Cramp and electrolytes

The theory everyone knows, and the trouble with it

The dehydration-and-electrolyte account says that sweating depletes fluid and sodium, the fluid around the muscle changes, and the muscle becomes hyper-excitable. It is plausible, it fits the fact that cramp happens in hot races, and it has been the standard explanation for decades.

The trouble is that it has not held up well when tested. Studies comparing crampers with non-crampers in the same race have generally not found the difference in hydration or blood sodium that the theory needs. Cramp also happens in cool conditions, in swimmers, and in muscles that were not the ones working hardest.

And there is a simpler problem: cramp usually strikes one muscle group while the rest of the body, drinking the same fluid and losing the same salt, is unaffected. A whole-body deficiency is a poor explanation for a local event.

The account that fits better

The current leading explanation is neuromuscular rather than chemical. When a muscle is fatigued, the reflexes that govern it drift out of balance — the signal telling it to contract stays elevated while the one telling it to relax weakens — and the muscle locks.

This account predicts what the electrolyte one struggles with: that cramp arrives at the end of hard efforts rather than the start, in the specific muscles being worked, in a shortened position, and that it is relieved by stretching. Stretching does nothing to your blood sodium and everything to the reflex loop, and stretching is what actually stops a cramp in the moment.

It is also consistent with the best single predictor found so far, which is not a blood value at all: a history of cramping, and going out faster than usual.

Where magnesium comes in, and why it mostly does not

Magnesium is genuinely involved in muscle relaxation, which is why the story is so appealing. But the trials do not support supplementing to prevent cramp — in people who are not deficient, reviews have repeatedly concluded there is no meaningful effect, and the effect in older adults with nocturnal leg cramp is small at best.

That is a narrower claim than "magnesium is useless". If your intake is genuinely low, fixing it is worth doing for reasons that have nothing to do with cramp, and roughly half of adults are below the reference intake. Fixing a real shortfall and treating a symptom are different projects.

What sodium is actually for

Sodium replacement matters, but for a different problem. Over long events, drinking large volumes of plain water while sweating out salt can dilute blood sodium — hyponatraemia — which is dangerous in a way cramp is not.

The sweat sodium range in the table is enormous, and that is the honest finding: people differ by a factor of ten in how salty their sweat is. Which means generic advice about how much salt to take during exercise is close to meaningless, and the salt tablet that transformed one runner may do nothing for the next.