Aspartic acid on trial: no, it doesn’t boost testosterone

When people hear the term anabolic steroid, testosterone often comes to mind. This association makes sense—testosterone treatment can drastically increase muscle mass and strength without any other intervention in both healthy^[1] and hypogonadal^[2] adults. There is also a clear dose-response effect^[3], meaning more testosterone leads to more muscle mass, although the effect is most pronounced when dealing with changes outside the normal range compared to within it. Essentially, if your goal is to build muscle and increase strength, then increasing testosterone levels is likely to help.

The sports supplement market has capitalized on young men’s drive for muscle gains. There are countless products available to buy that are claimed to boost testosterone levels. Many of these contain D-aspartic acid (DAA), an amino acid that has been shown to increase testosterone^[4] levels in rats through both direct actions^[5] on the testes and promoting the release of luteinizing hormone^[6] from the pituitary gland, which stimulates the testes to produce testosterone. DAA’s theoretical mechanism of action is summarized in Figure 1.

Initial research in humans was similarly promising. The first human trial^[7] to investigate the effects of DAA supplementation reported a significant 42% increase in testosterone levels when healthy men supplemented with three grams of DAA per day for 12 days. A follow-up study^[8] involving infertile men reported a significant 30-60% increase in testosterone levels with DAA supplementation (three grams a day for 90 days). However, two other studies involving resistance-trained young men found no effect on testosterone levels with three grams a day for 14 days^[9] or 28 days^[10].

Of particular concern is that the 14-day study reported supplementing with six grams of DAA actually led to a significant reduction in free testosterone compared to placebo (-15% vs +9.4%). Not only does this finding not support the idea that DAA is a testosterone booster, it suggests that DAA might have the exact opposite effect if taken in higher doses. The study under review is a follow-up to this previous study by the same research lab. The goal of this study was to evaluate the effects on testosterone of supplementing six grams of DAA per day over a longer time—for 12 weeks—in healthy, resistance-trained men.

The supplement market is full of purported testosterone boosting supplements, such as D-aspartic acid (DAA). Animal and early human research has shown that DAA supplementation increases testosterone levels in both healthy and infertile men, but subsequent research in resistance-trained men reported no effect. One study even found that high doses of DAA (six grams a day) reduces testosterone levels over the short-term (two weeks). The study under review sought to evaluate the effects on testosterone of supplementing six grams of DAA per day over a longer time period (12 weeks) in healthy, resistance-trained men.