Skip to content
Spiral Staircase
Buy the book
Menu

Glossary

Hyponatremia

Defined in topic 17

Blood sodium at or below 135 millimoles per liter. In endurance events it comes mainly from drinking more than the body loses, and at its extreme it swells the brain.

Published Updated

Where this comes from

In the reference topic that owns this term

Reference topic 17, The Water You Forgot to Drink, states the mismatch as a card: what the system evolved to expect, what it gets instead, and what happens in the gap.

Mismatch card

What it expects

Wet food and drinking to thirst, in an ape that turns over 30% to 50% less water than its cousins, size for size.

What it gets

Taps, climate control, a 1945 allowance stripped of its caveats, and a market selling hydration.

What happens in the gap

Little, for most healthy people. In long events, overdrinking can dilute blood sodium and swell the brain; thirst lags in older people and in heat.

The full card, and the topic behind it

What that chapter recommends

3 of the 7 graded recommendations in Reference topic 17, strongest evidence first. How the grades work.

  1. Treat suspected overdrinking as an emergency

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Headache, vomiting, confusion or seizures after heavy drinking during a long event, especially with weight gain, need urgent care, not more fluid.

    Read it in full

  2. For diarrhea, use oral rehydration solution

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    a mixture of clean water, salt and sugar. Severe dehydration needs medical care, sometimes with intravenous fluids.

    Read it in full

  3. Drink when thirsty and with meals

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Coffee, tea, milk and water-rich foods such as fruit and vegetables all count; healthy adults in mild conditions need no glass count (Chapter 40).

    Read it in full

The argument behind this term, its studies and its notes are in Reference topic 17 of the reference library. The book gives the practical version. Get the book.