Hydration ยท about four minutes
You can drink too much water. People do.
Exercise-associated hyponatremia is driven by taking in more fluid than you lose. It is the one hydration failure that gets worse the harder you try to fix it the usual way.
What it is
Hyponatremia means the sodium concentration in your blood has fallen too low. During long efforts the common cause is not losing too much sodium. It is taking on so much fluid that what sodium you have is diluted.
The symptoms are unhelpfully ordinary at first. Headache, nausea, feeling swollen or puffy, confusion. They look enough like dehydration that the instinctive response, drinking more, makes the actual problem worse.
How often it shows up
A continuous 246km race reported between 52% and 65%. Multistage events run lower per stage, between 1.6% and 10.1%, but accumulate to about 14.8% across the event.
Most of those cases are biochemical rather than symptomatic. The estimate for symptomatic EAH is far smaller, in the range of 0.1% to 1.0%. That is the number worth holding onto: this is not common, and it is not something to be anxious about on a normal training run.
Severe hyponatremia is a medical emergency and can be fatal. If someone is confused, vomiting repeatedly, or deteriorating during or after a long effort, that is a situation for medical help, not for a drink of any kind. Nothing on this page is medical advice, and nothing we sell treats or prevents this condition.
Why it is worth knowing about at all
Because the entire hydration category, us included, spends its time telling you to drink. That advice is right for most people on most days and it stops being right somewhere, and almost nobody in this aisle tells you where.
The useful takeaway is not a number of millilitres. It is that thirst is a reasonable guide for most efforts, that gaining weight during a long event is a warning sign rather than a good one, and that drinking to a schedule regardless of thirst is the behaviour most associated with getting this wrong.
What we will not tell you
We are not going to tell you that drinking an electrolyte mix prevents hyponatremia. The research does not support that, the mechanism is about total fluid balance rather than about what is dissolved in the fluid, and it is exactly the kind of claim that could get somebody hurt.
We put this page up because it is true and because it is the sort of thing you should know before you decide what to carry. That is all it is for.
Sources
Read what the research says about getting on top of the water