The Importance of Testosterone
Gaining muscle mass and its mechanisms is a very complex system.
Most people involved in bodybuilding, weight training, and exercise in general want to understand what will maximize their muscle growth potential, how their workouts and exercises should be structured to provide this, and what ingredients in their nutrition and supplements can provide the desired edge.
To start or "trigger" the muscle growth process, you always need a training stimulus through exercise.
However, there is one problem: these factors are known as mediators.
Mediators essentially regulate the degree of muscle growth and the amount of muscle that can be reached as a maximum (or minimum).
Over the years, various mediators have been discovered in the human body, but the most famous and influential mediator involved in the muscle growth process is the hormone testosterone.
Testosterone and other anabolic steroids that are derivatives and analogs (altered forms of testosterone) all work in much the same way as testosterone itself in mediating the muscle growth process.
Testosterone, specifically known as an anabolic androgenic steroid hormone (AAS for short), acts on a variety of tissues in the human lower system, but it is its anabolic muscle-building properties that are of particular note and interest to bodybuilders and athletes.
Anabolic and androgenic mean different things, but the essential properties of testosterone and all anabolic steroids include both.
Anabolic effects are distinct from androgenic properties, but they cannot be completely separated: anabolic properties refer to tissue-building effects (in this case, muscle tissue), while androgenic properties refer to the hormone's masculinizing effects (i.e., development of male secondary sex characteristics, deepening of the voice, development of facial and body hair, maturation of the genitals, etc.
Testosterone is produced endogenously by Leydig cells in the testes, and is actually controlled through an endocrine axis known as the HPTA (hypothalamic-pituitary-testicular axis) via a series of signaling hormones that originate in the hypothalamus.
The hypothalamus signals the pituitary gland to release LH (luteinizing hormone), which in turn releases GnRH (gonadotropin-releasing hormone), which signals the Leydig cells in the testes to produce testosterone.
To use a testosterone booster is to maximize this process or take it beyond normal functioning.
Testosterone boosters exist in many forms, from synthetic drugs to herbs that occur naturally in plants found on the planet, to actual activities and lifestyle habits.
For example, the hypothalamus is a component of the brain, so a variety of mechanisms related to the brain can actually affect the increase or decrease of testosterone (both emotional and physical stress), including emotional states, energy, and all types of physical and emotional stress.
Testosterone levels in the body are regulated by HPTA through a mechanism known as a negative feedback loop: when the body detects excessive levels of testosterone circulating through the bloodstream, it reduces endogenous testosterone production.
The HPTA then adjusts its output signal accordingly when it senses an excessive (or insufficient) amount of testosterone.
This process of recognition and change is the basic explanation for the operation of a negative feedback loop.
Excessive estrogen levels affect this negative feedback loop in the same way.
Testosterone acts as a gatekeeper for muscle growth and strength development, and without adequate levels of testosterone, no amount of training or food intake will result in muscle growth.
The risks of exogenous testosterone use and why testosterone booster alternatives should be preferred
Since the early 1990s, the availability of testosterone boosters has skyrocketed, and the popularity and availability of testosterone boosters has increased proportionally.
There are many reasons for this dynamic shift in the supplement market.
First of all, the use of anabolic steroids, which almost always use exogenous testosterone and related hormones, is associated with several risks.
While most of these risks are not life-threatening, they can be uncomfortable enough to discourage individuals from thinking about using anabolic steroids, which is understandable.
Even for those who are confident enough to deal with these side effects and risks, the complexity of how to properly use anabolic steroids can still be overwhelming for some people.
While anabolic steroid use can be done very safely, it is very complex and requires a tremendous amount of research, knowledge, and know-how.
These requirements can be too much for many people to handle, and these individuals may want to find a way to maximize their testosterone levels without the complexities associated with anabolic steroid use.
Another reason to favor alternative testosterone boosters is the legal status of anabolic steroids in the United States.
While most countries on the planet have no or lax laws regarding anabolic steroids, there are some countries that have very strict and severe penalties for using, possessing, buying, and selling testosterone and anabolic steroids without a valid prescription.
In countries like South Korea, many may wish to avoid these legal repercussions associated with exogenous testosterone use and opt for a testosterone booster.
Legality is one of the main reasons why testosterone boosters have become so popular since the early 1990s.
With the enactment of the Anabolic Steroid Control Act of 1988, testosterone and all anabolic steroids were included in Schedule III of the Controlled Substances Act in 1990.
The supplement market then had to find ways to maximize testosterone levels legally, and the demand for anabolic steroids increased significantly as awareness of anabolic steroids grew and they were considered Class A banned substances.
Soon after, the supplement market exploded with testosterone boosters and legal steroid alternatives at a very fast pace.
In the supplement industry, the primary goal of supplement companies has always been to create the most complete anabolic environment within the human body.
Maximizing testosterone, the key hormone needed to create the perfect anabolic environment, has always been key.
There are many different types of testosterone boosters both within and outside of the supplement industry, and there are also other methods of boosting testosterone that may or may not involve ingesting substances.
Basically, the different categories of testosterone elevators can be summarized as follows:
1. training, nutrition, and lifestyle
2. drugs and synthetic compounds
3. herbal products
4. vitamins and minerals
In this article, we'll take a closer look at these testosterone boosters and how they work to boost testosterone.
Training, Nutrition, and Lifestyle as Testosterone Boosters
First and foremost, the most effective testosterone booster is a proper, healthy, and regular lifestyle. It's a well-known fact that the average individual is out of shape, leads a sedentary lifestyle, and has extremely poor nutrition.
It is not surprising then that studies[1] [2] have shown a continuous and steady decline in average testosterone levels in men over the past two decades.
While many point to external factors such as the environment and chemicals ingested during modern food processing, the decline in testosterone levels also directly correlates to the rise in sedentary men who do not engage in adequate exercise or proper nutritional habits.
While external factors such as the environment and chemicals undoubtedly play a role, the overwhelming cause seems to be the decline in proper healthy lifestyle habits.
A proper and balanced diet consisting of clean, wholesome foods with adequate calorie intake should normalize and minimize the amount of body fat, and it is a well-known fact that adipose tissue (fatty tissue) is responsible for increasing the amount of aromatization (the conversion of testosterone to estrogen) in the body.
Clinical studies have shown that the higher the body fat percentage, the higher the risk of estrogenic side effects due to increased levels of estrogen in the body [3].
This is because aromatase enzymes (the enzymes responsible for aromatization) are very abundant in adipose tissue (fat), so it stands to reason that the higher an individual's body fat percentage, the higher the rate at which androgens are aromatized into estrogen.
Increased estrogen levels then further decrease the body's natural endogenous testosterone production and compound the problem.
It's pretty clear that this is true when you consider that studies have shown that excess estrogen in men can suppress the secretion of endogenous testosterone, leading to hypogonadism [4].
In addition, poor diet and nutrition is a big contributor to low testosterone levels in individuals.
Extensive research has concluded that incorrect nutritional protocols, including fasting or insufficient calorie intake, significantly reduce testosterone levels [5].
This lends credence to the fact that the vast majority of Americans and individuals with poor nutritional habits today are actually consuming far fewer calories than they need for normal maintenance, and when they do eat, they are eating the wrong, unhealthy foods that further interfere with the body's proper endocrine production.
This is also why starvation diets or extremely low-calorie diets should not be practiced, as they cause testosterone levels to drop dramatically.
In addition to nutrition, it is a very well-documented and well-known fact that training and exercise, especially the intensity of training and exercise, acts as a very impressive testosterone booster.
A study conducted in 1997 investigated the effects of resistance exercise (weight training) on testosterone levels by having 12 men perform squats, bench presses, and jump squats of varying intensities [6].
The results showed that men who performed jump squats increased their testosterone levels to levels significantly higher than baseline (15% increase) than men who performed bench presses (7% increase).
It's clear from here that performing compound exercises, which tend to recruit the maximum amount of muscle fibers for a lift, leads to the greatest increase in the body's endogenous testosterone.
This also reinforces the idea that stronger compound exercises (deadlifts, squats, bench presses, etc.) should be the main focus of any resistance training routine, as these larger compound exercises produce greater testosterone increases than isolation exercises.
The intensity of resistance exercise is also a factor in determining the amount of endogenous testosterone.
As exercise intensity increases, testosterone levels in response to exercise stimuli also increase.
A 2000 study corroborated this effect in nine male strength athletes, who performed back squats, front squats, and leg extensions [7].
In this study, each subject performed the exercises at 100% intensity, followed by 70% intensity, defined as training to failure and stopping after a certain number of reps without failure (or working with less weight), respectively.
Subjects who performed six reps at 100% intensity showed significantly greater increases in testosterone compared to those who performed six reps at 70% intensity.
In conclusion, performing resistance training near failure does not increase testosterone as significantly as training to 100% failure, and there are several reasons for this difference.
Not only does resistance exercise stimulate an immediate increase in testosterone, but there is a long-term increase in testosterone as a result of weight training.
In one study of 28 Olympic junior elite weightlifters, not only did testosterone increase with more training experience, but the testosterone increases experienced by athletes with more than two years of training experience were significantly greater than those with less training experience [8].
This provides a significant amount of evidence to support the idea that exercise not only acts to increase testosterone in the short term, but also induces a greater amount of testosterone increase the longer you continue to train.
Drugs and synthetic compounds as testosterone boosters
Science and medicine have made great strides in developing synthetic compounds that can be effectively utilized as test boosters to provide effective treatment for people experiencing low testosterone (hypogonadism and male menopause). There are two main classes of compounds:
- SERMs (selective estrogen receptor modulators)
- AIs (aromatase inhibitors)
Some of these synthetic compounds can be purchased legally over-the-counter (OTC) in supplement stores, while others are only available by prescription.
Many of these compounds vary in strength, regardless of whether they are prescription or OTC.
For example, there are very strong aromatase inhibitors, such as Femara (Letrozole), while there are also weaker aromatase inhibitors, such as 6-OXO(4-AT), which can be purchased over-the-counter in supplement stores.
SERM
A synthetic compound that is typically non-steroidal and is instead a triphenylethylene compound.
They include products such as Nolvadex (tamoxifen), Clomid (clomiphene), Fareston (toremifene), and Evista (raloxifene).
All of these work by blocking estrogen at its objective receptor sites in a variety of specific tissues (especially breast tissue).
The term used to describe the estrogen blocking action of these compounds, where they bind to estrogen's target receptor and either push estrogen out of the receptor site or prevent estrogen from binding to the target receptor site, is called estrogen antagonism.
This is called estrogen antagonism.
In other tissues, such as liver tissue, it acts as an estrogen agonist, exerting its own estrogenic effects at the site.
Some of the positive effects seen with Nolvadex (tamoxifen), for example, the positive cholesterol changes seen when it acts as an estrogen agonist in the liver, can be attributed to this effect.
Specifically, to act as testosterone boosters, many of these SERMs act as estrogen antagonists, albeit to varying degrees, at receptors located on the pituitary gland, which stimulates gonadotropin (LH and FSH release) and ultimately leads to an increase in testosterone levels.
In fact, SERMs are often used in the medical community as a treatment for HPTA recovery due to their estrogen antagonism on the pituitary gland, which results in a testosterone boosting effect through a process that stimulates Leydig cells in the testes [9] [10] [11] [12] [13].
AI
Aromatase inhibitors are synthetic compounds, some of which may be steroidal in nature and some of which may not.
These include compounds such as Aromasin (Exemestane), Arimidex (Anastrozole), Femara (Letrozole), and Proviron (Mesterolone), as well as many others that are available over-the-counter and prescription.
Each of these compounds essentially blocks aromatization, which is the cause of elevated estrogen levels.
As mentioned earlier in this article, aromatization is a metabolic process in which testosterone (or any androgen that can undergo this process) is converted to estrogen through contact with the aromatase enzyme.
The aromatase enzyme is the sole cause of aromatization, and this enzyme is inhibited (or neutralized) by the aromatase inhibitors mentioned earlier.
Some of these inhibitors only temporarily disable the aromatase enzyme, such as Arimidex or Letrozole, while others permanently inhibit the enzyme, such as aromasin or 6-OXO(4-AT), known as suicide inhibitors.
Many of these aromatase inhibitors range in strength from very mild, such as Proviron, to very strong, such as Letrozole.
In terms of testosterone boosting effects, AIs decrease total plasma levels of estrogen, which modulates and stimulates the negative feedback loop of HPTA.
As mentioned earlier in this article, when excess estrogen is detected in the bloodstream, the hypothalamus responds by signaling a decrease in testosterone production through a negative feedback loop, but when estrogen levels are abnormally lower than standard physiological levels, HPTAs have an antagonistic effect that produces higher levels of testosterone.
Studies have demonstrated the ability of these compounds to increase testosterone, with some aromatase inhibitors increasing serum testosterone levels in subjects by 58% from baseline [14], while other studies of other aromatase inhibitors have shown increases of over 60% [15], some by 84% [16], and even 90% [17].
Herbal Products as Testosterone Boosters
The most common and popular types of testosterone boosters on the market are herbal products, the most common and popular of which include
Tribulus Terrestris, Avena Sativa, Eurcoma Longifolia (LJ100), Tongkat Ali, and many others.
The opinions and reviews of these herbs and plant extracts that claim to be able to boost testosterone production in men are very mixed.
Many supplement companies may sell supplements that contain proprietary blends of ingredients that include one, several, or all of the aforementioned herbs.
These herbal ingredients can also be purchased individually at a much lower cost than the name-brand proprietary blends that are commonly found and sold in supplement stores.
The problem with these herbal ingredients is that while there are quite a few studies on rats, mice, and primates, there are few or no studies on humans.
While the data is very reliable, it remains to be seen if these products can produce the same type of testosterone-boosting effects in humans.
Tribulus is the most popular product and the most studied, but opinions are still mixed on its effectiveness as a testosterone booster.
Tribulus has many other benefits in the human body, but for the purposes of this article, we'll focus on its testosterone-boosting properties.
Tribulus is a vine that is grown in moderate temperatures and tropical environments and is believed to contribute to healthy testosterone levels by stimulating the pituitary gland to release LH and FSH, the gonadotropins that signal the Leydig cells in the testes to produce testosterone.
Tribulus is actually well known for its use by athletes as an alternative to anabolic steroids, and one study found no effect on endogenous testosterone two days later through urine analysis of two athletes who took 500 mg of tribulus three times daily [18].
However, in another study involving primates, rabbits, and rats, androgen levels actually increased in primates but were not statistically significant, while in rats the increase was statistically significant, and the same was true for rabbits, which had a significant increase in androgen levels compared to controls [19].
Another study in male rats also concluded that rats respond very positively to testosterone increases when given tribulus [20].
The fact that primates are a species more closely related to humans than rats or rabbits may lend further credence to the idea that tribulus doesn't work as well in humans as it does in other species.
Furthermore, a study conducted in humans, elite rugby players, concluded that while there was a significant increase in muscle mass over a five-week training period, there was no significant difference between the control and tribulus treatment groups, and also concluded that tribulus did not act to alter urinary testosterone levels, nor did it induce the increases in strength or lean body mass commonly claimed by manufacturers [21].
The definitive conclusion about tribulus is that it works very weakly or not at all as a testosterone booster in humans and is not worth spending a lot of money on at the supplement store.
Tongkat Ali is probably the most popular herbal testosterone booster after tribulus, and is often combined with tribulus in many testosterone booster products.
Like tribulus, clinical data on tongkat ali is abundant in the form of studies demonstrating testosterone boosting effects via HPTA stimulation in rats and mice [22] [23] [24].
However, as with tribulus, human studies are lacking, but the studies that have been conducted on tongkat ali activity in humans have actually shown at least some form of positive improvement over tribulus.
For example, one study investigated the use of tongkat ali as a test booster in men with late-stage hypogonadism (commonly referred to as andropause), which concluded that after one month of administration of 200 mg water-soluble tongkat ali extract, subjects who previously had lower-than-normal testosterone levels showed normal levels after treatment [25].
However, it is important to note that this study did not demonstrate the effectiveness of tongkat ali in raising testosterone levels above baseline or approaching the supraphysiological levels required for individuals to experience significant body composition and strength changes.
The bottom line on herbal-based testosterone boosters is that they are speculative at best, and there isn't enough research to definitively conclude that they provide significant testosterone boosting effects that are comparable to supraphysiological levels of testosterone.
While many animal studies have demonstrated this, most of the human studies that have been done have not.
Vitamins and Minerals as Testosterone Boosters
The two main items we'll look at here are vitamin D (cholecalciferol) and ZMA, which stands for zinc and magnesium aspartate, which were once claimed to boost testosterone levels to very high levels.
There is a lot of solid research evidence that high doses of vitamin D (cholecalciferol) have a significant effect on increasing testosterone levels in men, and that it also has a significant effect on suppressing SHBG levels in the body.
There are many clinical studies that demonstrate that low testosterone levels lead to a decrease in endogenous testosterone production (especially during the winter months for obvious reasons).
In one study conducted in Austria with nearly 200 subjects, divided into a group receiving 3332 IU of vitamin D daily and a placebo group, men with sufficient vitamin D levels had significantly higher testosterone levels and significantly lower SHBG levels compared to vitamin D-deficient subjects [26]. Androgen levels and vitamin D levels in men are correlated and show consistent seasonal variation [27].
Several other studies have reported similar results, with subjects who received higher doses of vitamin D over time showing significant increases in total testosterone levels and decreases in SHBG.
Anecdotal evidence from people who supplement with vitamin D and have regular blood tests done by their doctors suggests that they see a significant increase in total testosterone levels and free testosterone levels after about 1-2 months of vitamin D supplementation.
Developed by Victor Conte, founder of the Bay Area Laboratory Cooperative (BALCO), ZMA is a blend of zinc monomethionine and three other ingredients - zinc aspartate (30 mg), magnesium aspartate (450 mg), and vitamin B6 in the form of pyridoxine hydrochloride (10.5 mg) - that was initially claimed to act as a special testosterone booster and could equally efficiently increase strength levels in athletes.
While all three minerals are very important in biological processes, and zinc in particular has been proven to be a fertility aid and gonadal antioxidant/protector[28][29], none of them in the specific combination advertised have ever been reported to work as claimed for ZMA.
Initially, a study was conducted that claimed ZMA increased muscle strength in soccer players, but it was later revealed that the study was poorly conducted and that one of the study's authors, Victor Conte, had an ownership stake in the company that funded and ran the study.
Further studies have since been conducted on ZMA and have proven that it does not affect androgen levels in the body at all, nor does it increase strength or endurance [30] [31].
Medical references:
[1] Population-level declines in serum testosterone levels in US 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. 1196-202.
[2] Persistent decline in male testosterone and sex hormone binding globulin serum levels in a Danish population survey. Andersson AM, Jensen TK, Juul A, Petersen JH, Jørgensen T, Skakkebaek NE. J Clin Endocrinol Metab. 2007 Dec;92(12):4696-705. Epub September 25, 2007.
[3] Aromatization of androstenedione and 19-nortestosterone in human placenta, liver, and adipose tissue (abstract). Nippon Naibunpi Gakkai Zasshi 62 (1986:18-25)
[4] "The dangers of excess estrogen in aging men". Faloon, William. Life Extension Magazine, November 2008.
[5] Modulation of reproductive hormone secretion in primates by short-term nutritional changes. JL Cameron. University of Pittsburgh, Departments of Psychiatry, Neuroscience, Cell Biology and Physiology,
Pittsburgh, PA 15213, USA. May 1, 1996 117-126.
[6] Testosterone and cortisol in relation 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 training. 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] Rhythmic patterns of gonadotropins and testosterone in men: Effect of clomiphene in the presence and absence of testosterone. Naftolin F, Judd HL, N SSC. J Clin Endocrinol Metab 1973;36:285-8.
[10] Evidence for a role for endogenous estrogen in the hypothalamic regulation 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 regulation of gonadotropin concentration in men. II. the use of clomiphene citrate, an estrogen antagonist. 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] Estrogen in the feedback regulation of gonadotropin secretion in men: Effects of estrogen administration to hypogonadal subjects and the anti-estrogen tamoxifen and the aromatase inhibitor D1-testolactone to gonadal subjects. Gooren LJ, Van der Veen EA, van Kessel H, Harmsen-Louman W. Andrologia 1984;16:568-77.
[14] Estrogen suppression in men: 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 aromasin (Exemestane), a potent aromatase inhibitor, in young men. 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] "Eight-week effects of the nutritional supplement 6-OXO (androst-4-ene-3,6,17-trione) aromatase, a purported inhibitor of aromatase, on serum hormonal profiles and clinical safety indicators in resistance-trained eugonadal men". 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 effects of Tribulus terrestris ingestion on doping control assays of endogenous steroids. Saudan C, Baume N, Emery C, Strahm E, Saugy M. Forensic Science Int. 2008 Jun 10;178(1):e7-10. doi: 10.1016/j.forsciint.2008.01.003. Epub February 20, 2008.
[19] Hormonal effects of Tribulus terrestris and its role in the management of male erectile dysfunction - an evaluation using primates, rabbits and rats. Gauthaman K, Ganesan AP. Phytomedicine. 2008 Jan;15(1-2):44-54.
[20] Free serum testosterone levels in male rats treated with Tribulus alatus extract. El-Tantawy WH, Temraz A, El-Gindi OD. Int Braz J Urol. 2007 July-Aug;33(4):554-8; Discussion 558-9.
[21] Effects of 5 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 Condition 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 December 20.
In vivo effects of Eurycoma longifolia Jack (Tongkat Ali) extract on reproductive function in rats. Solomon MC, Erasmus N, Henkel RR. Andrologia. March 6, 2013 doi: 10.1111/and.12082.
[24] Effects of Eurycoma longifolia on testosterone levels and bone structure in an aged orchiectomy rat model. Tajul Ariff AS, Soelaiman IN, Pramanik J, Shuid AN. Evid-based supplementation Alternat Med. 2012;2012:818072. doi:10.1155/2012/818072. Epub August 26, 2012.
[25] A standardized water-soluble extract of Eurycoma longifolia, Tongkat ali, as a testosterone booster for the management of men with hypogonadism? Tambi MI, Imran MK, Henkel RR. Andrologia. May 2012;44 Suppl 1:226-30. doi: 10.1111/j.1439-0272.2011.01168.x. Epub 15 June 2011.
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. Mar 2011;43(3):223-5. doi:10.1055/s-0030-1269854. Epub December 10, 2010.
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, Endocrinology and Nuclear Medicine, Medical University of Graz, Graz, Austria. Aug 2010;73(2):243-8
A possible mechanism by which zinc protects testicular function in 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] Effects of folic acid and zinc sulfate on endocrine parameters and seminal antioxidant levels after varicocelectomy. Nematollahi-Mahani SN, Azizollahi GH, Baneshi MR, Safari Z, Azizollahi S. Andrologia. January 28, 2013. 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). International Journal 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 high-dose zinc supplementation”. Köhler, 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.
