What is D-Aspartic Acid?
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 .
It is not safe to shut off, weaken, or remove in humans the genes controlling the levels of D-Asp our bodies make. 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 .
DAA Mechanisms of Action
Naturally Produced D-Asp Improves Brain Development
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].
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, especially with high doses and long-term use.
D-Asp Affects Hormones
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 functions, reproduction, and blood pressure in mice [4, 6, 2].
Health Benefits of D-Aspartic Acid
1) May Boost Testosterone Levels
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 boosts in humans is weaker, scarcer, and less consistent .
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 .
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 .
For 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 .
So, D–Asp supplementation boosts human testosterone levels only in healthy but relatively inactive, non–resistance–training men during shorter time periods. More studies should be done in men to verify these results. Likewise, studies of women are needed to determine if D–Asp supplementation has similar effects as in men .
2) May Increase Sperm Quality
Many claims that consuming D–Asp supplements or D–Asp–rich foods greatly increases male fertility. D–Asp made by the body increases levels of testosterone and other sex hormones in male animals .
In fact, 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].
Naturally produced and supplemental D-Asp increases fertility and sperm quality in animals. Studies in humans show similar trends .
Taking D-Asp food supplements (2.66 grams D-Asp daily) for 90 days increased sperm counts 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 .
Thus, properly administered D-Asp dietary supplements may potentially help men with infertility problems.
3) May Be A Measure of Female Fertility
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 .
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 .
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].
4) D-Aspartic Acid Levels Indicate Aging
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 could also be useful to predict aging [26, 27, 28, 29].
5) May Be a Nootropic
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, which boost memory and thought processes. This is in contract with studies showing that L-Asp can damage the brain and impair learning [7, 28, 30].
D-amino acids altered by DAO act similarly to D-Asp. It’s possible that D–Asp may act as a naturally occurring nootropic .
Side Effects, Safety & Precautions
No long-term, harmful side-effects in humans consuming dietary D–Asp supplements in the amounts (2.66 – 6.00 grams daily) have been reported. The typical dosage is ~3 g/day and the safety of consuming more than 6 grams daily has not been confirmed.
In one study, the testosterone level dropped in resistance-trained men consuming 6 grams of D-Asp daily for two weeks. The effects of consuming such amounts for longer periods of time are unknown [1, 16].
2.66 grams of a D-Asp supplement daily for 90 days was safe for men with impaired fertility .
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 lack of scientific research about its effects. Product labels of D-Asp dietary supplements surveyed here insist that their supplements must not be consumed by pregnant or nursing women, persons under the age of 18, or children. Some state that no women should consume D-Asp dietary supplements.
People with medical conditions should consult their physicians before using D-Asp dietary supplements.
Limitations and Caveats
Scientific studies described here were well-planned, executed and interpreted. However, they do not cover many scenarios, effects, populations, contributing factors, and possible explanations.
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. While their conclusions are a good beginning, 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. Still, evidence about D–Asp as a fertility measure and egg–selection guide shows promise.
Caution must be used in extending conclusions from studies in men to D–Asp function in women.
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” .
Natural Sources, Supplementation & Dosing
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 [32, 33].
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. 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 needs to be researched in much more detail [34, 35].
D–Asp dietary supplements occur as D-aspartic acid or D-aspartate (salt of D-Asp) forms commonly sold as tablets, capsules, or powders. Studies did not find differences between results of consuming the acid forms vs the salt forms.
A wide variety of D-Asp dietary supplements exists. 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 weary of the fact that L-Asp may not have the same effects as D-Asp in the body.
D-Aspartic Acid Dosage
Based on results and conclusions from the studies discussed in this article, a safe dosage of D-Asp dietary supplement for a healthy adult is 2.66 grams to 3.00 grams per day.
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.
D-Aspartic Acid In Combination with Other Supplements
In one study of human blood plasma, a mixture of D–Asp and D–Glu (D-glutamic acid) was more effective than D-Asp alone, D-Glu alone, or several other amino acids (alone or combined) in stopping allergic reactions to peanut allergens to human plasma .
One study shows 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 can also prevent damage to sperm DNA and lipids during IVF [ 37, 38, 39, 40].
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.
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.
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