Hydration: less complicated than the sports drink industry suggests

Hydration advice has swung between "you are chronically dehydrated and must drink constantly" and a backlash suggesting it barely matters. Both overstate their case.

What dehydration actually does

Meaningful fluid loss — commonly cited as around two percent of body mass or more — measurably impairs endurance performance, increases cardiovascular strain and perceived effort, and impairs cognitive function including concentration and mood.

Effects on strength and short high-intensity efforts are less pronounced. A dehydrated lifter can still produce close to a maximal single. The effect shows up more in the later part of a long session, in work capacity, and in how the session feels.

Heat magnifies everything. In hot conditions, sweat rates rise substantially and losses accumulate faster than people notice.

What overdrinking does

The under-discussed risk. Drinking large volumes of plain water while losing sodium through sweat can dilute blood sodium concentration, producing exercise-associated hyponatraemia.

Symptoms range from nausea, headache and confusion to seizures and death, and they overlap with dehydration symptoms — which is dangerous, because the instinctive response of drinking more makes it worse.

It is most common in long endurance events, particularly among slower participants who drink at every station out of diligence. It is rare in resistance training. But the general lesson matters: more fluid is not automatically better.

The practical approach

Drink to thirst, most of the time. Thirst is a reasonably good regulator for people in ordinary conditions, and drinking to a schedule regardless of thirst is where overdrinking comes from.

Check urine colour. Pale straw is about right. Dark yellow suggests you are behind. Completely clear all day suggests you are drinking more than you need. It is a crude marker and it is affected by supplements — B vitamins turn urine bright yellow — but it is free and roughly informative.

Weigh yourself before and after long or hot sessions if you want a real number. Weight lost during a session is overwhelmingly fluid. Replacing something like one and a quarter to one and a half times that volume over the following hours is the usual guidance.

Start sessions hydrated rather than trying to catch up during them. A glass of water with your pre-training meal does most of the work.

Electrolytes

Sweat contains sodium primarily, plus smaller amounts of potassium, calcium and magnesium. Sodium losses vary enormously between individuals — some people are markedly salty sweaters, visible as white residue on clothing and skin.

For a normal hour-long gym session in temperate conditions, electrolyte replacement is unnecessary. Normal food contains plenty of sodium.

Electrolyte replacement becomes genuinely useful for: long sessions in heat, high sweat rates, multiple sessions in a day, endurance events, and people on low-sodium or very low-carbohydrate diets who may be running low.

When it is warranted, the important one is sodium. Elaborate multi-mineral formulations mostly add cost.

What counts as fluid intake

All of it. Water, tea, coffee, milk, soft drinks, and the water content of food, which is substantial — fruit, vegetables, soups and yoghurt all contribute meaningfully.

The idea that caffeinated drinks dehydrate you does not hold up. The mild diuretic effect of caffeine is more than offset by the fluid volume consumed, and habitual coffee drinkers show tolerance to it. Coffee counts.

Alcohol is the genuine exception. It suppresses antidiuretic hormone and produces net fluid loss, which is part of why a heavy night affects the next day's training so much.

The specific case of weight-class sports

Acute dehydration to make weight is widespread in combat sports and, to a lesser extent, in powerlifting. It carries real risks — impaired thermoregulation, cardiovascular strain, and in extreme cases death — and it degrades performance if rehydration time is short.

If it is part of your sport, it should be done under experienced supervision with a proper protocol and adequate rehydration time, not improvised from internet advice.

The summary

Drink when thirsty. Have water available during training. Check that your urine is pale rather than dark. Add sodium if you sweat heavily or train long in heat. Do not force large volumes of plain water on a schedule.

That covers essentially everyone who is not competing in an endurance event, and it costs nothing.