Cell culture (in vitro) experiments and animal studies suggest D-aspartic acid may play a role in testosterone synthesis and male fertility.[1][2] While D-aspartic acid can increase plasma testosterone levels in male rodents, the evidence for this effect in humans is limited[3][4][1] and the effect of supplementation with D-aspartic acid on human fertility is unclear.[2][1]
Although there is little to no evidence that D-aspartic acid increases testosterone and growth hormone levels in humans, because these effects have been observed in rodents,[3] [5] D-aspartic acid supplements are marketed to increase muscle mass and strength when combined with resistance training. However, the current evidence does not support such claims in humans.[6][4][7] In fact, the small number of human trials that do exist have actually found small reductions in testosterone.