Post-exercise rehydration with sodium and potassium (with optional carbs for long sessions)
Sims contrasts this targeted formula with commercial sports drinks, which are designed to supply fuel during exercise, not to optimise rehydration afterward. Gatorade at 6% carbohydrate, for instance, creates too high an osmotic pressure in the small intestine; this forces the body to draw water from the bloodstream and interstitial spaces into the gut to dilute it, resulting in delayed rehydration and even dehydration. Plain water, conversely, requires the body to secrete its own sodium and glucose into the intestine before absorption can occur, slowing the process. The prescribed 360–400 mg sodium and 200 mg potassium per 16 oz, with an optional 2% carb, sits in the ‘sweet spot’ of osmotic balance, harnessing the intestine’s built-in glucose-sodium transporters for rapid fluid uptake without the counterproductive fluid shifts. For prolonged efforts, a slight bump to 3–4% carb supports energy while remaining within the safe osmotic window.
Sodium cotransports with glucose via SGLT1 and also moves independently through sodium channels in the small intestine, drawing water osmotically into the blood. Potassium separately facilitates water movement into interstitial compartments. A 2–4% glucose-sucrose solution maintains a slightly hypotonic osmolality that favours absorption, while higher concentrations (>6%) create an osmotic gradient that pulls water into the gut lumen, delaying absorption and potentially causing net fluid loss.
Sims shares that she works out 60–75 minutes daily and personally uses around 500 mg of sodium in her post-workout rehydration, slightly exceeding the 400 mg mark, noting that perimenopausal women may need a bit more due to declining estrogen-mediated sodium retention.
When we're rehydrating, we want to look for around 360 to 400 mg of sodium per 16 ounces. And with potassium, you want half that amount. So around that 200 mg mark of potassium and that's your adequate rehydration.