ZMA is a combination of two minerals, zinc and magnesium, and Vitamin B-6 or pyridoxine. All three of these compounds are important in biological processes, and while studies have shown that most Americans get enough zinc and Vitamin B6, more than 50% are deficient in magnesium.
An increase in exercise can lead to the loss of vitamins and minerals making it particularly important for bodybuilding due to the blood sugar level rises and urination increases, increasing the loss of magnesium, zinc, B12, B6, folic acid, and many other nutrients. Although drinking water re-hydrates an athlete, fruit juice, sports drinks or foods high in water such as vegetables are needed to replenish water-soluble nutrients.
The proportion of ingredients generally used in products is 20-30 mg Zinc, 400-500 mg Magnesium and ~10mg B6. According to the label directions, ZMA should be taken before bed on an empty stomach (2 hours after eating your last meal and at least 30 minutes prior to any other supplements). The product should not be taken with calcium (cheese,milk,etc.), the reason being that calcium blocks the absorption of zinc.
Another study in 2004 conducted jointly by the Exercise & Sport Nutrition Lab of Baylor University, IMAGINutrition, and the Journal of the International Society of Sports Nutrition, found that ZMA has no effect on strength, hormone levels, or anaerobic capacity.