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D-Aspartic Acid (D-Asp) has received a lot of attention in the strength training world for claims that it can raise testosterone levels. Research has found that D-Asp can increase testosterone but there is a lack of convincing evidence from studies done on humans.  After reviewing the literature, the question remains, is it worthwhile to take this supplement or are you better off making sure your magnesium, omega-3, and essential amino acids levels are at their best?

What is D-Aspartic Acid?
D-Asp is an endogenous amino acid and is found in the neuroendocrine tissues of humans. There is evidence that D-Asp plays a role in sperm production and that it is involved in the release of testosterone (T), growth hormone (GH), and luteinizing hormone (LH). It also modulates melatonin synthesis and is found in high concentration in the pineal gland. In boars and lizards D-Asp has also been shown to enhance the conversion of testosterone to estrogen.

What Ergogenic Effect Does D-Asp Have in Humans?
In men, D-Asp has been found to improve motility of sperm and raise T levels. In an Italian study, after supplementing with a 20 mM solution of D-Asp (about 3 grams of D-Asp) for 12 days, 87 percent of the subjects had significantly increased LH and T levels  (from 4.5 ng/ml serum to 6.4 ng/ml) by 33 and 42 percent, respectively.

Researchers point to the fact that six days into the dosing protocol, T was only slightly elevated, whereas after 12 days it was significantly higher and stayed higher for three days after stopping the supplementation. This was thought to be due to evidence that when supplementation is suspended, D-Asp levels are maintained for at least three days in the testes, continuing to trigger T production. A study conducted on rats at the same time, indicated that D-Asp regulates the synthesis of LH from the pituitary and T from the testes. This hormone response is mediated in the pituitary by cGMP and in the testis by cAMP.

Further support for D-Asp is seen with a recent study that gave a supplement of pycnogenol (60 mg/d), l-arginine (690 mg/d), and aspartic acid (552 mg/d) to Japanese patients with erectile dysfunction. Taking the supplement for eight weeks improved sexual function and increased testosterone slightly. The aspartic acid supplement was small, which may be one of the reasons there was only a small T increase.

D-Asp in Mammals: Does it Raise Estrogen Levels?
One of the first ever studies into the relationship between D-Asp and T found that there was a strong correlation between D-Asp and T levels in rat testes. Additionally, feeding the rats with D-Asp increased T levels, and there was also an increase in progesterone and estrogen. Another study demonstrated that D-Asp supplementation raises GH levels in rats as well.

In boars, D-Asp does lead to an increase in T production, but it also leads to a 17B-estradiol synthesis, which in turn generates estrogens (its production depended on a testosterone substrate and was enhanced by D-Asp—estrogen production was two times greater than controls).

D-Asp in Ducks: Confusing Evidence
Another study performed on male ducks looked at the relationship between D-Asp and T, and between nitric oxide (NO) on T. Ducks typically go through an annual mating cycle in which both D-Asp and T are elevated and NO is at a low level. During the non-reproductive cycle, the D-Asp and T were low, whereas the NO levels were high.
An in vitro study found that when testis cells had D-Asp added to them, T was increased, whereas when the cells had L-arginine, a precursor of NO, it was inhibited.  Research on humans does not appear to support the idea that L-arginine inhibits T production, yet there is significant evidence that in rats NO does inhibit T production by limiting release of a steroidogenic enzyme.

What Can We Take From This Research?
Due to a lack of studies done on humans, it’s probably best to stay skeptical about the benefits of D-Asp. The one study from 2008 hasn’t been replicated on humans and additional evidence isn’t currently available. For strategies to raise T levels naturally, check out Testosterone Boosters, and the Testosterone Response: Get Anabolic Part 1.
 
References:
Topo, E., Soricelli, A., D’Aniello, A., Ronsini, S., D’Aniello, G. The Role and Molecular Mechanism of D-Aspartic Acid in the Release and Synthesis of LH and Testosterone in Humans and Rats. Reproductive Biology and Endocrinology. October 2008. 7(120), 120-121.

D’aniello, A., Cosmo, A., Cristo, C., Annunziato, L., Petrucelli, L., George, R. Involvement of D-aspartic Acid in the Synthesis of Testosterone in Rat Testes. Life Sciences. 59(2), 97-104.

DiFiore, M., Lamanna, C., Assisi, L., Botte, V. Opposing Effects of D-aspartic Acid and Nitric Oxide on Turning of Testosterone Production in Mallard Testis During the Reproductive Cycle. Reproductive Biology and Endocrinology. July 2008. 4(6), 28.

D’Aniello, A. D-Aspartic Acid: An Endogenous Amino Acid with an Important Neuroendocrine Role. Brain Research Review. February 2001. 53(2), 215-234.

Nagata, Y., Homma, H., Matsumoto, M., Imai, K. Stimulation of Steroidogenic Acute Regulatory Protein (StAR) Gene Expression by D-Aspartate in Rat Leydig Cells. FEBS Letters. July 1999. 454(3), 317-320.

Ducsay, C., Myers, D. ENOS Activation and NO Function: Differential Control of Steroidogenesis by Nitric Oxide and its Adaptation with Hypoxia. Journal of Endocrinology. 2011. 210, 259-269.

Aoki, H., Nagao, J., Ueda, T., Strong, J., Schonlau, F., Yu-Jing, S., Lu, Y., Horie, S. Clinical Assessment of a Supplement of Pycnogenol and L-Arginine in Japanese Patients with Mild to Moderate Erectile Dysfunction. Phytotherapy Research. May 2011. Published Ahead of Print.

Lamanna, C., Assisi, L., Botte, V., DiFiore, M. Involvement of D-Asp in P450 Aromatase Activity and Estrogen Receptors in Boar Testis. Amino Acids. 2007. 32, 45-51.