Exercise-associated muscle cramping is not reliably explained by dehydration or sodium loss — race pace, cramp history, and muscle damage are the factors the evidence actually supports
Interpretation
Schwellnus et al.'s prospective study of 210 Ironman triathletes found no difference in electrolyte or body-weight change between athletes who cramped and those who didn't — what predicted cramping was a faster race time and a history of cramping in recent races. Martínez-Navarro et al.'s marathon study similarly found no hydration or sodium difference in crampers, but did find significantly elevated muscle-damage markers (creatine kinase, LDH). Miller et al.'s evidence-based review concludes cramping likely has multiple causes and recommends individualised prevention over generic 'drink more' advice — consistent with both direct studies above.
Practical implication
Stop framing cramping primarily as a hydration or sodium problem to athletes — the direct Ironman evidence doesn't support it. Address pacing discipline and muscle-damage management (e.g. appropriate taper and strength preparation) as the better-evidenced levers, while treating acute cramps with gentle stretching per Miller et al.
Sources
3 sources, independently verified.
Pre-to-post changes in serum electrolytes and body weight (a dehydration proxy) did not differ between athletes who cramped and those who didn't. What did predict cramping: a faster overall race time (crampers were actually faster despite similar preparation) and a history of cramping in the last 10 races. The single most direct piece of evidence against the dehydration/sodium-loss explanation of cramping, in triathletes specifically.
Synthesises the cramping literature to conclude cramps likely arise from a confluence of intrinsic and extrinsic factors rather than one single cause (dehydration/electrolyte loss being only one candidate among several, including altered neuromuscular control). Recommends gentle static stretching as acute treatment and individualised, rather than generic 'drink more', prevention strategies.
24% of finishers cramped, with no significant differences in body-mass change, urine specific gravity, or serum sodium/potassium between crampers and non-crampers — but crampers had significantly higher post-race and 24-hour creatine kinase and LDH (muscle-damage markers), and slowed more from 25km onward. Points toward muscle damage/fatigue, not hydration status, as the better-supported mechanism.