This question is about Running
There is some evidence that strenuous exercise may increase magnesium losses in sweat and urine by 10-20% [ 1
1]. Although the research on magnesium depletion and running is lacking, it can be assumed that high mileage may lead to some magnesium depletion.
A magnesium intake less than 260 mg/day 220 mg/day for male and female athletes, respectively, may result in a magnesium-deficient status [ 1
1]. Evidence suggests that magnesium supplementation or increased dietary intake of magnesium may have beneficial effects on exercise performance in magnesium-deficient individuals.
Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnes Res. 2006;19(3):180-189.