Testosterone Booster


Introduction – The Importance of Testosterone

Gaining muscle mass and the mechanisms behind it is a very complicated system. The majority of people who engage in bodybuilding, weight training, and athletics in general want to understand what it is that can maximize the potential to grow muscle, how to structure their exercises and workouts to provide this, and what components of their nutrition and supplementation can provide them the edge desired. A training stimulus is always required through exercise to initiate or “spark” the muscle growth process into motion. The one problem that exists, however, is the fact that there are these factors known as mediators. Mediators essentially control the extent of muscle growth and how much can be maximally (or minimally) reached. Although there are many different mediators that have been discovered in the human body over the years, the most famous and influential mediator involved in the muscle growth process is the hormone Testosterone. Testosterone, and the other anabolic steroids, which are all derivatives and analogues of Testosterone (modified forms of Testosterone) all work more or less in the same manner as Testosterone itself in mediating the muscle growth process.

Testosterone is what is specifically known as an anabolic androgenic steroid hormone (abbreviated as AAS), and it acts on many different tissues on the subsystems of the human body, but what bodybuilders and athletes are especially focused on and interested in are its anabolic muscle building properties. The two terms anabolic and androgenic mean two different things, but they are both intrinsically part of the nature of Testosterone and all anabolic steroids. Although the anabolic effects can be distinctly distanced from the androgenic properties, they cannot be completely separated and excluded. The anabolic properties refer to the tissue building effects (in this case, muscle tissue) while androgenic properties refer to the masculinizing effects of the hormone (i.e. development of male secondary sex characteristics, deepening of the voice, development of facial and bodily hair, maturation of sexual organs, etc.). Testosterone is endogenously manufactured by the Leydig cells of the testes, which is actually controlled through an endocrine axis known as the HPTA (Hypothalamic Pituitary Testicular Axis) by way of a cascade of signaling hormones that start at the hypothalamus. The hypothalamus releases GnRH (Gonadotropin Releasing Hormone), which signals the pituitary gland to release LH (Luteinizing Hormone), which signals the Leydig cells in the testes to manufacture Testosterone.


The idea of using a Testosterone booster is to maximize this process, or possibly take it beyond its normal functioning capability. Test boosters can come in many forms, from synthetic drugs, to naturally occurring herbs in plants found on our planet, to actual activities and lifestyle habits. For example, the hypothalamus is a component of the brain, and therefore a variety of mechanisms related to the brain can actually influence increases or decreases of Testosterone, such as emotional state, energy, and physical and emotional stress of all types (both emotional and physical). The levels of Testosterone in the body, through the HPTA, is controlled through what is known as a negative feedback loop. What this means is that the body will decrease its endogenous Testosterone production if what it deems as excessive levels of Testosterone are detected circulating through the bloodstream. The HPTA will then adjust the output signals accordingly if excessive (or insufficient) amounts of Testosterone are detected. This recognition and alteration process is the basic description of the operation of the negative feedback loop. Excessive Estrogen levels also affect this negative feedback loop in the same manner as well.

Testosterone acts as the gatekeeper to muscle growth and strength development – without adequate levels of it, muscle growth cannot occur despite any amount of training or food.

The Risks of Exogenous Testosterone Use and Why Testosterone Booster Alternatives Might Be Preferred

The prospect of using Testosterone boosters have skyrocketed since the early 1990s, and the popularity and availability of them has increased in proportion to this. There are many reasons for such a dynamic shift in the supplement market. First of all, the use of anabolic steroids, which is pretty much the use of exogenous Testosterone and related hormones, is associated with several risks. Many of these risks are not life threatening, but they can be enough of an inconvenience to deter individuals from the idea of anabolic steroid use, which is understandable. For those who might be confident enough to be able to deal with these side-effects and risks, the imposing complexity of how to properly utilize anabolic steroids might still be too much for some people. Anabolic steroid use can be performed very safely, but it is extremely complex to do so, and requires an immense amount of research, knowledge, and know-how. Such a requirement might be too much for many people to handle, and these individuals may want to find ways of maximizing their Testosterone levels without the complexity associated with anabolic steroid use. One other reason for the preference of the alternative Testosterone boosters is simply due to the legal status of anabolic steroids in the United States. While the majority of nations on earth have either no laws or lax laws where anabolic steroids are concerned, there are nations in which the laws are very harsh and serious penalties exist for the use, possession, buying, and selling of Testosterone and anabolic steroids without a valid prescription. In these countries, such as the United States, many people might wish to avoid these legal implications associated with exogenous Testosterone use and opt for Test boosters.

The legality is one of the primary reasons as to why Testosterone boosters have become very popular since the early 1990s. The Anabolic Steroid Control Act was brought into legislation in 1988, which effectively placed Testosterone and all anabolic steroids in 1990 under schedule III of the Controlled Substances Act. Following this, the supplement market had to now find methods of appealing to the quest to maximize Testosterone levels legally, which there was a massive demand for now that the awareness of anabolic steroids had increased, and that anabolic steroids were now considered an A-list prohibited drug. It was not very long until the supplement market skyrocketed with Testosterone boosters and legal steroid alternatives at a very fast rate.

The primary goal of the supplement companies within the supplement industry has always been the goal of creating the best and most perfect anabolic environment within the human body. Maximizing the key hormone necessary to creating the perfect anabolic environment – Testosterone – has always been the key. There are many categories of Testosterone boosters both inside and outside of the supplement industry, as well as other methods of boosting Testosterone that does not involve the intake or ingestion of any substances. Essentially, the different categories of Testosterone Elevator can be summed up as follows:

1. Training, Nutrition, and lifestyle habits
2. Drugs and synthetic compounds
3. Herbal products
4. Vitamins and minerals

Each of these will be explored in this article, as well as the efficacy of these Testosterone boosters and Testosterone boosting methods.

Training, Nutrition, and Lifestyle Habits as Testosterone Boosters

First and foremost, the most effective Testosterone booster is that of a proper, healthy, and regimented lifestyle. It is a well-known fact that the average individual is out of shape, sedentary, and engages in extremely poor nutritional habits. It is no surprise that there has been a constant and steady decline in the average Testosterone levels in men over the last 20 or more years, as evidenced by studies[1] [2]. Many people point to external factors such as the environment and chemicals ingested in modern food processing, but the decline in Testosterone levels is also directly correlated with the increasing amount of out-of-shape sedentary men that do not engage in proper exercise or proper nutritional habits. No doubt external factors such as the environment and chemicals play a factor, but the overwhelming cause seems to be the decline in proper healthy lifestyle habits. A properly adjusted and balanced diet consisting of clean health wholesome foods with a proper caloric intake should normalize and minimize the amount of body fat, and it is a well-known fact that adipose tissue (fat tissue) is responsible for increasing the amount of aromatization (the conversion of Testosterone into Estrogen) in the body. Clinical studies have demonstrated that a higher body fat percentage will present a higher risk of Estrogenic side effects resulting from increased Estrogen levels in the body[3]. This is because the aromatase enzyme (the enzyme responsible for aromatization) is highly abundant in adipose tissue (fat), and so it stands to reason that the higher an individual’s body fat percentage is, the greater the rate of aromatization of androgens into Estrogen will result. It then follows suit that an increase in Estrogen levels will further decrease the body’s natural endogenous production of Testosterone, and further compound the issue. It is very evident that this is the case, considering studies have demonstrated that excess Estrogen in men can and does suppress the output of endogenous Testosterone, leading to hypogonadism[4].

Furthermore, under-eating and under-nutrition are massive causes of reduced Testosterone levels in individuals. Extensive research has concluded that poor nutritional protocols that involve fasting or consuming inadequate amounts of calories results in significantly diminished Testosterone levels[5]. This lends credence to the fact that the majority of Americans and individuals today who engage in poor nutritional habits are actually also consuming far less calories than they require for normal sustainment, and when they do eat, they consume the wrong unhealthy foods that further hamper the body’s proper endocrine production. This also is the reason why starvation diets or extremely low calorie diets should not be engaged in, as they will plummet Testosterone levels.

Tesotsterone Booster

Aside from nutrition, it is a very well documented and well-known fact that training and exercise, especially the intensity of the training and exercise, acts as an extremely impressive Testosterone booster. One study performed in 1997 examined the effect of resistance training exercise (weight training) on Testosterone levels in 12 men, whereby squats, bench press, and jump squats performed in varying degrees of intensity was performed[6]. The results concluded that Testosterone levels rose to a much higher level than baseline (15% higher) following the jump squats than those who performed bench press (a 7% increase). It is evident here that the compound exercises performed, which tend to recruit the maximal amounts of muscle fibers for the lift, will stimulate maximal endogenous Testosterone boosts in the body. This also reinforces the idea that the stronger compound lifts (such as deadlifts, squats, bench press, etc.) should be the primary focus of any resistance training routine, as these bigger compound lifts will exhibit a larger Testosterone boost than isolation exercises will.


The intensity of the resistance training exercise is also a determining factor in how much endogenous Testosterone is boosted. As the intensity of training increases, so too does the level of Testosterone in response to the training stimulus. One particular study conducted in 2000 on 9 male strength athletes has demonstrated such an effect and correlation, in which these athletes performed back and front squats, as well as leg extensions[7]. In this study, each test subject performed exercises at 100% intensity, and then at 70% intensity defined by training to failure versus stopping a certain amount of repetitions short of failure (or working with less weight) respectively. The test subjects what performed 6 repetitions at 100% intensity exhibited far superior increases in Testosterone compared to the test subjects that performed 6 repetitions at 70% intensity. The end conclusion is that performing resistance training short of failure will not elicit as great of a Testosterone boost as training to full 100% failure will, and there are a few reasons behind such a change in difference.

Testosterone Boosts

Not only does resistance training exercise stimulate Testosterone boosts in the immediate sense, but Testosterone increases occur in the long term as well as a result of weight training. One particular study examining 28 Olympic elite junior weight lifters had determined that not only did the more novice trainees experience Testosterone increases, but the Testosterone increases experienced by those athletes who possessed greater than 2 years of training experience were far greater by a great deal than those athletes with less training experience[8]. This lends a significant amount of evidence to back up the fact that exercise not only works to act as a Testosterone increaser in the short term, but will only continue to elicit Testosterone increases in greater amounts as training continues in the long term.

Drugs and synthetic Compounds as Testosterone Boosters

Science and medicine has made vast headway in the development of synthetic compounds that can effectively be utilized as Test boosters, which can provide effective treatment for those experiencing low Testosterone (hypogonadism and andropause). The compounds include the primary two categories:

SERMs (Selective Estrogen Receptor Modulators)
– AIs (Aromatase Inhibitors)

Some of these synthetic compounds can be obtained legally over the counter (OTC) in supplement stores, and others are available via prescription-only. The difference between many of these compounds, whether or not they are prescription or OTC, is that of a variation in strength. There exist, for example, very strong aromatase inhibitors such as Femara (Letrozole), while there are more mild aromatase inhibitors such as 6-OXO (4-AT), which is available over the counter in many places in supplement stores.

SERMs: These are synthetic compounds that are generally non-steroidal in nature, and are instead commonly triphenylethylene compounds. They include products such as Nolvadex (Tamoxifen), Clomid (Clomiphene), Fareston (Toremifene), Evista (Raloxifene). All of these serve to block Estrogen at Estrogen’s objective receptor sites in various particular tissues (specifically breast tissue). This procedure is known as Estrogen antagonism, which is the terminology describing the Estrogen blocking activity of these compounds whereby they will bind to Estrogen’s target receptors and then essentially force Estrogen out of the receptor sites, or prevent Estrogen from being able to bind to its target receptor sites. This is what is known as Estrogen antagonism. In other tissues, they serve to function as Estrogen agonists, such as within liver tissue, where they exert Estrogenic effects on their own accord in these areas. Some positive effects can be seen from such effects, for example the positive cholesterol changes that Nolvadex (Tamoxifen) exhibits when it acts as an Estrogen agonist in the liver. Specifically for the purpose of acting as a Testosterone elevator, many of these SERMs, to varying degrees, will function as Estrogen antagonists at receptors located at the pituitary gland, which will result in a stimulation of gonadotropin (LH and FSH release), and as a result, Testosterone levels will ultimately increase. SERMs are in fact used as a frequent treatment for HPTA recovery within medicine due to their Estrogen antagonistic actions on the pituitary gland, which consequently results in a Testosterone boosting effect through their process in causing stimulation of the Leydig cells of the testes[9] [10] [11] [12] [13].

AIs: Aromatase inhibitors are synthetic compounds, whereby some might be steroidal in nature and others might not be. These are compounds such as Aromasin (Exemestane), Arimidex (Anastrozole), Femara (Letrozole), and Proviron (Mesterolone) as well as many other compounds that can be available both OTC as well as prescription-only. Each of these compounds disable the cause of rising Estrogen levels at the root: aromatization. Aromatization, as previously mentioned in this article, is a metabolic process in which Testosterone (or any androgen with the capability of experiencing this procedure) is converted into Estrogen by means of contact with the aromatase enzyme. The aromatase enzyme is the sole cause for aromatization, and this enzyme will becomes inhibited (or disabled) by the aforementioned aromatase inhibitors. Some of these inhibitors will only disable the aromatase enzyme temporarily, such as Arimidex and Letrozole, while others will inhibit the enzyme permanently, such as Aromasin and 6-OXO (4-AT), which are known as suicidal inhibitors. Many of these aromatase inhibitors will vary in strength from very mild, such as Proviron, to very strong, such as Letrozole. In terms of their Testosterone boosting effects, AIs will adjust and stimulate the negative feedback loop of the HPTA by way of plummeting total blood plasma levels of Estrogen. As previously mentioned in this article, if excess Estrogen is detected in the bloodstream, the hypothalamus will respond through the negative feedback loop by way of signaling a reduction of Testosterone production, however, if Estrogen levels are unusually lower than standard physiological levels, it causes an antagonizing effect whereby the HPTA will generate higher levels of Testosterone. Studies have demonstrated the effectiveness of these compounds on boosting Testosterone, where some aromatase inhibitors have increased serum Testosterone levels in test subjects by 58% above baseline[14], while other studies on other aromatase inhibitors have increased it upwards of 60% or greater[15], and some as great as 84% and even 90%[16] [17].

Herbal Products as Testosterone Boosters

One of the more common and prominent types of Testosterone boosters on the market are herbal products, of which the most commonly seen and popular are: Tribulus Terrestis, Avena Sativa, Eurycoma Longifolia (LJ100), and Tongkat Ali, as well as several others. There are very mixed opinions and reviews in regards to these herbal and plant extracts that have claimed to be able to stimulate Testosterone production in men. Many supplement companies might sell a supplement that contains a proprietary blend of ingredients that might include one, multiple, or all of the aforementioned herbs. These herbal components can also be purchased on their own for much cheaper costs than the big brand name proprietary blends that are often seen and sold on supplement store shelves. The problem with the majority of these herbal compounds is that there are very little to no human studies, although there do exist a considerable number of studies on rats, mice, and primates. This data is very reliable, but it is yet largely unknown if these products will elicit the same types of Testosterone boosting effects in humans.

Tribulus seems to be the most popular of all of them, and does happen to have the most amount of research behind it, but opinions are still quite mixed in regards to its efficiency at acting as a Testosterone increaser. Tribulus does contain plenty of other benefits that it exhibits in the human body, but for the purpose of this article, its ability to boost Testosterone will be examined. Tribulus is a vine plant that is cultivated in moderate temperature and tropical environments and is believed to contribute to supporting healthy androgen levels by way of acting as a Test booster through the stimulation of the pituitary gland to release LH and FSH, the gonadotropins responsible for signaling the Leydig cells of the testes to manufacture Testosterone.

It is well known that Tribulus is indeed utilized by athletes as an alternative to anabolic steroids, and one particular study noted that, through urine analysis of two athletes who ingested 500mg of Tribulus 3 times daily, no impact on endogenous Testosterone was noted after two days[18]. However, another study conducted on primates, rabbits, and rats determined that in primates, androgen levels had indeed increased but they had been statistically insignificant, while in rats, androgen increases were statistically significant, and the same was true for rabbits which experienced significant increases in androgen levels compared to control groups[19]. Other studies on male rats have also concluded that rats do indeed respond very positively to Testosterone increases when administered Tribulus[20]. The fact that primates are a closer species to humans than rats or rabbits are might lend more credence to the fact that Tribulus does not work as well in humans as in other species. Furthermore, a study conducted on humans – elite rugby players – for 5 weeks concluded that although muscle mass had increased significantly during the 5 week training period, there was no significant difference between the gains made between the control group and the group being administered Tribulus, and furthermore, Tribulus did not serve to alter the levels of Testosterone in urine, nor did it elicit any significant strength or lean mass gains that manufacturers commonly claim[21].

A solid conclusion on Tribulus seems to be that it either works very mildly as a Testosterone booster in humans or not at all, and certainly not worth spending the amounts of money on them in the supplement stores.

Tongkat Ali is perhaps the second most popular herbal based Testosterone booster next to Tribulus, and is frequently combined with Tribulus in many Testosterone booster products. As with Tribulus, there exist an overabundance of clinical data on Tongkat Ali in the form of studies that demonstrate its effectiveness on boosting Testosterone by way of HPTA stimulation in rats and mice[22] [23] [24]. However, much like Tribulus, human studies are lacking, but those studies that have been conducted on Tongkat Ali activity in humans have indeed shown at least some form of positive improvement than with Tribulus. For example, one particular study investigated Tongkat Ali’s use as a Test booster in men with late-onset hypogonadism (commonly referred to as andropause), and the study concluded that after being administered a 200mg water-soluble Tongkat Ali extract for 1 month, the test subjects that had previously exhibited lower than normal Testosterone levels were then displaying normal values following treatment[25]. It is important to note, however, that the study did not demonstrate any effect of Tongkat Ali on raising Testosterone levels above baseline, or anywhere near the supraphysiological levels required for individuals to experience significant body composition and strength changes.

The conclusion to make with herbal based Testosterone boosters is that the idea is dodgy at best, and there is not enough research to definitively conclude that they provide any Testosterone boosting effect significant enough to match the effects of significant supraphysiological levels of Testosterone. Many animal studies demonstrate this, but the vast majority of human studies that have been conducted do not.

Vitamins and Minerals as Testosterone Boosters

The two primary items to be examined here will be vitamin D (Cholecalciferol) and ZMA, which stands for Zinc and Magnesium Aspartate, a product that was once claimed to raise Testosterone levels to very significant levels.


There is plenty of established solid evidence through studies that mega dosing Vitamin D (Cholecalciferol) exhibits a significant effect on increasing Testosterone levels in men and also has a significant ability to suppress SHBG levels in the body. One of the best Testosterone boosting products is indeed Vitamin D. There is an overabundance of clinical studies in existence demonstrating that low Vitamin D levels corresponds with a low level of endogenous Testosterone production (particularly in the winter months for obvious reasons). In one study conducted in Austria where about 200 subjects were involved with one group administered 3332iu daily of vitamin D, and a placebo group, results had shown that men with sufficient Vitamin D levels had significantly higher levels of Testosterone and significantly lower levels of SHBG when compared to the D-deficient subjects[26]. Androgen levels and Vitamin D levels are associated in men and reveal a concordant seasonal variation[27]. In various other studies, similar findings were reported where subjects who were administered higher amounts of Vitamin D over time demonstrated vast increases in total Testosterone levels, and decreases in SHBG. Anecdotal evidence of people who have been supplementing with vitamin D and getting regular blood work at their doctors are observing large increases in their total and free testosterone levels approximately 1 – 2 months after supplementing with Vitamin D.

ZMA was developed by BALCO (Bay Area Laboratory Cooperative) founder Victor Conte, and is a blend of three ingredients in a particular mixture: zinc monomethionine and aspartate (30 mg), magnesium aspartate (450 mg), and vitamin B6 as pyridoxine hydrochloride (10.5 mg) and was initially claimed to act as an extraordinary Testosterone booster, and to increase strength levels in athletes equally as efficiently. Although all three minerals are very important in biological processes, and zinc in particular has demonstrated to be a fertility aid and gonadal antioxidant/protectant[28] [29], none of these in the particular combination advertised have been reported to be effective what so ever in the claims that have been made surrounding ZMA. Initially, a study was done that claimed that ZMA increased strength in football players, but it was later discovered that the study was performed incorrectly and that Victor Conte, who was one of the authors of the study, had ownership equity in the company that had funded and run the study. Later on, further studies were conducted on ZMA that demonstrated it had absolutely no effects on androgen levels in the body, and no effects on strength or endurance increases either[30] [31]

Medical References:


[1] A Population-Level Decline in Serum Testosterone Levels in American Men. Thomas G. Travison, Andre B. Araujo, Amy B. O’Donnell, Varant Kupelian and John B. McKinlay. Travison et al. 92 (1): 196. The Journal of Clinical Endocrinology & Metabolism January 1, 2007 vol. 92 no. 1 196-202.

[2] Secular decline in male testosterone and sex hormone binding globulin serum levels in Danish population surveys. Andersson AM, Jensen TK, Juul A, Petersen JH, Jørgensen T, Skakkebaek NE. J Clin Endocrinol Metab. 2007 Dec;92(12):4696-705. Epub 2007 Sep 25.

[3] Aromatization of androstenedione and 19-nortestosterone in human placental, liver, and adipose tissues (abstract). Nippon Naibunpi Gakkai Zasshi 62 (1986:18-25

[4] “Dangers of Excess Estrogen In the Aging Male”. Faloon, William. Life Extension Magazine, November 2008.

[5] Regulation of reproductive hormone secretion in primates by short-term changes in nutrition. JL Cameron. Departments of Psychiatry, Neuroscience, and Cell Biology and Physiology, University of Pittsburgh,

Pittsburgh, PA 15213, USA. May 1, 1996 1 117-126.

[6] Testosterone and cortisol in relationship to dietary nutrients and resistance exercise. Volek JS, Kraemer WJ, Bush JA, Incledon T, Boetes M. J Appl Physiol. 1997 Jan;82(1):49-54.

[7] Hormonal responses to high- and moderate-intensity strength exercise. Raastad T, Bjøro T, Hallén J. Eur J Appl Physiol. 2000 May;82(1-2):121-8.

[8] Acute hormonal responses in elite junior weightlifters. Kraemer WJ, Fry AC, Warren BJ, Stone MH, Fleck SJ, Kearney JT, Conroy BP, Maresh CM, Weseman CA, Triplett NT, et al. Int J Sports Med. 1992 Feb;13(2):103-9.

[9] Pulsatile patterns of gonadotropins and testosterone in man: the effects of clomiphene, with and without testosterone. Naftolin F, Judd HL, Yen SSC. J Clin Endocrinol Metab 1973;36:285–8.

[10] Evidence for a role of endogenous estrogen in the hypothalamic control of gonadotropin secretion in men. Winters SJ, Troen P. J Clin Endocrinol Metab 1985;61:842–5.

[11] Studies on the role of sex steroids in the feedback control of gonadotropin concentrations in men. II. Use of the estrogen antagonist, clomiphene citrate. Winters SJ, Janick JJ, Loriaux DL, Sherins RJ. J Clin Endocrinol Metab 1979;48:222–7.

[12] Short- and long-term effects of clomiphene citrate on the pituitary–testicular axis. Santen RJ, Leonard JM, Sherins RJ, Gandy HM, Paulsen CA. J Clin Endocrinol Metab 1971;33:970–6.

[13] Estrogens in the feedback regulation of gonadotropin secretion in men: effects of administration of estrogen to agonadal subjects and the antiestrogen tamoxifen and the aromatase inhibitor d1-testolactone to eugonadal subjects. Gooren LJ, Van der Veen EA, van Kessel H, Harmsen-Louman W.  Andrologia 1984;16:568–77.

[14] Estrogen suppression in males: metabolic effects. Mauras N; O’Brien KO; Klein KO; Hayes V. J Clin Endocrinol Metab 2000 Jul;85(7):2370-7   (ISSN: 0021-972X)

[15] Pharmacokinetics and dose finding of a potent aromatase inhibitor, aromasin (exemestane), in young males. Mauras N, Lima J, Patel D, Rini A, di Salle E, Kwok A, Lippe B. J Clin Endocrinol Metab. 2003 Dec;88(12):5951-6.

[16] “Effects of eight weeks of an alleged aromatase inhibiting nutritional supplement 6-OXO (androst-4-ene-3,6,17-trione) on serum hormone profiles and clinical safety markers in resistance-trained, eugonadal males”. Rohle D, Wilborn C, Taylor L, Mulligan C, Kreider R, Willoughby D. (2007). J Int Soc Sports Nutr. 4: 13. doi:10.1186/1550-2783-4-13. PMC 2100070. PMID 17949492.

[17] “The science of 6-OXO”. Muscle & Fitness. July, 2004. http://www.findarticles.com/p/articles/mi_m0801/is_7_65/ai_n6074719.

[18] Short term impact of Tribulus terrestris intake on doping control analysis of endogenous steroids. Saudan C, Baume N, Emery C, Strahm E, Saugy M. Forensic Sci Int. 2008 Jun 10;178(1):e7-10. doi: 10.1016/j.forsciint.2008.01.003. Epub 2008 Feb 20.

[19] The hormonal effects of Tribulus terrestris and its role in the management of male erectile dysfunction–an evaluation using primates, rabbit and rat. Gauthaman K, Ganesan AP. Phytomedicine. 2008 Jan;15(1-2):44-54.

[20] Free serum testosterone level in male rats treated with Tribulus alatus extracts. El-Tantawy WH, Temraz A, El-Gindi OD. Int Braz J Urol. 2007 Jul-Aug;33(4):554-8; discussion 558-9.

[21] The effect of five weeks of Tribulus terrestris supplementation on muscle strength and body composition during preseason training in elite rugby league players. Rogerson S, Riches CJ, Jennings C, Weatherby RP, Meir RA, Marshall-Gradisnik SM. J Strength Cond Res. 2007 May;21(2):348-53.

[22] Standardized quassinoid-rich Eurycoma longifolia extract improved spermatogenesis and fertility in male rats via the hypothalamic-pituitary-gonadal axis. Low BS, Das PK, Chan KL. J Ethnopharmacol. 2013 Feb 13;145(3):706-14. doi: 10.1016/j.jep.2012.11.013. Epub 2012 Dec 20.

[23] In vivo effects of Eurycoma longifolia Jack (Tongkat Ali) extract on reproductive functions in the rat. Solomon MC, Erasmus N, Henkel RR. Andrologia. 2013 Mar 6. doi: 10.1111/and.12082.

[24] Effects of Eurycoma longifolia on Testosterone Level and Bone Structure in an Aged Orchidectomised Rat Model. Tajul Ariff AS, Soelaiman IN, Pramanik J, Shuid AN. Evid Based Complement Alternat Med. 2012;2012:818072. doi: 10.1155/2012/818072. Epub 2012 Aug 26.

[25] Standardised water-soluble extract of Eurycoma longifolia, Tongkat ali, as testosterone booster for managing men with late-onset hypogonadism? Tambi MI, Imran MK, Henkel RR. Andrologia. 2012 May;44 Suppl 1:226-30. doi: 10.1111/j.1439-0272.2011.01168.x. Epub 2011 Jun 15.

[26] Effect of vitamin D supplementation on testosterone levels in men. Pilz S, Frisch S, Koertke H, Kuhn J, Dreier J, Obermayer-Pietsch B, Wehr E, Zittermann A. Horm Metab Res. 2011 Mar;43(3):223-5. doi: 10.1055/s-0030-1269854. Epub 2010 Dec 10.

[27] Association of vitamin D status with serum androgen levels in men. Wehr E, Pilz S, Boehm BO, März W, Obermayer-Pietsch B. Department of Internal Medicine, Division of Endocrinology and Nuclear Medicine, Medical University Graz, Graz, Austria. 2010 Aug;73(2):243-8

[28] Possible mechanism by which zinc protects the testicular function of rats exposed to cigarette smoke. Sankako MK, Garcia PC, Piffer RC, Dallaqua B, Damasceno DC, Pereira OC. Pharmacol Rep. 2012 Nov;64(6):1537-46.

[29] Effect of folic acid and zinc sulphate on endocrine parameters and seminal antioxidant level after varicocelectomy. Nematollahi-Mahani SN, Azizollahi GH, Baneshi MR, Safari Z, Azizollahi S. Andrologia. 2013 Jan 28. doi: 10.1111/and.12067.

[30] “Effects of Zinc Magnesium Aspartate (ZMA) Supplementation on Training Adaptations and Markers of Anabolism and Catabolism”. Wilborn, Colin D; Kerksick, Chad M; Campbell, Bill I; Taylor, Lem W; Marcello, Brandon M; Rasmussen, Christopher J; Greenwood, Mike C; Almada, Anthony et al. (2004). Journal of the International Society of Sports Nutrition 1 (2): 12–20. doi:10.1186/1550-2783-1-2-12. PMC 2129161. PMID 18500945.

[31] “Serum testosterone and urinary excretion of steroid hormone metabolites after administration of a high-dose zinc supplement”. Koehler, K; Parr, M K; Geyer, H; Mester, J; Schänzer, W (2007). European Journal of Clinical Nutrition 63 (1): 65–70. doi:10.1038/sj.ejcn.1602899. PMID 17882141.