In 2004, a study funded by a research grant from Cytodyne (another supplement producing company) with 42 resistance trained males showed that ZMA supplementation had no significant effects on total and free testosterone, IGF-1, growth hormone, cortisol, the ratio of cortisol to testosterone, or muscle and liver enzymes in response to training. No significant effects were observed in changes in strength, upper or lower body muscle endurance, or anaerobic sprint capacity [3].
In another study done in 2006, a team of German scientists conducted a study on the effect of ZMA and testosterone levels in the body[4]. The result showed an increase in zinc secretions in urine making it much darker like blood, but no effect on the level of testosterone in the body.