Fuel 101: Hydration and Electrolytes

Fuel 101: Hydration and Electrolytes

The Variable Nobody Talks About.

Of all the things that affect endurance performance, hydration is probably the most underestimated. Not because athletes don't know they should drink, but because the advice most people follow is far too generic to be actually useful.

"Drink when you're thirsty" works reasonably well for moderate conditions and moderate efforts. It breaks down rapidly in heat, humidity, and anything lasting more than 90 minutes. Understanding what's actually happening when you sweat, and why the response varies so much between individuals, is what turns a rough guideline into a proper strategy.

What dehydration actually does

Even a relatively small fluid deficit has measurable effects on performance. A loss of around two percent of body weight through sweat, which is less than 1.5 litres for most people, is associated with a noticeable reduction in aerobic capacity, elevated heart rate at a given effort and impaired thermoregulation.

By the time you feel thirsty, you're typically already 1 to 1.5 percent dehydrated. In a long race or training session, that's fine if you're drinking consistently. If you've been ignoring fluid intake for the first hour, you're playing catch-up in conditions where catch-up is very difficult.

The practical message: thirst is a useful signal but not a reliable early warning system. Consistent fluid intake throughout exercise matters more than responding to thirst after the fact.

Why sweat rate varies so much

Here's the thing that catches most athletes out: individual sweat rates vary by a factor of five or more. Some people lose 0.5 litres per hour during moderate exercise. Others lose 2.5 litres or more. There's no way to know where you fall on that spectrum without testing it.

Variables that affect sweat rate include body size, fitness level, heat acclimatisation status, ambient temperature and humidity, exercise intensity, and genetics. Two athletes doing an identical session side by side on a warm day can finish it with dramatically different fluid deficits.

This is why blanket hydration advice (drink 500ml per hour, or drink X bottles per hour on the bike) is a starting point at best. The only way to personalise your hydration strategy is to measure how much you actually sweat.

Electrolytes: why sodium is the one that matters most

Sweat isn't just water. It contains electrolytes, primarily sodium, along with potassium, magnesium, chloride and calcium in smaller amounts. When you replace fluid losses without replacing electrolyte losses, you dilute your blood sodium concentration, which creates its own set of problems.

Sodium is the principal electrolyte in the fluid outside your cells. It regulates fluid balance, drives thirst and helps maintain blood volume. When sodium levels fall, cells absorb excess water and swell. In the brain, this causes hyponatraemia, which in mild cases produces nausea, headaches and confusion, and in severe cases is life-threatening.

Most cases of exercise-induced hyponatraemia occur not from excessive sweating but from drinking large volumes of plain water during prolonged exercise, particularly at lower intensities where thirst isn't strongly activated and people drink out of habit rather than need. The fix is straightforward: include sodium in your fluid intake during sessions lasting more than 90 minutes.

Salt concentration in sweat also varies significantly between athletes, which is why some people finish sessions with visible white salt lines on their kit and others don't. Salty sweaters need to pay particular attention to sodium replacement.

Pre-hydration, during exercise and recovery

Before: Going into a session already dehydrated puts you behind before you've started. If your urine is a pale straw colour, you're well hydrated. Dark yellow means drink more in the hours before training.

During: For sessions under 60 minutes in moderate conditions, plain water is sufficient. For longer sessions or hot conditions, a drink containing 400 to 700mg of sodium per litre is a good target. Sports drinks, electrolyte tablets or even a small amount of salt in a water bottle all work. The carbohydrates in sports drinks serve double duty as both fuel and fluid.

After: Recovery hydration should match losses. As a rough guide, drink 1.2 to 1.5 litres of fluid for every kilogram of body weight lost during exercise. Including sodium in your recovery drink or eating a salty meal with your recovery food accelerates the process.

Personalising your approach

The most useful thing any endurance athlete can do for their hydration strategy is a basic sweat test: weigh yourself before and after a known session, note conditions and fluid intake, and calculate approximate sweat rate. Do this a few times across different conditions and you'll build a far more accurate picture than any generic recommendation can provide.

From there, matching fluid intake to sweat rate, and including adequate sodium for longer sessions, is what a genuinely personalised hydration strategy looks like.

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