D-Aspartic Acid Raised Testosterone by 42 Percent in 12 Days in 23 Untrained Men. Every Trial in Men Who Lift Found Nothing, and the Highest Dose Pushed It Down.
Turn over almost any bottle marketed as a testosterone booster and D-aspartic acid is on the label, usually at around 3 grams a serving. The reason is a single trial. In 2009, Enza Topo, Antimo D'Aniello and colleagues in Naples gave 23 healthy men aged 27 to 37 a daily 3.12 gram dose of sodium D-aspartate for 12 days, with 20 men on placebo, and reported that luteinizing hormone rose 33 percent and total testosterone rose 42 percent in the treated group. A 42 percent increase from an amino acid, in under two weeks, was a story the supplement industry could sell, and it did. What it did not sell was the follow-up. Between 2013 and 2017, three randomized, placebo-controlled trials tested D-aspartic acid in 66 resistance-trained men at the same dose and at double the dose, for two weeks, four weeks and twelve weeks. None found a rise in testosterone. One found a fall. This article walks through the mechanism that made the original hypothesis reasonable, what the 2009 trial actually measured, what the three trials in lifters found and why, what the one fertility series showed and what it lacked, how the supplement market handled the evidence, and what a man should do with it.
Key Takeaways: What the D-Aspartic Acid Evidence Actually Shows
The only randomized trial to report that D-aspartic acid raises testosterone in men was published by Topo and colleagues in Reproductive Biology and Endocrinology in 2009: 23 healthy, sedentary men aged 27 to 37 took 3.12 grams of sodium D-aspartate a day for 12 days, and total testosterone rose 42 percent, from roughly 4.5 to 6.4 nanograms per millilitre, with luteinizing hormone up 33 percent and no change in the 20 men on placebo. Three randomized placebo-controlled trials in resistance-trained men followed. Willoughby and Leutholtz in 2013 gave 20 lifters 3 grams a day for 28 days and found no change in total testosterone, free testosterone or luteinizing hormone. Melville, Siegler and Marshall in 2015 gave 24 lifters placebo, 3 grams or 6 grams a day for 14 days and found no change at 3 grams and a 12.5 percent fall in total testosterone and a 15.4 percent fall in free testosterone at 6 grams. The same group in 2017 gave 22 lifters 6 grams a day for 12 weeks of training and found no change in testosterone, luteinizing hormone, strength or lean mass. The 2013 trial also found that D-aspartate oxidase, the enzyme that breaks D-aspartate down, rose in the men taking the supplement, which is the most likely reason a higher dose did not produce a higher hormone. A 2012 series in 60 sub-fertile men reported improvements in sperm count and motility over 90 days but did not include a placebo arm. A 2020 review of 50 best-selling testosterone booster supplements found that only about a quarter of their ingredients had any published data supporting a testosterone increase, and about one in ten had data showing a decrease. The mechanism by which D-aspartate stimulates luteinizing hormone and testosterone is well established in rats. In men, and especially in men who train, the supplement has not been shown to act on it.
One Trial Moved Testosterone Up. One Moved It Down. Three Arms Moved It Nowhere.
Reported percent change in total testosterone relative to placebo across the randomized trials of D-aspartic acid in men. Topo and colleagues in 2009 gave 23 sedentary men aged 27 to 37 a daily 3.12 g dose of sodium D-aspartate for 12 days and reported a 42 percent rise, with no change in 20 placebo men. Willoughby and Leutholtz in 2013 gave 20 resistance-trained men 3 g a day for 28 days and found no difference from placebo. Melville, Siegler and Marshall in 2015 randomized 24 trained men to placebo, 3 g or 6 g a day for 14 days: the 3 g arm did not differ from placebo and the 6 g arm showed a 12.5 percent fall in total testosterone. The same group in 2017 gave 22 trained men 6 g a day for 12 weeks and found no difference from placebo. The null arms are plotted at zero. The 2012 fertility series is not shown because it had no placebo group.
What is D-aspartic acid and why would it raise testosterone?
D-aspartic acid is the mirror-image form of the common amino acid L-aspartic acid, and unlike most D-amino acids it is made in the body on purpose, concentrated in the tissues that run the male hormone axis. The two forms share a chemical formula and differ only in three-dimensional arrangement. L-aspartate is a building block of proteins and is in every meal. D-aspartate is not used in proteins. It is produced from L-aspartate by an enzyme called aspartate racemase and is found in the brain, the pituitary gland and the testes, at concentrations that in rats climb through sexual maturation and track the rise in testosterone. That distribution is what made researchers in Naples, led by Antimo D'Aniello, suspect a signalling role, and a series of rat studies through the 1990s and 2000s built the case. Injected D-aspartate accumulates in the hypothalamus, where it increases the release of gonadotropin-releasing hormone. In the pituitary it increases luteinizing hormone release. In the Leydig cells of the testis it increases the expression of steroidogenic acute regulatory protein, the protein that moves cholesterol into the mitochondria to start testosterone synthesis. D-aspartate also acts on the NMDA glutamate receptor, which is present at every level of the axis. In rats the whole chain fires: give the amino acid, and luteinizing hormone and testosterone go up within hours. The body also has a dedicated enzyme for removing D-aspartate, called D-aspartate oxidase, and its presence turns out to matter a great deal for what happens when a man swallows three grams of the stuff. The hypothesis was not a marketing invention. It was a reasonable extrapolation from good animal work, and the 2009 trial was the obvious next step.
What did the 2009 trial that started the claim actually find?
The Topo trial found that 12 days of sodium D-aspartate raised luteinizing hormone by about a third and total testosterone by about 42 percent in 23 healthy young men, with no change in 20 men on placebo. The paper, published in Reproductive Biology and Endocrinology in October 2009 by Topo, Soricelli, D'Aniello, Ronsini and D'Aniello, had a human arm and a rat arm. The human arm enrolled 43 men aged 27 to 37 who were healthy, non-smoking and, by the authors' description, not engaged in sport. Each man in the treatment group drank 3.12 grams of sodium D-aspartate dissolved in water every morning for 12 days. Blood was drawn at baseline, on day 6 and on day 12, and again three days after the last dose. Luteinizing hormone rose from about 4.2 to 5.6 milli-international units per millilitre. Total testosterone rose from about 4.5 to 6.4 nanograms per millilitre, which is roughly 15.6 to 22.2 nanomoles per litre, and 20 of the 23 treated men showed an increase. Three days after stopping, testosterone was still above baseline. Serum D-aspartate itself also rose, confirming the supplement was absorbed. The rat arm showed increased luteinizing hormone and testosterone release and mapped the steroidogenic acute regulatory protein mechanism. Several features of the human arm deserve attention. Twelve days is short. The men were sedentary and in their late twenties to mid thirties, with a baseline testosterone in the middle of the normal range. Testosterone was measured as a single morning value at each visit, which is standard but noisy. The sample was 23 treated men. None of these features make the result wrong. All of them make it the kind of result that needs to be repeated before it becomes a supplement category, and the repetition is where the story changes.
What the 2009 Trial Reported in Its 23 Treated Men
Luteinizing hormone and total testosterone at baseline and after 12 days of 3.12 g sodium D-aspartate a day in the treated arm of the Topo trial, published in Reproductive Biology and Endocrinology in 2009. Luteinizing hormone rose from about 4.2 to 5.6 mIU/mL, a 33 percent increase, and total testosterone from about 4.5 to 6.4 ng/mL, a 42 percent increase, with 20 of the 23 men showing a rise. Total testosterone of 4.5 ng/mL corresponds to roughly 15.6 nmol/L, in the middle of the normal range for men in their late twenties and thirties. The 20 placebo men showed no change on either measure. Three days after the last dose, testosterone in the treated men was still above baseline. The men were described as healthy, non-smoking and not engaged in sport, which turned out to be the population detail that mattered most when other groups tried to repeat the result.
- Baseline
- Day 12 of D-aspartate
Did D-aspartic acid raise testosterone in men who lift weights?
No, in three separate randomized placebo-controlled trials in a combined 66 resistance-trained men, at the same dose as the Italian trial and at double that dose, for two weeks, four weeks and twelve weeks. Darryn Willoughby and Brian Leutholtz at Baylor published the first in Nutrition Research in 2013. Twenty men with at least a year of consistent training took 3 grams of D-aspartic acid or placebo daily for 28 days while following a four-day-a-week heavy resistance programme. Total testosterone, free testosterone, luteinizing hormone, estradiol and estrone did not differ between groups at the end. Neither did body composition or one-repetition maximum strength on the bench press or leg press. Serum D-aspartate rose in the supplement group, so absorption was not the problem. Geoffrey Melville, Jason Siegler and Paul Marshall at Western Sydney published the second in the Journal of the International Society of Sports Nutrition in 2015. Twenty-four trained men were randomized to placebo, 3 grams or 6 grams a day for 14 days. The 3 gram group showed no change in total or free testosterone. The 6 gram group showed a 12.5 percent fall in total testosterone and a 15.4 percent fall in free testosterone, both statistically significant, with no change in luteinizing hormone. The same group ran the third and longest trial, published in PLoS One in 2017. Twenty-two trained men took 6 grams a day or placebo for 12 weeks alongside a supervised resistance programme. Total testosterone, free testosterone, luteinizing hormone and estradiol did not differ between groups at any point, and neither did strength gains or lean mass. Three trials, three research groups on two continents, three durations, two doses, and not one rise in testosterone in a man who trains.
Forty-Three Sedentary Men Said Yes. Sixty-Six Lifters Said No.
Number of men randomized in each placebo-controlled trial of D-aspartic acid that measured testosterone, coloured green for the trial that reported an increase, grey for trials that reported no difference, and rose for the trial whose higher-dose arm reported a decrease. The positive result came from the only trial in men who did not train. Willoughby and Leutholtz at Baylor enrolled 20 men with at least a year of consistent resistance training and ran them through a four-day-a-week heavy programme for 28 days. Melville and colleagues at Western Sydney enrolled 24 trained men in 2015 and 22 in 2017, the second under a supervised 12-week resistance programme. All three trials confirmed that serum D-aspartate rose in the supplemented men, so the absence of a hormone response was not a failure of absorption. Every trial in this literature is small, and the total of 109 men across four trials is less than a single arm of a typical drug trial, which is the normal state of the supplement evidence base.
Does a higher dose of D-aspartic acid work better?
The trials say no, and the one head-to-head comparison found the higher dose was the one that lowered testosterone. Melville and colleagues built their 2015 trial specifically to test dose. The reasoning was that trained men might have a higher baseline drive on the hormone axis, so a 3 gram dose that worked in sedentary Italians might need to be doubled to move anything in lifters. The 3 gram arm over 14 days replicated Willoughby's null. The 6 gram arm produced a decrease in both total and free testosterone of roughly an eighth to a sixth, which was not what anyone expected. Luteinizing hormone did not fall, which argues against the pituitary simply switching off in response to a stronger signal. The 2017 trial then ran 6 grams for 12 weeks, six times longer, and found that the decrease seen at two weeks did not persist and no increase appeared either. The most economical reading of the three results together is that the body treats supplemental D-aspartate as something to be cleared, that clearance ramps up with the dose, and that whatever transient disturbance a large dose causes settles back to baseline within weeks. There is no dose in the human literature that has been shown to raise testosterone in trained men, and the highest dose tested has the only negative signal. For a man deciding whether to take more because 3 grams did nothing, the evidence points the other way.
More Dose Did Not Mean More Hormone
Percent change in total and free testosterone relative to placebo across the four dose-and-duration arms tested in resistance-trained men. The 3 g arms at 14 days (Melville 2015) and 28 days (Willoughby 2013) showed no difference from placebo on either measure. The 6 g arm at 14 days (Melville 2015) showed total testosterone down 12.5 percent and free testosterone down 15.4 percent, both statistically significant, with luteinizing hormone unchanged. The 6 g arm at 84 days (Melville 2017) showed no difference from placebo on total testosterone, free testosterone, luteinizing hormone, estradiol, strength or lean mass. The simplest reading is that a large dose disturbs the axis briefly before the body's clearance adapts, and that no dose tested produces the rise seen in sedentary men in 2009. Null arms are plotted at zero.
- Total testosterone
- Free testosterone
Why would the body resist a D-aspartic acid supplement?
Because it has an enzyme whose job is to destroy D-aspartate, and the 2013 Baylor trial found that enzyme rising in the men taking the supplement. D-aspartate oxidase is a flavoenzyme, present in the kidney, liver and brain, that breaks D-aspartate down into oxaloacetate, ammonia and hydrogen peroxide. Its purpose in normal physiology appears to be keeping D-aspartate levels in tissues within a tight range, since the amino acid is a signalling molecule rather than a nutrient and its concentration is regulated rather than simply accumulated. Willoughby and Leutholtz measured serum D-aspartate oxidase activity alongside the hormones and reported that it increased significantly in the D-aspartic acid group over the 28 days, while serum D-aspartate also rose. Their interpretation, which has held up as the most plausible explanation for the whole literature, is that a daily 3 gram oral dose triggers the body to upregulate its clearance machinery, so that the supplement raises circulating D-aspartate without raising it in the hypothalamus, pituitary or testis where it would need to act. This fits the timing of the trials. The Italian result appeared at 12 days in men who had never been exposed. The Baylor result at 28 days showed the enzyme already elevated and the hormones unmoved. The 12-week Sydney trial showed nothing at all. It also fits the higher-dose result: more substrate, more enzyme induction, and more hydrogen peroxide and ammonia as by-products, which is at least a candidate explanation for why 6 grams produced a transient dip rather than a rise. The mechanism that made D-aspartic acid interesting is intact. The body simply has a mechanism for the mechanism.
Does D-aspartic acid improve sperm quality or fertility?
One published series reported that it does, and that series had no placebo group, so the question is open. Gemma D'Aniello and colleagues published a study in Advances in Sexual Medicine in 2012 in which 60 men attending a fertility clinic, 30 with low sperm count and poor motility and 30 with poor motility alone, took 2.66 grams of sodium D-aspartate a day for 90 days. The authors reported that sperm concentration and motility rose substantially in both groups, that serum testosterone rose by roughly 30 to 60 percent, and that a number of the men's partners became pregnant during the study period. The findings are consistent with the animal work, which shows D-aspartate concentrated in the testis and involved in spermatogenesis. What the study lacks is a comparison group. Semen parameters vary substantially from sample to sample in the same man, men attending a fertility clinic tend to be enrolled when their numbers are at their worst and to drift back toward their own average with time, a statistical pattern called regression to the mean, and pregnancies happen in untreated sub-fertile couples at a rate of several percent a month. Without a placebo arm none of these can be separated from a drug effect. No randomized trial of D-aspartic acid for male fertility has been published since. For comparison, the largest randomized trial of vitamin D for the same question, in 307 infertile men in Copenhagen in 2018, found no effect on any semen parameter, which is a reminder that supplements with excellent mechanistic stories and positive uncontrolled series often do not survive a control group. A man with abnormal semen results who wants to try D-aspartic acid is experimenting on the strength of a single uncontrolled series from the group that discovered the molecule.
What does the testosterone booster market do with this evidence?
It cites the 2009 trial and omits the three that followed, and D-aspartic acid remains one of the most common ingredients in the category. Chase Clemesha, Dhanashree Theva and Lawrence Wyner at the Massachusetts General Hospital published an audit in the World Journal of Men's Health in 2020 of the 50 top-selling testosterone booster supplements on the largest online retailer. Ninety percent of the products claimed to boost testosterone. Between them they contained 109 distinct ingredients. When the authors searched the published literature for each one, about 25 percent of the ingredients had at least one study reporting a testosterone increase, about 10 percent had at least one study reporting a testosterone decrease, and the majority had no human data on testosterone at all. D-aspartic acid was one of the ingredients that appeared in both the increase and the decrease columns, because the same literature reviewed here contains both. The audit also found that a number of products delivered doses of zinc, magnesium or vitamin B3 above the tolerable upper limits set by the Food and Nutrition Board. Several of the most frequently included ingredients, including fenugreek, tribulus and zinc, have the same shape of evidence as D-aspartic acid: a mechanism, one or two small positive trials, and null results in larger or better-controlled ones. A 2017 systematic review by Roshanzamir and Safavi in the International Journal of Reproductive BioMedicine reached the same conclusion for D-aspartic acid specifically, noting that the positive evidence came from sedentary men over 12 days and the null and negative evidence from trained men over two to twelve weeks. The supplement industry's target customer is a man who trains. That is exactly the population in which the supplement has never worked.
What Is Actually Behind the Label
Approximate evidence status of the 109 distinct ingredients found across the 50 top-selling testosterone booster supplements audited by Clemesha, Theva and Wyner in the World Journal of Men's Health in 2020. Ninety percent of the products claimed to raise testosterone. When the authors searched the published literature ingredient by ingredient, about a quarter had at least one study reporting a testosterone increase, about a tenth had at least one study reporting a decrease, and the majority had no human testosterone data or data showing no effect. D-aspartic acid sits in both the increase and decrease categories because the trials reviewed in this article contain both. Shares are rounded from the published figures and some ingredients fall into more than one category, so the segments are illustrative rather than exact.
- Data supporting an increase
- Data showing a decrease
- No human testosterone data or no effect
Is D-aspartic acid safe to take?
At 3 grams a day for four weeks and 6 grams a day for 12 weeks, the published trials reported no serious adverse events, but the total exposure is small and nothing is known past three months. D-aspartic acid is an amino acid the body makes and clears, which is a point in its favour, and the trials in trained men measured liver enzymes, kidney markers and blood counts without finding changes attributable to the supplement. The Sydney 6 gram arm in 2015 recorded a decrease in testosterone, which reversed by 12 weeks in the follow-up trial, and some participants in the higher-dose arms reported mild symptoms such as headache and irritability that were not clearly different from placebo. The theoretical concerns come from the clearance chemistry. D-aspartate oxidase produces hydrogen peroxide and ammonia when it degrades D-aspartate, and while the body handles both routinely, chronic high-dose supplementation induces the enzyme and increases the load in a way that has not been studied beyond 12 weeks in 11 men. D-aspartate is also an NMDA receptor agonist, which is the reason it signals in the hypothalamus, and NMDA overactivation is something neuroscientists generally try to avoid rather than encourage. None of this amounts to evidence of harm. It amounts to an absence of the long-term safety data that would justify daily use of a compound whose only demonstrated benefit lasted 12 days in men who did not train. A man who takes it is accepting an unmeasured risk for a benefit the trials in his population did not find.
How should a man over 40 use this evidence?
Take D-aspartic acid off the list of things expected to raise testosterone, spend the money on a testosterone measurement instead, and put effort into the interventions that have randomized support. The 2009 result is genuine data and it may well reflect a real short-term effect in sedentary young men who have never been exposed to the compound. It is also the only positive randomized result in the literature, it lasted 12 days, and it has never been reproduced. The three trials in men who train, which is the population buying the product, found nothing at 3 grams and nothing or a decrease at 6 grams, and the enzyme data explain why. A man over 40 who is worried about his testosterone should have it measured on two separate mornings before 10 a.m. If both results are low with symptoms, that is a conversation with a physician about causes and about treatment, and no supplement on the booster shelf is a substitute for that conversation. If the results are normal, the interventions that measurably move testosterone in randomized trials are not supplements. Weight loss in obese men raises total testosterone by roughly 2 to 3 nanomoles per litre for every 10 percent of body weight lost. A week of five-hour nights lowers daytime testosterone by 10 to 15 percent in healthy young men, and restoring sleep restores it. Resistance training raises it acutely and supports the lean mass that keeps the axis working. Reducing alcohol and treating sleep apnea where present each have trial support. Every one of those has a larger and more durable effect than the best result D-aspartic acid has ever produced, and none of them induces an enzyme to cancel itself out.
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