We included all types of studies describing any association between D-Asp and testosterone until October Additional articles were identified from reference lists of included studies and relevant reviews. The flow diagram of the studies throughout the selection process is shown in Figure 1. We retrieved 96 references from the PubMed database, from Web of Science databases, 76 from the ProQuest and 97 from the Scopus database. The title and abstract of various articles were initially screened and evaluated and only 96 articles were included as potentially relevant.
An examination of the full text of these articles resulted in the exclusion of 25 studies. After hand searching of the bibliographies of these articles and relevant reviews, we identified 2 additional articles, leaving 27 articles for final inclusion: 4 in vivo human studies, and 23 animal studies. Quantitative meta-analysis was not done because of various designs of included studies and small amount of human studies. In vivo animal studies were carried out on male rats, male green frogs, male lizards, and male mallards 4 , 5 , 7 , 14 , 25 , 31 , 38 - These studies showed that D-Asp increases testosterone levels in the blood.
These results are contrary from those were carried out on females green frog and female lizards, in which these differences assume different effect of D-Asp depending on species, sex and organ-specific 4 , 6 , In addition, in a study by Huang et al. Ddo is an endogenous enzyme and is responsible for the metabolizing of D-Asp In vitro studies clarified the functions of D-Asp in steroidogenesis.
Nagata et al demonstrated that D-Asp increases human chorionic gonadotropin hCG -stimulated testosterone synthesis in purified rat Leydig cells. Also they have shown that D-Asp and hCG synergistically up-regulate the production of testosterone in rat Leydig cells, apparently by increasing gene expression StAR in purified rat Leydig cells 31 , 34 , 45 , Other studies showed that D-Asp directly inhances aromatase activity 5 , 6 , 31 , 36 , 42 , 43 , Topo et al carried out a clinical trial on healthy male volunteers aged between 27 and 37 years at the IVF in vitro fertilization unit In the intervention group, 23 participants consumed 3.
Topo et al suggested that consumed D-Asp may also be remained in the testis, and it continued to stimulate the testosterone production in the testis. Inclusion and exclusion criteria were not specified and it is a methodological limitation for this study. In addition, except the age, no details of the participants were reported e.
Significant differences between two groups have not been analyzed. It seems the allocation of participants to the study groups was not random. Thus, the two groups may differed in various baseline characteristics, such as diet, drugs or other supplement consumption, weight, physical activity. These baseline variables were not controlled. No adjustment for potential confounders was performed. No adverse events were reported. In a randomized, double-blind interventional study by Willoughby et al , twenty apparent healthy and heavy resistance-trained men were enrolled After 28 days D-Asp supplementation had no effect on muscle strength, body mass and serum hormones total testosterone, free testosterone, LH, GnRH and estradiol.
Unlike the previous study, participant demographics had been well described such as age, weight and height. All participants passed a mandatory medical screening. Exclusion criteria were consuming any nutritional supplements excluding multivitamins three months before the study. Adverse events were reported. There were no significant differences between groups for total calories or for the intake of protein, carbohydrate, and fat.
The limitations of this study are the small sample size and the reliance on self-report for dietary intake and supplement compliance. Contrary to the study of Topo et al in this study D-Asp supplementation had no effects on serum testosterone Thus, D-Asp supplementation may only be effective with lower testosterone levels. This is contrary with the hypothesis that D-Asp supplementation is effective with lower testosterone levels.
A major limitation of the study was the small size of the sample groups. In addition, this study was open label and was not placebo controlled. However, no differences of statistical significance were observed between groups, hence it seems the allocation of participants in these groups have not been random.
The latest study by Melville et al was a randomized, double-blinded, and placebo controlled design Twenty four participants were randomly allocated to one of three experimental groups: placebo, 3 gr of D-Asp and 6 gr of D-Asp. Three gr of D-Asp acid had no significant effect on the testosterone levels after two weeks. It is similar finding by Bloomer et al and Willoughby et al and contrary to the study of Topo et al 40 , 49 , Baseline testosterone levels of the current study were higher than values found in Topo et al and Bloomer et al experiments 6.
In the six-gram group, total and free testosterone was significantly reduced from baseline. The authors hypothesized that 6 gr of D-Asp per day may affect negative feedback mechanisms of the HPG axis, thus reducing pituitary initiated production of luteinizing hormone and also testosterone levels.
However, in the study of Bloomer et al , 6 gr daily of D-Asp had no effect on the testosterone levels Probability this report revoked Melville et al hypothesis. The limitations of this research are the study length and small sample size. The observed reduction in testosterone may rebound, or even decrease further than expected.
To our knowledge, this was the first systematic review to collect recent evidence on association between D-Asp and testosterone level. Although articles identified from the search strategy, but only 27 articles; including 23 animal studies and 4 human studies satisfied search criteria.
In vivo and in vitro animal studies revealed the mechanisms regulating D-Asp in the synthesis of steroid hormones with different effect of D-Asp depending on species, sex and organ-specific. The bulk of evidence revealed by these experiments suggests that the D-Asp in male animals alerts testosterone production in different ways by acting either directly on Leydig cells or indirectly on the hypothalamus-pituitary-testis axis HPG axis :.
D-Asp can directly increase the release of GnRH from the hypothalamus. D-Asp acts directly on the pituitary gland inducing an increase of LH releasing 14 , D-Asp is capable of boosting the action of human chorionic gonadotropin hCG , thereby it is able to induce testosterone synthesis in the Leydig cells 34 , D-Asp inhances aromatase activity, with consequent production of estradiol from testosterone 6 , 42 , D-Asp participates in steroidogenesis through increasing the expression of some subunits of NMDA receptor in testes D-Asp administration induces an increase of c-kit receptor expression and of tyrosine kinase activity D-Asp induces up-regulation of androgen receptor and down-regulation of estrogen receptor expression D-Asp may delay LH receptor membrane trafficking mediated by hCG, resulting in an increased testosterone production Taken together, these data strongly support a prominent role of D-Asp in the neuroendocrine control of steroidogenesis activity.
Therefore, the investigators assumed that this amino acid can be used as a testosterone booster for infertile men, and by athletes to increase muscle mass and strength. In our review, we were able to identify four studies on D-Asp supplementation have been conducted on humans that their results are contrary.
The age, training status and basal testosterone values of subjects may be responsible for the difference in outcome between these studies. The small sample sizes most likely precluded the examination of important moderator variables such as age, weight status and basal testosterone levels.
Burrone et al showed that D-Asp increases the expression of superoxide dismutase 1 in the kidney and enhances caspase 3 levels in brain and heart tissues These data indicate that exogenous D-Asp may induce oxidant stress and apoptosis in several tissues. Among included studies in this review only one study reported adverse effects of exogenous D-Asp Additional data are not only required regarding the actual association of D-Asp with testosterone levels but also concerning the safe and maximum effective dose of D-Asp supplements for humans.
This study has several strengths. Frist, the comprehensive search was conducted on multiple databases and hand searching reference lists. Second, it may be the first systematic review focused on the relationship between D-Asp and testosterone concentration. Third, we included all type studies, which could more clearly explain this relationship. Some limitations also need to be considered. Frist, we only included English and published studies. Second, it is possible that we have missed some published studies.
Finally, we do not enable to conduct meta-analysis due to high heterogeneity of the included studies. This comprehensive systematic review showed that exogenous D-Asp enhances testosterone levels in male animal studies, whereas studies in human yielded inconsistent results. The evidence for this association in man is still sparse, mostly because of the lack of good quality studies. Based on limited and contrary of the data in humans, the paucity and often low quality of primary studies, it is clear that more and high-quality randomized controlled trials need to be conducted on D-Asp supplementation regarding their ability to increase endogenous levels of testosterone and elucidating the mechanisms of its action in human research.
National Center for Biotechnology Information , U. Int J Reprod Biomed. Farzad Roshanzamir , Ph. Author information Article notes Copyright and License information Disclaimer. Box ,Email: safavimorteza yahoo. Copyright notice. This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This article has been cited by other articles in PMC. Abstract Background: D-Aspartic acid D-Asp is in invertebrate and vertebrate neuroendocrine tissues, where it carries out important physiological functions. Objective: The aim of this review is to summarize available evidence related to the effects of D-Asp on serum testosterone levels.
Results: With retrieved records, 23 animal studies and 4 human studies were included. Conclusion: There is an urgent need for more and well-designed human clinical trials with larger sample sizes and longer duration to investigate putative effects of D-Asp on testosterone concentrations. Introduction D-Aspartic acid D-Asp is an endogenous amino acid occurring in several tissues and cells of both invertebrates and vertebrates 1.
Materials and methods We included all types of studies describing any association between D-Asp and testosterone until October Table I Search strategy. Search strategy to identify relevant exposures and outcomes: 1. Open in a separate window. Results The flow diagram of the studies throughout the selection process is shown in Figure 1. Figure 1. Table II The characteristics and outcomes of the included animal studies.
Table III The characteristics and outcomes of the included human studies. First author, year reference country Design Participants Treatment Group Mean participants characteristics and age year Exclusion criteria Other participant demographics and comments Topo et al. Discussion To our knowledge, this was the first systematic review to collect recent evidence on association between D-Asp and testosterone level.
The bulk of evidence revealed by these experiments suggests that the D-Asp in male animals alerts testosterone production in different ways by acting either directly on Leydig cells or indirectly on the hypothalamus-pituitary-testis axis HPG axis : 1. Conclusion This comprehensive systematic review showed that exogenous D-Asp enhances testosterone levels in male animal studies, whereas studies in human yielded inconsistent results. Conflict of interest The authors declare that there are no conflicts of interest.
References 1. D'Aniello A. Our results suggest a regulatory role for d -Asp in the steroido-genesis and spermatogenesis of the testis of the lizard Podarcis s. In the family of neuroexcitatory amino acids, it is now well known that d -Asp has important neurotransmission and neurosecretion roles Schell et al. An extremely high content of d -Asp was detected in human brain Fisher et al. In addition, d -Asp may participate in brain development, differentiation and functioning Hashimoto et al.
Most recent investigations have examined the endocrine system of mammals where d -Asp is well represented in the pineal gland Imai et al. Maximal content and transient emergence of d -Asp in these tissues correspond to their morphological and functional maturation Hashimoto et al. In the adenohypophysis, pineal gland and testis in particular, d -Asp is involved in hormone synthesis and release Dunlop et al.
The relationship between d -Asp and endocrine activity has been well documented. Developmental changes in levels of d -Asp and testosterone in rat testis parallel each other closely: they increase to maximum levels at sexual maturity.
Moreover, d -Asp is contained in spermatids Sakai et al. These data indicate that d -Asp may act as a novel putative regulator of hormonal synthesis. In support of findings in mammals, other studies on lower, seasonal breeding vertebrates have shown the effects of d -Asp on gonads. In the amphibian Rana esculenta, d -Asp occurs in the ovary where it is involved in the control of testosterone release during the sexual cycle Di Fiore et al.
In the female lizard Podarcis s. Raucci et al. On the other hand, d -Aspartate may play a role in hormonal regulation, as it stimulates testosterone synthesis in the testis, increasing the mRNA level of a steroidogenic acute regulatory protein StAR Nagata et al. Testosterone is a well-known prerequisite for normal spermatogenesis see for reviews, Zirkin and Sharpe In the roe deer Capreolus capreolus , a typical seasonal breeder, the peak in testosterone coincides with maximal meiotic activity of the testis and with spermatogonial proliferation.
This evidence, already known in most vertebrates, strongly suggests the importance of testosterone for sperm production Roelants et al. Furthermore, the protein proliferating cell nuclear antigen PCNA is essential for the proliferation of the spermatogonia. For these reasons, PCNA is utilized as an endogenous and molecular marker of mitotic and testicular epithelial proliferation Chieff et al. Except for the data reported on male frogs, there are no studies yet available on d -Asp presence and its effect on the testis of lower vertebrates.
Seasonal breeders are good models for studying the involvement of d -Asp on the testis because the effects of this molecule can be compared in response to the different phases of spermatogenesis. Therefore, to gather information on this aspect of d -Asp function, we investigated the occurrence of endogenous d -Asp in the testis of lizard, Podarcis s. To gain insight into the functional significance of d -Asp in this organ, we studied the role of this amino acid in lizards collected during the main phases of their reproductive cycle.
We studied the uptake of d -Asp in the testis and its putative role in both steroidogenesis sex hormones in the testis and plasma evaluated during the cycle and from in vivo experiments and spermatogenesis immunohistochemistry technique using PCNA antibody.
Finally, we attempted to determine whether the d -Asp present in the gonad could come from a local conversion of l -Asp by a specific racemase. Generally, at the beginning of March the lizards emerge from winter shelter; gonads and secondary sexual characters SSCs begin to develop and are functional until the end of June—beginning of July.
From March to April, male lizards are engaged in fights aggressive phase that are linked to reproductive territory assessment reproductive period. At the end of April—beginning of May, courtship and mating begin and last for several weeks mating phase. In July, when the temperature is still favorable for reproduction, a refractory period induces a block of spermatogenesis and the regression of SSCs refractory phase : this phase is considered the post-reproductive period. In October, spermatogenesis resumes and some sperms are produced, but there is no spermiation or SSCs.
From November to March, external temperatures decline and the lizards undergo semi-hibernation pre-reproductive period. Taking into account the reproductive characteristics, adult Podarcis s. The animals used were 2—3 years old and had a body weight of about 8—9 g. The animals were given a regular supply of mealworms and fresh vegetables and were allowed to feed ad libitum. The experiments were carried out on lizards caught in the three main phases of their reproductive cycle.
Lizards were assigned to different groups according to treatments see below. Each group was composed of five animals. Soon after capture, several animals were anesthetized by short cold exposure; blood was collected through a heparinized glass capillary inserted into the heart. From each animal, liver and testes were rapidly dissected out. The methods of capture and dissection and the captive rearing conditions were in accordance with Italian law D.
Short-term experiments were carried out by injecting d - and l- forms of amino acids into lizards caught in three main phases of their reproductive cycle. Lizards, sorted into 5 groups, 25 animals in each, were treated as follows: lizards from groups 1, 2, 3 and 4 received i.
This dose was chosen on the basis of preliminary experimental tests. Namely, group 1 was injected with l -Asp, group 2 with d -glutamate d -Glu , group 3 with l -glutamate l -Glu , group 4 with d -alanine d -Ala. The lizards from group 5 were injected with vehicle alone saline solution and used as controls. Plasma, liver and testes were collected at different times after the last injection 0, 3, 6, 15 and 24 h and utilized as described above. The amino acids were purchased from Sigma.
In other experiments, the d -Asp uptake by testis and the concomitant levels of testicular and plasma steroid hormones were studied. Lizards, belonging to main phases of their reproductive cycle, were distributed in two groups 25 animals each. Animals from the first group were injected i. Five injected lizards from each group were killed at set times within a period of 24 h 0, 3, 6, 15 and 24 h after the injection respectively.
The lizards were utilized as previously reported for experiments on d -Asp uptake, sex steroid concentrations and immunoreaction assay see below. Sex steroid determinations in the plasma were conducted utilizing enzyme immunoassay EIA kits Adaltis Italia, spa, Italy. The addition of d -Asp to the standard curve did not modify the assay sensitivity. The upper phase ethyl ether was transferred to a glass tube. Two extractions were performed. The residue was dissolved in a 0.
Three extractions were performed. Pooled ether extracts were dried and then utilized for the enzyme immunoassays as previously reported Di Fiore et al. Steroid recovery was assessed by parallel processing of tissue or plasma samples to which known amounts of steroids had been added prior to extraction and assay.
Testis and liver samples were homogenized with 0. The supernatant was brought to pH 7. The supernatant was adjusted to a pH of about 2. The dry eluates were dissolved in 1 ml of 0. They were then purified by slowly passing through a Seppak C cartridge mg; Waters, Milan, Italy which had been previously activated with methanol or acetonitrile and washed with distilled water.
To recover the amino acids from these eluates, the cartridge was eluted twice with 2 ml of 0. This method has been fully described in a previous paper Di Fiore et al. In this study the d -AspO enzyme was obtained by overexpression and purified according to the procedure described previously Negri et al. To verify whether d -Asp is biosynthesized from l -Asp, via an aspartate racemase, we measured the racemase activity by evaluating the in vitro conversion rate of l -Asp into d -Asp.
Control samples contained all components except l -amino acids. Incubations were stopped by rapid freezing in an ice-bath. The amino acids present in the samples were extracted with 1. To assess cell proliferation, PCNA immunohistochemistry was performed according to the procedure reported in Chieffi et al.
Fixed lizard testes were serially dehydrated in ethanol and cleared in xylene. The conventional avidin—biotin complex ABC procedure was used Hsu et al. Sections were mounted with a synthetic medium. The following controls were performed: 1 omission of the primary antibody; 2 substitution of the primary antiserum with pre-immune serum Dako diluted in blocking buffer; no immunostaining was observed after any of the control procedures. A section of testis from Rana esculenta was used as the positive control, as described in Raucci et al.
The morphological parameter measured was the number of immunoreactive elements for PCNA in 1 mm 2 of the testis germinal epithelium. In addition, the correlation coefficients r between d -Asp content in the testis and both plasma and testicular concentrations of steroid hormones were calculated.
The plasma levels of sex hormones are also reported. It should be noted that steroid hormones are synthesised by the gonads, rapidly released into the plasma, and do not accumulate in the testis. Endogenous d -Asp occurred in the testis during all periods of the cycle and its level underwent significant variations depending on the reproductive phase.
The d -Asp level was significantly higher during the reproductive period than in pre- and post-reproductive periods 1. Likewise, testosterone concentration in the testis was higher in the reproductive phases and low in pre- and post-reproductive phases 1. The sex hormone levels in the plasma reflected those in the gonads, although in the plasma testosterone level was much higher in the reproductive phase.
In adult male Podarcis s. In the pre-reproductive phase Fig. In the reproductive period Fig. Furthermore, basal values were rapidly reached within 15—24 h. At 6 h after injection its levels were still high, but were successively decreasing at 15 and 24 h.
In the liver used as control tissue the concentration of d -Asp in each period of the cycle was significantly higher than in the testis. Following injection of 2. The d -Asp administration affected the levels of sex hormones. In the pre-reproductive period Fig. This effect also appeared in circulation plasma, Fig. Successively, in both testis and plasma, the testosterone concentration reached baseline within 24 h. Estradiol concentrations in testes decreased at 3 h from In the reproductive and post-reproductive period, similar sex hormone profiles were observed.
Moreover, it is interesting to note that during the reproductive period the variation of hormone concentrations was less than in the other phases of the cycle. This phenomenon could be due to the highest endogenous physiological concentration of d -Asp in testis during the reproductive period. In fact, d -Asp was at the highest level in the cycle and therefore further stimulation with exogenous d -Asp had no effect on steroid response. In order to verify whether d -Asp is locally synthesized by l -Asp through an aspartate racemase we measured the racemase activity by evaluating the in vitro rate conversion of l -Asp into d -Asp during the reproductive cycle Fig.
Testis and liver tissue homogenates were incubated with l -Asp or other amino acids under different pH values. Testicular tissue converted l -Asp into d -Asp in all phases of the cycle and d -Asp biosynthesis only significantly varied during the reproductive period, reaching its maximal level In the liver control tissue racemase activity was observed but remained unchanged through the cycle data not shown.
The histological morphology of the testis shows that in the seminiferous tubules of treated animals Fig. Spermatogonia SPG are observed near the basement membrane of the seminiferous ephitelium in all experimental groups Fig. Their nuclei contain prominent nucleoli and heterochromatin concentrated close to the nuclear membrane.
The nuclei are spherical, centrally localized and have a distinct acrosome vesicle in direct contact with the nuclear envelop. Elongation of the apical region of the nucleus, nuclear condensation and cytoplasmic elimination produce elongated spermatids with short flagella. At 3 and 6 h after d -Asp injection the elongated spermatids can be seen at the lumen proximity Fig. No mature spermatozoa are observed in the lumen of seminiferous tubules either in controls or treated animals.
Immunohistochemistry reactions, carried out on serial sections of testis, revealed the presence of PCNA protein in the gonad in each period of the reproductive cycle. We found that immunoreactivity for PCNA was abundant in the cytoplasm and nucleus of SPG and in the nucleus of early stage I SPG during spermatogenesis reproductive period and in the post-reproductive period data not shown.
Figure 4 shows testis sections of a pre-reproductive lizard stained for PCNA together with the negative control for immunoreaction Fig. Immunopositive material was found in the actively dividing germinal epithelium and was localized in the cytoplasm and nucleus of type I and II SPG of both control Fig. Moreover, at time 0 the positivity was observed in SPG Fig. At 3 Fig. The immunohistochemical results were assessed by morphometric analyses revealing an increase of immunopositive elements in the testis treated with d -Asp.
We provide physiological evidence of naturally occurring free d -Asp in the testis of the lizard Podarcis s. Gonadal levels of this amino acid undergo fluctuations throughout the sexual cycle: the highest concentration of d -Asp is present in the testis of lizards during the reproductive period.
The testis shows a very high and relatively rapid ability to take up and accumulate exogenously administered d -Asp, particularly in the pre- and post-reproductive phases rather than in the reproductive phase, probably because of the highest endogenous d -Asp present in the gonad during the latter period. Cultured rat pinealocytes can take up exogenous d -Asp intensively Takigawa et al.
The l -Glu transporter, which has been identified in the rat pineal gland Yamada et al. The capacity of the testis to concentrate d -Asp suggests the presence of l -Glu receptors, whose expression could vary in correlation with gonadal activity.
This indicates that d -Asp is the only amino acid that is actively taken up by the testis. The d -Asp profile in the testicular tissue and its correlation with testosterone titer led us to propose that d -Asp could be endowed with the control of the synthesis and release of steroid hormones by the gonad.
In vivo experimental results support this hypothesis. The exogenous d -Asp and its parallel uptake in the gonad induce a significant increase in both plasma and testicular testosterone concentration. This effect is observed in all phases of the sexual cycle although it is particularly evident in the pre- and post-reproductive periods. This hormonal trend is summarized in Fig.
A similar trend in sex hormone levels is observed in the blood although the peak is shifted by 3 h, i. Sex hormones were restored to basal levels within 15—24 h.
Experiences with pure D-Aspartic Acid were not as hopeful; those with calcium and sodium salts of the amino acid faired better. The body absorbs these forms of D-Aspartic Acid, it seems, better. However, more recent research is of the contrary.
According to a small human study published by sports scientists Darryn Willoughby and Brian Leutholtz in Nutrition Research, the effect of D-aspartic acid on testosterone levels is minimal. The same can be said about its effects on muscle building qualities and its ability to burn body fat, according to the study.
Because the Italian study was done on elderly males with an already low testosterone level, Willoughby and Leutholtz were sceptical about its effects on healthy eugondal males. They conducted a study on healthy male bodybuilders for 28 days. The experiment was with twenty bodybuilders with an average age of 22 years. Ten of the bodybuilder were given 3g every day of D-Aspartic Acid and the other ten were given placebo containing no active ingredient.
The results can be found in the below tables and show very minimal effects on muscle growth, fat loss and changes in hormonal markers. What did change was the level of the enzyme D-aspartate oxidase. This is the enzyme responsible for breaking down D-Aspartic Acid in the liver, kidneys and intestines.
This recent study was done on the free form of D-Aspartic Acid. The study has little to say when it comes to the sodium and calcium bound analogues of D-Aspartic Acid , sodium-D-aspartate and calcium-D-aspartate, which the Italian study claimed to be effective. Limitations of this study were that it was only done on twenty subjects and the Italian study was done on elderly males, with an already low testosterone levels, indicating the studies analogues of D-Aspartic Acid could be good for increase male fertility.
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