Companies are advertising D-aspartic acid to fitness enthusiasts and men with low testosterone and fertility issues. But does D-aspartic acid (D-asp) have any science-based benefits or are data on its effects and side effects completely lacking? Read on to learn about the science behind D-asp as a dietary supplement.
What is D-Aspartic Acid?
Overview
Aspartic acid (Asp) is an amino acid that animals and humans naturally make in two forms: L–Asp and D–Asp. Since our bodies can make aspartic acid, it is a non-essential amino acid. L-Asp combines with other amino acids to form proteins vital to life. D-Asp exists freely as small individual molecules in various human and animal tissues [1, 2].
Individual L-Asp and D-Asp molecules are made of the same atoms. In fact, they share the same chemical formula (C4H7NO4). The main difference is how they align in space: L-Asp and D-Asp molecules are mirror images of each other, like your left hand and your right hand. These “mirrored” arrangements give L-Asp and D-Asp different properties [3].
What we currently know about the effects of D-Asp made by our bodies comes from animal research. Free D-Asp occurs naturally in nerve and hormone tissues in both animals and humans [4, 5, 1].
D-Asp is important for brain and nerve development and nerve communication. It also helps produce and release various hormones. However, scientists are still discovering all the ways in which D-Asp affects humans [4].
D-Asp supplements have not been approved by the FDA for medical use. In general, dietary supplements lack solid clinical research. Regulations set manufacturing standards for supplements but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing.
Additionally, no proper clinical studies on D-Asp as a supplement have been carried out. Thus, the effectiveness and safety of this compound remain unknown. We recommend against using D-asp until proper clinical data are published.
Mechanisms of Action
Effects on the Brain
High D-Asp levels promote proper brain development before and immediately after birth in mice. These levels later decrease, but D-Asp continues to spark nerve development, helping to create awareness of the surrounding environment in adult mice [6, 7, 8, 2].
Scientists consider D-Asp a neurotransmitter: It helps to produce or enhance nerve signals, stimulate new brain connections, store memories, and promote learning in mice [6, 9, 7, 2].
Although naturally produced aspartic acid is needed for proper brain development, some animal studies warn us about the potential dangers of supplementing.
In mice, both glutamate and aspartic acid can cause brain damage and impair learning. These so-called excitatory amino acids damaged the hypothalamus, an important brain region for hormone balance (affecting pituitary hormones) and a part of the limbic system involved in emotional control [10, 11].
The above studies do not refer to the D form of aspartic acid specifically. The L and the D form of this amino acid can have different effects. The term “aspartic acid” can commonly refer to a mix of both. The form used to make proteins and taken in from food is the L version. In fact, L-aspartic acid can oppose the effects of D-aspartic acid. And in one study, L-Asp specifically had toxic effects on the kidneys and salivary glands in rats. It’s important to have these opposing effects in mind [12, 13, 14].
Still, research is sparse and it’s unknown if D-Asp supplementation can cause brain damage in humans; caution is advised.
Hormonal Effects
D-Asp stimulates the production of hormones such as testosterone, progesterone, luteinizing hormone (LH), oxytocin, prolactin, growth hormone, and others involved in muscle development, sexual function, reproduction, and blood pressure in mice. These effects have not been studied in humans [4, 6, 2].
Claimed Health Benefits of D-Aspartic Acid
Possibly Ineffective for Increasing Testosterone
Many sources claim testosterone-boosting abilities for dietary supplements containing D-Asp.
Studies show that D-Asp supplements boost testosterone levels in male animals. However, scientific evidence for such effects in humans is weak, scarce, and inconsistent [15].
In a 12-day, controlled study, taking 3.12 grams of D-Asp daily increased testosterone levels in 23 men. All were healthy, did no resistance training, and had low initial testosterone levels [1, 16].
Conversely, in a 14-day study of 24 healthy men, taking 6 grams of D-Asp daily greatly reduced testosterone. All men had at least 2 years of resistance-training and resistance-trained for 4 days in both weeks [1].
During a 12-week study of 19 healthy, resistance-trained men, those consuming 6-grams of D-Asp supplement daily experienced no differences in muscle size or strength gains compared to the others. None had testosterone changes. All men had at least 2 years of resistance-training and resistance-trained for 4 days per week [17].
In a 28-day study of healthy, resistance-trained men, 3-grams of D-Asp daily did not affect resistance-training, muscle gain, or testosterone levels [18].
Therefore, D-Asp supplementation likely doesn’t increase testosterone levels. Future studies in healthy but relatively inactive, non-resistance-training men are needed. Likewise, studies of women are needed to determine the hormonal effects of D-Asp supplementation [15].
Insufficient Evidence for Improving Sperm Quality
Some companies are claiming that consuming D-Asp supplements or D-Asp-rich foods greatly increases male fertility. This is based on the misconception that D-Asp supplements increase testosterone.
It’s true that D-asp made by the body increases levels of testosterone and other sex hormones in male animals. This has never been proven in humans. What’s more, supplemental D-asp–as explained above–most probably has no effect on testosterone levels [19].
On the other hand, scientists speculate that naturally occurring D-Asp controls the development of healthy, fully-functional sperm needed for reproduction. For example, a study in male rabbits, a mix of L-Asp and D-Asp daily for two weeks increased the number, speed, and mobility of their sperm [19, 20].
In one study, infertile men had much lower levels of D-Asp in mature sperm (if present) and semen than fertile men according to a study of 10 fertile and 20 infertile men. The effects of supplementation were not investigated in this study [21].
In another small study, taking D-Asp food supplements (2.66 grams D-Asp daily) for 90 days increased sperm count and improved sperm swimming abilities in 60 men with low sperm count and/or poorly-swimming sperm. In addition, 26 female partners of these men became pregnant [22].
However, large-scale studies are needed to determine if D-Asp dietary supplements affect infertility in men.
Uses Lacking Evidence:
No clinical evidence supports the use of D-Asp for any of the conditions listed in this section.
Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies listed below should not be interpreted as supportive of any health benefit.
IVF Research
In-vitro fertilization (IVF) clinics seek new, more efficient and reliable ways to determine and increase the quality of egg cells in prospective mothers. Such methods decrease the number of fertilized embryos required for a successful pregnancy. They also reduce the need to store embryos for longer time periods [23].
In a study of 20 women (ages 22 to 40 years), concentrations of D-Asp in follicular fluid dropped with aging. This decline in D-Asp is linked to decreasing egg quality and reduced numbers of successful pregnancies for aging women [24].
Egg quality may be influenced by D-Asp occurring naturally in the follicular fluid that surrounds, protects, and nourishes egg cells in the ovaries. D-asp can improve egg quality by influencing growth hormone, prolactin, estrogen, progesterone, nitric oxide, amino acids, and proteins [24, 25, 23].
One study suggests that treating sperm with the mix of D-Asp, zinc, and CoQ10 (coenzyme Q10) found in a popular dietary supplement improves the swimming and movement ability of human (as well as bull) sperm used for IVF (in-vitro fertilization). This treatment may also prevent damage to sperm DNA and lipids during IVF. Further research is needed [26, 27, 28, 29].
IVF research using D-Asp is still in the early stages. No conclusions can be drawn from the available evidence.
Anti-aging Research
Levels of naturally-occurring D-Asp increase as people age in various human tissues and organs, such as teeth, bones, eyes, and brain. This is because natural L-Asp is converted to D-Asp over time. Such changes are used to understand aging, and although more research is needed, tracking D-Asp might turn out to be useful in anti-aging research [30, 31, 32, 33].
Nootropic Potential
In mice studies, free D–Asp boosted brain cell communication and memory. One human study also connected increased levels of an enzyme (DAO) that alters many D-amino acids with improved memory and thought processes. This is in contrast with studies showing that L-Asp can damage the brain and impair learning [7, 32, 34].
D-amino acids altered by DAO act similarly to D-Asp. Some scientists hypothesize that D-Asp may act as a naturally occurring nootropic, though their theories remain unproven [32].
D-Asp Side Effects, Safety & Precautions
Clinical studies did not report side effects of dietary D-Asp supplements at 2.66 – 6.00 grams daily. However, no proper safety studies have been carried out. The short- and long-term side effects of D-Asp are unknown.
The typical dosage recommended by manufacturers is ~3 g/day. The safety of consuming more than 6 grams daily is also unknown.
In one study, 6 grams of D-Asp daily for two weeks reduced testosterone levels in resistance-trained men consuming [1, 16].
2.66 grams of a D-Asp supplement daily for 90 days was safe for men with impaired fertility, according to a limited, small study [22].
In mice, aspartic acid caused brain damage and impaired learning. The effects of D–Asp — or the D form of this amino acid specifically on the brain — is not well understood. Previous studies have shown the L-Asp can be toxic to the kidneys and salivary glands as well. Since the clinical studies about the safety and efficacy of D-Asp are limited, we advise caution. The risks can increase with higher doses and long-term use [10, 11, 12, 13, 14].
D-Asp dietary supplements are not recommended in women and children due to a lack of safety data.
D-Asp supplement labels state that use is not recommended in pregnant or nursing women, persons under the age of 18, or children. Some state that no women whatsoever should consume D-Asp dietary supplements.
Consult your physician before using D-Asp dietary supplements.
Limitations and Caveats
Clinical studies on D-Asp are mostly small scale and low quality. Additional research about the effects and safety of D-Asp in various populations is needed.
Limitations include:
Humans differ significantly from animals used to model living processes; findings from animal studies cannot be assumed to have the same effects in humans.
While animal models give a general indication of how D-Asp supplements and natural D-Asp function in humans, future studies may show differences in humans.
However, little is known about D amino acids in general, in contrast to their more common L forms. This is an emerging field of research. Up until recently, the existence and relevance of D amino acids were largely downplayed.
Most human studies involve fairly small groups of people with limited diversity for important factors such as age, race, and gender. Studies of much larger human populations with greater demographic diversity are needed to verify and expand the research described in this article.
Very little research has been done about the role of naturally–occurring D–Asp in women. Less research exists about the role of dietary D-Asp supplements in women. Thus, extensive research is still needed on large, diverse groups of women to verify the roles of dietary D-Asp supplements and naturally-occurring D-Asp in women.
More studies involving women from numerous demographic backgrounds are needed to show how D-Asp affects female fertility. Caution must be used in extending conclusions from studies in men to D-Asp function in women.
Drug Interactions
Research on D-Asp interactions with drugs and medications is lacking. However, the following should be noted:
To safeguard against unwanted drug interactions, persons considering the use of D–Asp dietary supplements should first consult their physicians regarding their health status and possible interactions between D–Asp and their medications.
According to DrugBank (a comprehensive, online scientific database), drug interaction information for D-Asp is “Not Available” [35].
D-Asp Natural Sources, Supplementation & Dosing
Food Sources
Can you get D-Asp from food? The simple answer is: probably not. Aspartic acid naturally occurs in foods mostly only as L–Asp. Sources of L aspartic acid include turkey, eggs, soybeans, avocado, asparagus, molasses, oysters, sausage, beef, chicken, kidney beans, peanuts, green peas, yellowtail fish, catfish [36, 37].
L–Asp and other L–amino acids found in large amounts in food can possibly be converted to their D–amino acid forms by cooking, modern food preparation processes, or microorganisms [38, 39].
These conversions are affected by time, temperature and pH. Overall, it appears that cooking and other food preparation processes increase levels of D-Asp in many foods. The exact ways in which the L forms can be concentrated in foods need to be researched in much more detail [38, 39].
Dietary Supplements
D–Asp dietary supplements occur as D-aspartic acid or D-aspartate (salt of D-Asp) forms commonly sold as tablets, capsules, or powders. Data are lacking to determine whether any of these forms offer advantages.
Some D-Asp supplements contained metal (calcium or zinc) chelates. These chelates are promoted as improving the body’s absorption of D-Asp. Evidence doesn’t back up such claims.
A wide variety of D-Asp dietary supplements exist. Some combine D-Asp with other potential testosterone boosters, vitamins and/or other active ingredients. Others list D-Asp as the only active ingredient.
Some supplements contain mixtures of L- and D-Asp. Others just contain the L-Asp form. Be wary of the fact that L-Asp may not have the same effects as D-Asp in the body.
Dosage
Because D-Asp is not approved by the FDA for any condition and clinical studies are sparse, there is no official dose.
Studies discussed in this article used 2.66 grams to 3.00 grams per day in men.
Many brands and forms of D-Asp dietary supplements are commercially available. Depending on the brand and form, most products list dosages between 3.00 grams to 3.20 grams. All insist that the labeled daily dosage should not be exceeded.
User Experiences
Users report various experiences with D-Asp dietary supplements. Some describe having increased libido and energy levels, but also a few headaches when first taking D-Asp.
Some men mention fertility improvements, but also complain of increased anxiety and depression. Another often-reported effect was increased strength/energy, but with some gut issues and bad taste.
A small number of users reported no negative effects but felt no significant testosterone gains.
Overall, customer reviews seem neither overwhelmingly positive nor negative. Many attitudes vary with the brand and type (tablet, capsule, or powder, etc) used.
The opinions expressed in this section are solely those of the users who may or may not have medical or scientific training. Their reviews do not represent the opinions of SelfHacked. SelfHacked does not endorse any specific product, service, or treatment.
Do not consider user experiences as medical advice. Never delay or disregard seeking professional medical advice from your doctor or another qualified healthcare provider because of something you have read on SelfHacked. We understand that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.
Takeaway
D-Asp is the less common (“D”) form of aspartic acid, a non-essential amino acid created by our bodies. The more common form that is also found in food is called D-Asp.
Scientists suspect that D-Asp is important for nerve health, but we know little about its effects in humans.
Small-scale human studies suggest that it likely doesn’t increase testosterone in men, but it shows some potential against infertility. However, there’s still not enough research to recommend its use for any health condition.
Although D-Asp supplements are widely available and often misleadingly marketed, caution is advised. Animal studies point to D-Asp’s toxic potential, while human safety data are lacking.
