Magnesium did not reduce night-time leg cramps in older adults
Across 11 trials in 735 people, mostly older adults with night cramps, magnesium changed cramp frequency by 0.18 a week against placebo. Nothing here tests cramping during exercise.
What the study found
Magnesium did not meaningfully cut leg cramps in 735 people
This is the closest thing to a settled answer anywhere in the cramp literature. Pooling 11 trials, magnesium shifted idiopathic leg cramps by 0.18 cramps a week against placebo, an interval running from 0.84 fewer to 0.49 more. Minor stomach upset was commoner on magnesium.
The caveat · Not tested on cramps during sport
The people tested were mostly older adults with night-time leg cramps, not athletes cramping mid-race, so this settles the mineral rather than the sport. The pregnancy trials in the same review could not be pooled at all because their outcomes were reported inconsistently, and a separate Cochrane review graded those same trials as far less biased than this one did. Nothing here tests magnesium during exercise.