This is adapted from the book, Eat Smart, Play Hard, by RW USA Nutrition Editor Liz Applegate.
For years, researchers have known that magnesium plays a critical role in endurance performance. Magnesium exists mainly inside the cells of muscles and bones, where it assists with muscle contractions and energy metabolism. Studies with lab animals and people show that magnesium deficiency reduces endurance, and that low blood levels of magnesium are associated with decreased aerobic capacity. Unfortunately, extreme endurance events such as marathons can deplete magnesium from your body.
Researchers believe that low magnesium levels reduce production of a substance called 2,3-DPG (or just DPG), which is essential for delivering oxygen to exercising muscles. Theoretically, increasing your magnesium intake will boost your DPG level, thereby improving oxygen delivery and in turn boosting aerobic capacity and performance.
A study done at Lenox Hill Hospital in New York City tested this magnesium-DPG connection with trained runners, who took either a magnesium supplement (at two times the DV) or a placebo for 2 weeks. Runners were tested on the treadmill before and after supplementation. The results showed that the extra magnesium failed to boost circulating DPG level or max VO2 (the maximum amount of oxygen that can be removed from circulating blood and used by your working tissues during a specified period), and there was no difference in perceived exertion.
The magnesium supplementation may have been superfluous in this case, since the male runners diets already contained more than the DV of 400 milligrams of magnesium. In other words, more does not mean better.
My recommendation: Try to get your magnesium from food sources such as nuts, molasses, whole grains, and dark green, leafy vegetables because oversupplementation can cause diarrhea and can interfere with calcium absorption and metabolism.