Water during exercise: signs you need to drink more

What happens when water runs short

During exercise, the body loses water through sweat to control temperature. If that water is not replaced, blood volume drops, the heart works harder and performance falls, especially in the heat.

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What the American College of Sports Medicine says

The ACSM position on fluid replacement in exercise recommends avoiding excessive dehydration, defined as water loss above 2% of body weight, because it hurts performance. The need varies a lot with heat, intensity and how much each person sweats.

Source: ACSM, position on exercise and fluid replacement.

The scale test

Weigh yourself before and after a typical workout, lightly dressed and without counting the water you drank during it. A 70 kg person who leaves the workout at 68.5 kg lost 1.5 kg, about 2% of body weight: a sign they need to drink more during or right after. The number is an example; what counts is your own scale.

Signs during and after the workout

• Strong thirst, dry mouth and unusual tiredness.
• Cramps and a drop in performance.
• Dark urine or little urine in the following hours.
• Dizziness when standing up.

Drinking too much water also carries risk in long events. The goal is to replace what is lost, not to force liters. See signs of dehydration in daily life.

How much water loss starts to get in the way

A review published in Comprehensive Physiology, by researchers at the US Army Research Institute of Environmental Medicine, gathered what is known about dehydration and performance. Losing water equal to more than 2% of body weight tends to hurt aerobic exercise, such as running and cycling, especially in the heat.

For a 70 kg person, 2% is about 1.4 kg lost in sweat. That is why the scale test before and after a workout helps so much: it shows how much you usually lose and whether hydration is keeping up.

Source: Cheuvront and Kenefick, Dehydration: physiology, assessment, and performance effects, Comprehensive Physiology, 2014.

The risk on the other side: drinking too much

Drinking much more water than the body loses is also harmful. An international expert consensus, published in the Clinical Journal of Sport Medicine, deals with exercise-associated hyponatremia, when blood sodium falls too low. The main cause identified is precisely drinking fluid beyond what is lost in sweat, something more common in long events, such as marathons.

The signs include nausea, headache, swelling in the hands and confusion, and the condition can be severe. The consensus guidance is to drink according to thirst, without forcing large volumes. Gaining weight during a long event is a sign of having drunk too much.

Source: Hew-Butler and colleagues, consensus on exercise-associated hyponatremia, Clinical Journal of Sport Medicine, 2015.

When to seek help

Confusion, fainting, repeated vomiting, very hot and dry skin, or a strong headache during exercise in the heat call for immediate care. Anyone with a heart or kidney condition should follow the doctor's guidance on fluids.