Heat, fluid, and the mistake that runs the other way

For about thirty years, runners were told to drink early, drink often, and stay ahead of thirst. That advice produced a condition, and the condition is more dangerous than the thing it was meant to prevent.

The problem with drinking ahead of thirst

Exercise-associated hyponatremia is what happens when you take on more fluid than you are losing. Your blood sodium concentration falls, water moves into cells, and in the brain that causes trouble.

The condition has had its own international consensus process, now in its third round, which exists precisely because this keeps happening to people who thought they were doing the right thing².

Who actually gets it

The pattern is consistent and it is not the elite field.

It is people out on the course for a long time, who therefore pass many drink stations. It is people running slower than they expected, so they sweat less than their drinking plan assumed. It is people who were told a number of millilitres per hour and stuck to it regardless of conditions or thirst. Smaller runners reach the same absolute fluid excess faster than larger ones.

Hot conditions raise the risk, partly because they increase both drinking behaviour and the number of stations people stop at¹.

What to do instead

Drink when you are thirsty, roughly the amount your thirst asks for. For most runners in most races this is sufficient, and it has the advantage of self-correcting for pace, temperature and your own sweat rate without you having to calculate anything.

A useful check outside racing: weigh yourself before and after a long run in conditions similar to your race. If you finish heavier than you started, you drank too much. That is a more informative measurement than any general rule, because sweat rates vary enormously between people.

Salt does not reliably prevent this and is not a licence to drink more. Taking sodium alongside excessive fluid does not solve a problem caused by the fluid volume.

The heat itself

Separate from fluid, and worth expecting.

Running in heat is harder at any given pace, because a large part of your circulation is diverted to the skin to lose heat. Your heart rate at a given pace rises, your perceived effort rises, and your sustainable pace drops. That is physiology, not a fitness problem, and no amount of drinking changes it.

The practical consequence is to run by effort rather than by pace on hot days, and to expect a slower time in a hot race. Runners who try to hold goal pace in unexpected heat are the ones who end up walking the last ten kilometres.

Acclimatisation is real and reasonably quick. Repeated exposure to exercising in the heat over a couple of weeks produces meaningful adaptation, which is why the same temperature in May feels worse than in August.

When it is an emergency

Some heat problems are not something to manage with a drink and a sit down.

Altered mental state in a runner in the heat is the sign that matters most. It appears in both exertional heat stroke and in serious hyponatremia, both of which are emergencies, and neither of which should be treated by a bystander offering more fluid.

Where our advice stops

Fluid and electrolytes sit at the edge of what a physiotherapist should be advising on. The joint position statement from the American College of Sports Medicine and the dietetic associations says explicitly that athletes should be referred to a registered dietitian for a personalised nutrition plan³, and that includes hydration strategy for anyone whose situation is not straightforward.

What we will help with is the training side: heat acclimatisation built into a block, pacing expectations for a hot race, and the return to training after a heat-related event.