Lack Energy? Maybe It's Your Magnesium Level:
"A study by ARS physiologist Henry C. Lukaski and nutritionist Forrest H. Nielsen reveals important findings on the effects of depleted body magnesium levels on energy metabolism.
He and Nielsen, with the center's clinical nutrition support staff, showed that inadequate magnesium is associated with a need for increased oxygen during exercise. They found that during moderate activity, those with low magnesium levels in muscle are likely to use more energy—and therefore to tire more quickly—than those with adequate levels."