According to studies by the U.S. Department of Agriculture, women age 55 to 70 were able to ride an exercise bike for equal lengths of time regardless of whether they were getting enough magnesium. But they burned 15 percent more oxygen while on a magnesium-poor diet, and their heart rates averaged nine beats a minute faster.
In other words, they got fewer miles to the metabolic gallon.
To get more magnesium, eat more greens (especially spinach, Swiss chard, kale and lettuce) and grains (especially brans). You can also take a mineral supplement that includes magnesium -- but don't take more than 350 milligrams a day. An oversupply of magnesium will cause neurological problems and can also be dangerous to people who have reduced kidney function.