How Women Should Hydrate in Training: Sweat Rate, Sodium, Menstrual Cycle, and Perimenopause

Sims’s core argument is that plain water is not the body’s ideal hydration vehicle; the small intestine is sensitive and needs a small amount of salt and sugar to efficiently pull water into the bloodstream. This is especially relevant for women who experience bloating or gastrointestinal discomfort with large volumes of plain water. She references her PhD supervisor’s insight that drinking plain water kills the thirst stimulus upon first taste, so people stop drinking before they’ve actually absorbed enough. By pre‑loading the water with sodium and a touch of sugar, the absorption becomes faster and more comfortable, making hydration more effective even if one drinks less total volume. She emphasizes that this is not about sodium loading but about facilitating the physiological mechanism.

Sims notes that when we start exercising, we need a little more help because blood flow shifts to muscle. A teaspoon of maple syrup, which contains sucrose (a disaccharide that splits into glucose and fructose), activates all the fluid transport mechanisms. This is far more effective than plain water and avoids the high carbohydrate loads of commercial sports drinks that can sit in the stomach. She stresses that you don’t need a laundry list of electrolytes; the sodium plus the sugar is the key to hydration. The maple syrup also provides a small amount of fuel without trying to replace all the calories, which aligns with the idea that up to 5 hours, you can fuel with food. This protocol is flexible across weather patterns because the primary driver is fluid absorption, not sodium compensation for sweat.

Stacy Sims argues that two pervasive myths must be clarified: the 8–10 glasses of water daily, which like 10,000 steps was a marketing creation, and the idea that athletes must replace sodium aggressively with electrolyte drinks. She explains how Gatorade’s origin story involved a renal physiologist who suggested adding a little sugar and salt to help Florida football players, but once bought by a corporation, an artificial sweetener was added, then removed by the FDA, forcing them to double carbohydrate to maintain palatability. Research was then funded to prove it as a carbohydrate replacement with incidental hydration benefits, when the original data showed a 1–3% solution optimized plasma volume expansion. This historical inversion has led to an entire industry of high-carb, high-electrolyte products that are not primarily hydration tools. Sims says she’s been talking about sex differences in hydration for 20 years, and only recently is Gatorade launching a sex-difference research project. She sees the whole space as finally evolving.

Sims explains that there are clear sex differences in sweat rate, fluid accumulation, and fluid shifts during exercise. Women who do long endurance races will finish with normal blood sodium or lower, whereas men will finish with normal or higher. This is due to differences in how fluid moves between compartments and how the body handles sodium. She then emphasizes that across the lifespan, during perimenopause and postmenopause, thirst sensation decreases, yet the body is under more stress and needs slightly more water. She is excited that Gatorade is finally launching a sex-difference research project, something she’s been advocating for two decades. Her personal story from the Kona Ironman illustrates the real danger: during her high-hormone phase, she drank plain water, became bloated and dizzy, then took an electrolyte tablet (Gastrolyte) and urinated excessively — classic hyponatremia. That experience led her to investigate and discover phase-specific sodium differences.

Sims explains that fluid absorption in the small intestine is sensitive to pressure and requires a specific amount of glucose and sodium to pull water from the lumen into the body. If you drink plain water, the body has to first secrete sodium and glucose into the intestine to create the osmotic gradient, which is slower and can distort intestinal pressure, causing bloating. Adding a little sodium and sugar to the water supplies the co-transporters directly, enabling faster and more comfortable absorption. Moreover, her PhD supervisor noted that the moment water touches the tongue, the thirst stimulus is turned off, even if the body hasn’t actually absorbed enough fluid. This leads people to drink small amounts, kill thirst, but never truly hydrate. The extreme version is the gallon‑jug habit: the sheer volume overwhelms renal capacity, so it is rapidly excreted, making urine clear but leaving the body dehydrated. Sims states that people drinking gallons often report clear urine but persistent thirst, which is a sign that they are flushing out the water rather than absorbing it.

Sims revisits the Gatorade origin to show that the early research identified a 1–3% glucose‑sodium solution as ideal for increasing plasma volume — i.e., hydration. When the product was reformulated to taste better and later to replace lost carbohydrates, the concentration rose to 5–8%, which is still the standard for most sports drinks. Companies then funded studies to prove that this concentration was effective for carbohydrate replacement, effectively rebranding the product as a fuel source while retaining the hydration halo. For athletes, this means typical sports drinks are primarily sugar delivery systems, and relying on them for hydration can be suboptimal, especially if the athlete’s goal is fluid absorption rather than energy.

Sims asserts that sweat sodium testing is performed on a small patch of skin and does not represent whole‑body sodium requirements because sweat composition varies across body sites and over time. More importantly, humans have a large sodium reserve, and losing even half of it does not impair function. The marketing narrative that athletes must precisely replace sodium lost in sweat to avoid cramping or hyponatremia is, in her view, unfounded. Instead, adding a modest amount of sodium to fluid — just enough to facilitate absorption — is sufficient for nearly all exercise under five hours. She explicitly calls out LMNT and Precision Hydration as capitalizing on this fear, while noting that excess sodium is simply excreted in sweat and urine.