Running in the heat: drink when you're thirsty

For years the advice was to drink on a schedule, before feeling thirsty, to “get ahead of dehydration”. That recommendation has changed, and not because of a fad: because of the people who ended up in A&E from the opposite problem.

What drinking too much can do

It’s called exercise-associated hyponatremia: blood sodium is diluted by drinking more fluid than you lose. The early symptoms — nausea, headache, swelling, confusion — look a lot like dehydration, and that’s the danger: the instinctive response is to drink more, which is exactly what makes it worse.

The consensus recommendation

The international consensus conference on hyponatremia sums it up in one line: drink according to thirst, no more and no less. The reasoning is simple: how much you sweat varies enormously between people and between days, so any fixed amount is wrong for almost everyone. Thirst, on the other hand, is the mechanism your body has used all your life, and it works.

What actually matters when it’s hot

• The time of day. Half an hour earlier or later changes more than any drink.
• Slowing down. In heat, the same effort gives a worse pace. It isn’t that you’re out of shape.
• Shade and light clothing are worth more than they look.
• Acclimatising: in one or two weeks of short runs in the heat, the body adapts considerably. You don’t have to suffer through the first one.

What to do tomorrow

• For runs under an hour, water when you’re thirsty is plenty.
• Don’t force yourself to drink by the clock or weigh yourself chasing a number.
• If you’ll be out more than ninety minutes in the heat, carry something with electrolytes, but still drink to thirst.