The Science Behind Your Sweat — What Your Body Is Actually Doing

Every endurance athlete knows they sweat. Most know they lose sodium when they do. But very few understand what's actually happening at a physiological level — and why that makes individual differences so consequential.

Why we sweat

Sweating is your body's primary cooling mechanism during exercise. As you generate heat through muscle activity, your core temperature rises. To prevent it from reaching dangerous levels, your brain triggers sweat glands across your skin's surface. The sweat that emerges onto your skin evaporates, and the energy required for that evaporation draws heat away from your body.

It's an extraordinarily efficient system — and the reason humans are uniquely capable of sustained aerobic activity in warm conditions. No other mammal can outrun a human over long distances in heat.

What's in sweat

Sweat isn't just water. It contains a mix of electrolytes — primarily sodium, with smaller amounts of chloride, potassium, and magnesium. Sodium is by far the most significant electrolyte in sweat, both in terms of concentration and in terms of its physiological importance.

The sodium in your sweat comes from your blood plasma. As you sweat, sodium leaves the blood along with water. If you replace the water without replacing the sodium — by drinking plain water — your blood becomes progressively more diluted. This condition, hyponatremia, is one of the more serious risks in ultra-endurance events and is why "drink to thirst" and "drink lots of water" can actually be contradictory advice depending on the individual.

Why sodium concentration varies so much

This is the part most athletes find surprising.

Your sweat sodium concentration is largely determined by genetics — specifically, by the activity of sodium channels in your sweat glands. These channels reabsorb sodium from your sweat before it reaches the skin's surface. Some athletes have highly active channels that reabsorb most of the sodium, producing dilute sweat. Others have less active channels, losing far more sodium per litre.

This variation is not related to fitness level, training volume, or diet in any meaningful way. Two athletes training side by side for years can have a three to five times difference in their sweat sodium concentration. It's simply individual biology.

This is also why sweat sodium concentration doesn't change over time. It's essentially fixed. Get tested once, and your result is yours for life.

Sweat rate — the other variable

While sodium concentration is fixed, your sweat rate (how much you sweat per hour) is more variable. It's influenced by exercise intensity, ambient temperature and humidity, heat acclimatisation status, individual genetics, and body size and composition.

The practical implication is that a hot race in Melbourne in February will produce much higher total sodium losses than a cool morning run — not because your concentration changed, but because your sweat rate increased. Knowing your concentration lets you scale your sodium intake appropriately.

The total picture

When you combine sweat sodium concentration with sweat rate, you get your total sodium losses per hour. For some athletes in hot conditions, this can exceed 2,000mg per hour. For others, it might be under 400mg. Applying the same hydration strategy to both athletes — as generic advice inevitably does — produces dramatically different outcomes.

This is the core rationale behind sweat testing. The data isn't interesting for its own sake. It exists to help you build a hydration strategy that actually reflects how your body works.