D-aspartic acid and L-aspartic acid are the two naturally occurring forms of the amino acid aspartic acid. Both are synthesized in the human body, they are also obtained through the diet via any dietary protein source. While L-aspartic acid is used as a protein building block, D-aspartic acid is not. Instead, D-aspartic acid has direct effects on the central nervous system and endocrine tissues. This is why D-aspartic acid is also sold as a dietary supplement.
Cell culture (in vitro) experiments and animal studies suggest D-aspartic acid may play a role in testosterone synthesis and male fertility. While D-aspartic acid can increase plasma testosterone levels in male rodents, the evidence for this effect in humans is limited and the effect of supplementation with D-aspartic acid on human fertility is unclear.
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,  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. In fact, the small number of human trials that do exist have actually found small reductions in testosterone.
It’s important to note that rodent studies show that daily supplementation with D-aspartic acid does not cause toxicity. Furthermore, human studies have not reported evidence of toxicity or serious side effects. One resistance training study of 20 men with no health conditions did report irritability, nervousness, rapid heart rate, and headache in 2 participants receiving D-aspartic acid, but the same adverse effects were also reported by 1 participant in the placebo group. In the absence of toxicity or serious side effects, the main drawback of supplementation with D-aspartic acid is the lack of evidence for a beneficial effect in humans.
D-aspartic acid is present in neurons and synapses in the brain, has a similar structure to the neurotransmitter N-methyl-D-aspartate (NMDA), and can bind to NMDA receptors. Therefore, it acts as a neurotransmitter/neuromodulator. For example, D-aspartic acid directly affects neuroendocrine function in the hypothalamus and pituitary gland in the brain, causing the secretion of several hormones, including gonadotropin-releasing hormone, prolactin, luteinizing hormone, and growth hormone. Additionally, D-aspartic acid directly affects cells in the testes, causing testosterone secretion. However, many of these actions have only been detected in cell culture (in vitro) and/or animal studies, so their biological relevance in humans is not completely understood.
The standard dose for D-aspartic acid is between 2,000 – 3,000mg.
D-AA is taken daily.
Different studies have used different supplementation protocols. One study used 3,000mg for 12 days, taken daily, followed by a week with no supplementation. A different study did not cycle D-AA, and used 2,000mg of continual daily supplementation with no harm. Further study is needed to determine whether D-AA should be cycled.
It is possible that ZMA can cause weird dreams, and the anecdotes support this; however, since this has not been directly investigated the best 'proof' that can be given is weak.
This claim has not been investigated much, but a pilot study suggests that a dose of 250mg pyridoxine can alter dream perception in college aged men, through a hypothesized increased conversion of tryptophan to serotonin. This dose of B6, however, is much higher than that occurring in ZMA products; which tends to range in the 10-50mg range and usually at the lower end.
One other study has reported synergism between B6 and Magnesium in regards to anxiety reduction, when the subjects were women experiencing PMS; it is theoretically possible that the ZMA formulation enhances the actions of pyridoxine allowing the previous research's results to be relevant.
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