ZMA is typically pretty effective because a lot of people are deficient in either Zinc or Magnesium. Victor Conte(the guy who started BALCO) got his start by running mineral profiling tests on elite level swimmers, he found that almost all of them were deficient in both Zinc and magnesium. Once he started supplementing them with these there performance almost immediately improved, thus he created the ZMA supplement. However, if you are eating a large amount of calories you may not have these mineral deficiencies, and ZMA won't do anything.