to debunk the perception that women are just “small men” and to provide a rationale for why women shouldn’t be training, fueling, and hydrating in the same way as men.
Dr. Sims suggests that females should approach hydration differently from men based on differences in sodium and fluid balance. Those differences are attributed to the menstrual cycle and the fluctuating hormones which regulate it. Clearly, there are physiological differences between males and females, but to what extent should these impact how you hydrate?
The Menstrual Cycle and its Impact on Hydration
Estrogen and progesterone are the two main female sex hormones involved in regulating a woman’s monthly cycle. The effects of these reproductive hormones upon female physiological systems, and hydration, in particular, are not clear-cut. The fact that estrogen and progesterone have opposing effects on water and sodium regulation makes this topic even more complex. Let’s take a closer look.
Estrogen is the main regulator of the menstrual cycle, during which its production varies regularly. Around days five and six of the cycle, the ovaries gradually start ramping up estrogen production until around day 12, when the release of an egg causes estrogen levels to dip. If the egg is fertilized, estrogen levels will slowly rise; but in the more common instance that an egg isn’t fertilized, estrogen levels return to baseline and the cycle begins all over again.
High levels of estrogen have many other effects within the body, one of which is the upregulation of the antidiuretic hormone (aka, ADH, or vasopressin), which is responsible for water retention and the constriction of blood vessels. By increasing ADH and reducing the amount of fluid the body excretes via the kidneys, elevated levels of estrogen in the follicular and luteal phase have been suggested to alter plasma osmolality and blood plasma volume.
Therefore, estrogen is likely to have a water-retaining effect on the female body. But before jumping to conclusions, blood sodium levels also must be addressed, as this is another determinant of plasma osmolality. And, as mentioned before, high levels of progesterone have the opposite effect that estrogen does on water retention and sodium balance.
Progesterone is the hormone released by the ovaries in the second half of the menstrual cycle (aka, the luteal phase) and is important in the body’s preparation for pregnancy. Progesterone influences the body’s sodium balance by acting as a blocker of the hormone aldosterone, whose job is to retain sodium in the kidneys. Therefore an increase in progesterone results in an increase in urinary sodium excretion. Dr. Sims rightly reports that days of high progesterone will result in sodium loss, but research suggests that this only tells half of the story.
Studies have shown that urinary sodium excretion only lasts for a short time, and is quickly offset by greater aldosterone secretion. More aldosterone being released and binding at the receptors in the kidneys lessens the ‘blocking’ effect of progesterone, meaning more sodium is retained in the body. So although progesterone initially increases sodium loss, the female body responds quickly enough to mitigate any significant impact.
Hydration for Women
Dr. Sims suggests that an increased shedding of sodium would be problematic to women’s athletic performance because of the direct relationship between blood sodium level and blood plasma volume. Low total-body sodium leads to a reduction in blood plasma volume, which in turn leads to low cardiovascular pressure and an elevated heart rate to compensate for this during exercise.
Reduced plasma volume also affects the thermoregulatory system. So, less fluid on board means less is available for sweating (i.e., cooling) and this results in a greater core body temperature during exercise. Dr. Sims’s advice to compensate for this supposed loss of sodium and subsequent shift in core body temperature is to consume a high-sodium hydration product before exercise to pull fluid back into your bloodstream.
This preloading technique is arguably something that most athletes — regardless of sex — can reap the benefits of before a long, hot, or sweaty bout of exercise or competition. In fact, a study published in 2016 showed that 31% of more than 400 male and female amateur athletes were starting their exercise dehydrated.
What is neglected by Dr. Sims in her discussion of this topic, and what will undoubtedly skew the data, is the crucial factor of sodium ingestion. Dietary intake causes daily fluctuations in the secretion of sodium- and fluid-regulating hormones, which adds to the difficulties of trying to accurately assess the effects of the menstrual cycle. Therefore, the relationship between dietary sodium ingestion and the menstrual cycle requires further investigation.
In young and healthy women, estrogen and progesterone do not induce excess fluid retention or loss; neither do they induce excess sodium retention or loss. Instead, they appear to alter the homeostatic set point (i.e., the range of values that help maintain balance) around these systems. Ultimately, the overall effect of the menstrual cycle on sodium and fluid balance appears minimal. Therefore, separate hydration for men and women isn’t warranted, which is what the American College of Sports Medicine also concluded: “sex differences in renal water and electrolyte retention are subtle and probably not of consequence.”