The Science of Sodium: Your Endurance Baseline (Part 1)

Most athletes track carbohydrate intake closely and give sodium far less attention, even though heat and long sessions raise sodium losses through sweat.

Sweat rate and sodium loss vary widely. In the heat you lose both fluid and sodium. Sweat rates commonly range from 0.5 L to 2 L per hour, and sweat sodium concentration varies widely between athletes, from about 200 mg to 2,300 mg per litre, with an average near 950 mg. That variation is why one number never fits every athlete.

Your gut has a practical fluid limit. During hard exercise, roughly 1 L of fluid per hour is the upper limit for most athletes, and many tolerate less. Drinking beyond that does not hydrate you faster. The fluid sits in your stomach and causes bloating and gastrointestinal distress.

Overdrinking can cause hyponatremia. Drinking more than you sweat, especially plain water over many hours, can dilute your blood sodium and cause exercise-associated hyponatremia. Symptoms include nausea, headache, bloating and swelling. Severe cases cause confusion and seizures, which is a medical emergency. Consensus guidelines recommend drinking to thirst rather than forcing fluid.

Sodium helps you absorb fluid. In the small intestine, sodium is absorbed together with glucose, and water follows. Sodium and carbohydrate at suitable concentrations move fluid into your bloodstream more effectively than plain water, which helps maintain plasma volume. Stable plasma volume supports stroke volume and limits cardiovascular drift, the gradual rise in heart rate at a constant pace.

My starting point is 800 mg to 1000 mg per hour. My starting point for long efforts in the heat is 800 mg to 1000 mg of sodium per hour. General guidelines for efforts over two hours list 300 mg to 600 mg per hour. I start higher because sweat sodium losses vary widely and heavy salty sweaters lose far more. We then adjust for each athlete’s sweat rate, sweat sodium losses and gut response.

This is an endurance protocol. For a 45 to 60 minute run in moderate conditions, your body’s sodium reserves cover the loss and plain water is enough. In extreme heat and humidity, or once a session passes 90 minutes to 2 hours, sodium replacement becomes much more important.

In Part 2, I show how I adjust off-the-shelf hydration powders to reach these numbers without overloading your stomach with sugar.