Drinking too much: hyponatraemia and why more water isn't safer

Almost all hydration advice for cyclists is about not drinking enough. That is the common problem and the one most riders should think about. But there is a second, rarer failure that runs in the opposite direction, and it is the one that occasionally kills people: drinking so much that the sodium concentration in your blood falls to a dangerous level.

Exercise-associated hyponatraemia is uncommon in cycling and uncommon in short events. It is not, however, exotic. It has been documented repeatedly in marathons, ultramarathons, long triathlons and multi-day endurance events, and there have been deaths. The mechanism is simple enough to explain in a paragraph, the symptoms overlap almost exactly with dehydration, and the instinctive response to those symptoms — drink more — makes it worse.

This piece is about that failure mode specifically: what it is, who gets it, why it is hard to spot, why salt tablets do not reliably protect you, and the point at which it stops being a nutrition topic and becomes a call to emergency services. If you want the ordinary side of the question, how much you should actually drink on the bike covers it.

What is actually going wrong

Blood sodium concentration is a ratio: how much sodium, divided by how much water it is dissolved in. Hyponatraemia is defined as a serum sodium concentration below 135 mmol/L, and you can arrive there by losing sodium or, far more commonly in this setting, by adding water.

The dominant cause in endurance events is fluid overload. You drink more than you lose, the excess is retained rather than passed as urine, and everything in the blood is diluted. Consensus documents on the condition are explicit that over-drinking of hypotonic fluid — water, or sports drink, which is much more dilute than blood — is the primary driver.

Two things make it worse than simple arithmetic suggests. First, prolonged exercise can leave the antidiuretic hormone inappropriately elevated, so your kidneys hold onto water at exactly the moment they should be getting rid of it. Second, sodium lost in sweat adds to the deficit, and in some people sodium appears to become temporarily unavailable within the body rather than simply being lost. Work describing the condition identifies several independent mechanisms acting together, which is part of why a single rule of thumb does not prevent it.

The clinical danger is not the number itself. It is that water follows sodium by osmosis, so as blood becomes dilute, water moves into cells. In the skull, where there is no room to expand, that becomes cerebral oedema. That is the mechanism behind the confusion, seizures and, in severe cases, death.

Who it happens to

The intuition that this is a problem for the fastest, hardest-working athletes is exactly backwards. It happens to people who are out for a very long time, are not working especially hard, and have plenty of opportunity to drink.

In the best-known survey of the condition, blood samples from runners finishing the Boston Marathon found around 13% with hyponatraemia, and the strongest associations were with substantial weight gain during the race, longer finishing times and lower body mass index. Weight gain during an endurance event is the signature. You cannot gain weight across six hours of exercise unless you have taken on more fluid than you have lost.

Why it is dangerous: it looks like dehydration

This is the crux. Mild hyponatraemia and dehydration present with almost the same list: headache, nausea, weakness, poor performance, feeling generally wrong. A rider who feels bad on a long ride reaches for the bottle, because that is what everyone has been taught. If the problem is dilution, that response deepens it.

Two of those rows are actually useful in the field: swelling, and weight change. Everything else is ambiguous. If you have been drinking steadily for hours, you feel unwell, your rings are tight and you have not needed to urinate much, stop drinking. That is the opposite of the reflex, and it is the right move.

Why salt tablets are not the answer

The obvious fix seems to be adding sodium. It does not work as reliably as it sounds, for two reasons.

The first is concentration. Sports drinks contain far less sodium than blood does — typically a small fraction of it. Drinking them adds sodium and a larger relative volume of water, so the ratio still moves in the wrong direction. You cannot out-salt an excess of fluid using a hypotonic drink.

The second is that it has been tested. A field study of sodium supplementation during a 161 km ultramarathon found supplementation did not prevent hyponatraemia; fluid intake relative to losses remained the determining factor. Consensus guidance reflects this: sodium replacement has a role in some contexts, but it is not a substitute for not over-drinking, and it should not be relied on as protection.

None of which means sodium is irrelevant. Replacing sweat sodium matters for very salty sweaters on long, hot days, and it is a reasonable part of a plan — our pieces on electrolytes, sodium and cramp and sweat testing and sodium go into where it does help. The point here is narrower: sodium supplements are not a licence to drink more, and taking them does not make over-drinking safe.

What the consensus guidance actually says

The international consensus conferences on this condition converge on a short and slightly unfashionable recommendation: drink according to thirst. Not to a schedule, not to a fixed hourly volume, and not pre-emptively to "stay ahead of it".

Thirst is a reasonably well-calibrated signal over the timescales that matter, and drinking to it tends to land you a little short of replacing losses — a small deficit, which is well tolerated and is the region the middle of that chart sits in. The behaviour that causes trouble is the deliberate override: a rule that says one bottle per hour regardless of conditions, effort or how you feel.

Practically, for a long ride:

• Drink when thirsty, not on a timer. On a cold, slow, long day this may be substantially less than you expect.
• Adjust for the day rather than repeating last week's plan. Ten degrees and rain is a different fluid requirement from thirty degrees and still air.
• Weigh yourself before and after a long event if you can. Going up is a problem. A small loss is normal and fine.
• Be suspicious of a headache that worsens as you keep drinking. That pattern points the wrong way.
• Eat on the bike. Real food carries sodium and calories that fluid alone does not, which matters on the sort of long day where this arises. Planning a 200 km ride covers building that into the route rather than improvising.

When this is an emergency

Severe exercise-associated hyponatraemia is rare. Most riders will never see it, and nobody should finish this article frightened of their bottle. The reason it is worth knowing is that it is one of a small number of situations in endurance sport where the intuitive response — drink more, you must be dehydrated — is precisely the wrong one, and where recognising that early is the whole of the treatment.

Drink when you are thirsty. Stop when you are not. Be particularly careful on the long, cool, slow days when it feels like nothing much is happening, because those are the days it happens on.