Introduction

Testosterone is a critical hormone that plays a key role in muscle growth, fat loss, and overall health. Many factors influence testosterone levels, including age, genetics, lifestyle choices, and environmental factors. All of this, I broke down in my last article on the site called, “How Your Testosterone Levels Influence Muscle Growth”. So if you have no clue what testosterone really is and what it does for your body composition, you may want to start there first. However, I’ll still touch on a few of those things here…

Now, the primary focus of this article will be helping you boost your own natural testosterone levels, to help optimize your body composition and progress from the gym! So get ready, because we are going to dive deep into the science behind testosterone and its relationship with fitness, exercise, nutrition, and supplements, so that you can fully understand it’s role in your fitness journey. Then I’ll show you HOW to boost your own testosterone levels, naturally through exercise, diet, sleep, supplements, and stress management!

What Are The Basic Functions and Health Effects Of Testosterone? 

Testosterone is a vital hormone produced primarily in the testes in men and, to a lesser extent, in the ovaries in women. It plays a crucial role in various physiological processes throughout the body. Testosterone is responsible for the development of secondary sexual characteristics during puberty, such as facial and body hair growth, deepening of the voice, and increased muscle mass. Additionally, it influences bone density, fat distribution, and red blood cell production. Testosterone is also essential for maintaining libido, mood, cognitive function, and overall energy levels. 

Testosterone production is regulated by a complex feedback loop known as the hypothalamic-pituitary-gonadal (HPG) axis, which involves the hypothalamus, pituitary gland, and gonads (testes in men and ovaries in women). This process begins when the hypothalamus releases gonadotropin-releasing hormone (GnRH), which then signals the pituitary gland to produce and release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH is responsible for stimulating the Leydig cells in the testes or the theca cells in the ovaries to produce testosterone. As testosterone levels rise, the hormone exerts negative feedback on both the hypothalamus and pituitary gland, causing a decrease in GnRH and LH secretion. This self-regulating mechanism ensures that testosterone levels remain within a healthy range, maintaining balance and preventing excessive or insufficient hormone production.

When testosterone levels are too too low, it can lead to a range of health issues, including reduced muscle mass, increased body fat, mood disturbances, and decreased sexual function. On the other hand, excessively high testosterone levels can contribute to increased risk of cardiovascular disease and occasionally prostate enlargement in men (Dobs et al., 2013; Xu et al., 2005). 

What Happens To Testosterone Levels As You Age? 

As we go through different stages of life, hormone levels fluctuate, with testosterone being no exception. In children, testosterone concentrations remain relatively low until they hit puberty. During this phase of rapid growth and development, boys experience a significant increase in testosterone levels, while girls see a more modest rise (Bhasin et al., 2018).

As we age beyond ~50 years, our testosterone levels begin to decline gradually. Men, in particular, face a 1-3% decrease in circulating testosterone concentration per year (1.6% in total and 2-3% in bioavailable testosterone) (Harman et al., 2001). This decline can ultimately lead to very low resting concentrations of circulating testosterone, a condition often referred to as andropause (Lunenfeld & Gooren, 2002). Similarly, women experience a gradual decline in circulating testosterone concentrations until they reach menopause. Following menopause, there is a striking 60% reduction in testosterone levels within 2-5 years (Davison et al., 2005). Ultimately, this decrease in testosterone production can contribute to various age-related changes, such as reduced muscle mass, increased body fat, decreased bone density, and a decline in cognitive function, making it essential to monitor and address hormone imbalances as we grow older.

Does Exercise Boost Testosterone Levels?

Testosterone plays a vital role in the development and maintenance of muscle mass, bone density, and overall physical performance. The relationship between testosterone and exercise has been widely studied, revealing that engaging in regular physical activity can have a significant impact on testosterone levels. 

Resistance training, in particular, has been shown to be an effective stimulus for increasing testosterone levels. Research by Ahtiainen et al. (2015) demonstrated that men who engaged in progressive resistance training for 21 weeks experienced significant increases in both total and free testosterone levels compared to a control group. The study also found that the hypertrophic response (i.e., muscle growth) was closely related to the increase in testosterone levels, highlighting the importance of testosterone in muscle development.

High-intensity interval training (HIIT), characterized by short bursts of intense exercise followed by brief recovery periods, has also been associated with increased testosterone levels. A study by Kraemer et al. (2017) found that participants who performed HIIT workouts for six weeks experienced significant increases in total testosterone levels. These findings suggest that incorporating HIIT into a fitness routine may help improve hormonal balance and overall physical performance.

The duration and intensity of exercise also play a role in influencing testosterone levels. A study by Vingren et al. (2010) found that performing resistance training at higher intensities (i.e., using heavier weights and fewer repetitions) led to greater increases in testosterone levels compared to training at lower intensities (i.e., using lighter weights and more repetitions). Additionally, research has shown that exercising for longer durations can lead to a decrease in testosterone levels, as the body’s energy stores become depleted, and cortisol levels rise (Hill et al., 2008). Therefore, it is essential to strike a balance between exercise intensity and duration to maximize the benefits of training on testosterone levels.

Another critical factor to consider when examining the relationship between testosterone and exercise is the timing of hormone measurement. Testosterone levels tend to fluctuate throughout the day, with the highest levels typically observed in the morning and a gradual decline over the course of the day (Brambilla et al., 2009). Research has shown that acute bouts of exercise can lead to a transient increase in testosterone levels; however, these levels typically return to baseline within a few hours (Cook et al., 1992). Therefore, it is important to consider the timing of exercise and hormone measurement when evaluating the impact of physical activity on testosterone levels.

What Foods Boost Testosterone Levels?

Maintaining optimal testosterone levels is essential for overall health, and diet plays a significant role in supporting hormone production. Several key foods have been reported to boost testosterone levels naturally by providing essential nutrients required for testosterone synthesis. Here, we will discuss some of these foods and the scientific evidence supporting their role in promoting testosterone production. 

1. Cruciferous vegetables: Cruciferous vegetables, such as broccoli, cauliflower, cabbage, and Brussels sprouts, are rich in indole-3-carbinol (I3C), a compound that has been shown to have a positive effect on testosterone levels. I3C helps to reduce the activity of an enzyme called aromatase, which converts testosterone into estrogen. By inhibiting aromatase, I3C may help maintain higher levels of circulating testosterone (Michnovicz et al., 1997).
2. Oysters: Oysters are an excellent source of zinc, an essential mineral required for testosterone production. Studies have shown that zinc deficiency can lead to decreased testosterone levels, while zinc supplementation has been shown to increase testosterone levels in men with low zinc status (Prasad et al., 1996).
3. Fatty fish: Omega-3 fatty acids, found in fatty fish such as salmon, mackerel, and sardines, have been linked to improved testosterone levels. A study conducted by Pischon et al. (2008) found that men with higher levels of omega-3 fatty acids in their blood had higher testosterone levels than those with lower levels. These fatty acids can help reduce inflammation and improve blood flow, which can contribute to better overall health and hormonal balance.
4. Extra virgin olive oil: Extra virgin olive oil is a healthy monounsaturated fat known for its numerous health benefits, including supporting healthy testosterone levels. A study conducted by Derouiche et al. (2013) found that consuming extra virgin olive oil led to an increase in testosterone levels in healthy young men.
5. Pomegranate: Pomegranate is a powerful antioxidant-rich fruit that has been shown to boost testosterone levels. A study conducted by Al-Dujaili et al. (2012) found that daily consumption of pomegranate juice for two weeks increased salivary testosterone levels by an average of 24% in both men and women.
6. Brazil nuts: Brazil nuts are an excellent source of selenium, a trace mineral that plays a vital role in testosterone production. A study conducted by Hawkes & Hornbostel (1996) found that selenium supplementation increased testosterone levels in men with low baseline levels of the mineral.
7. Garlic: Garlic contains a compound called diallyl disulfide, which has been shown to stimulate the release of luteinizing hormone (LH), a hormone that signals the testes to produce testosterone (Ouattara et al., 2010).

In summary, incorporating these key foods into a well-balanced diet may help support healthy testosterone production by providing essential nutrients and promoting hormonal balance. However, not all of the studies are casual – some of them are correlational – meaning that we can’t just eat these foods and magically improve testosterone levels. It’s important to remember that a well-rounded, nutrient-dense diet, combined with regular exercise, adequate sleep, and stress management, is crucial for maintaining optimal hormone levels and overall health.

How Does Sleep Impact Testosterone Levels?

Sleep plays an important role in maintaining optimal testosterone levels, which in turn influence overall health, muscle growth, fat loss, and cognitive function. The significance of adequate sleep cannot be overstated when it comes to regulating testosterone levels and ensuring hormonal balance.

Research has consistently shown that sleep deprivation has a negative impact on testosterone levels. One study conducted by Leproult and Van Cauter (2011) found that men who slept less than five hours per night for one week experienced a significant drop in testosterone levels compared to when they had a full night’s sleep. The study demonstrated that testosterone levels decreased by 10-15% with sleep restriction, emphasizing the importance of getting sufficient sleep to maintain healthy testosterone levels.

The decline in testosterone levels due to sleep deprivation has been attributed to the disruption of the normal sleep cycle. Testosterone production follows a circadian rhythm, with levels peaking during sleep, particularly during the rapid eye movement (REM) stage (Axelsson et al., 2005). In addition to its direct impact on testosterone production, sleep deprivation can also indirectly affect testosterone levels by increasing cortisol, a stress hormone that has been shown to suppress testosterone synthesis. A study by Leproult et al. (1997) found that sleep restriction resulted in elevated cortisol levels the following evening, which can have a negative impact on testosterone production.

Sleep quality is another important factor to consider when examining the relationship between sleep and testosterone levels. A study by Andersen et al. (2011) found that fragmented sleep, characterized by frequent awakenings and arousals, was associated with lower testosterone levels in men. This suggests that not only the duration but also the quality of sleep is essential for maintaining optimal testosterone levels.

To ensure adequate testosterone production, it is recommended that adults aim for seven to nine hours of sleep per night (Hirshkowitz et al., 2015). Adopting healthy sleep habits, such as maintaining a consistent sleep schedule, creating a sleep-friendly environment, and avoiding stimulants like caffeine and electronic devices before bedtime, can help improve sleep quality and duration, ultimately supporting healthy testosterone levels.

Does Stress Decrease Testosterone Levels? (Cortisol and Testosterone)

Cortisol and testosterone are two essential hormones that play significant roles in the human body. Cortisol, often referred to as the “stress hormone,” is produced by the adrenal glands and helps regulate the body’s response to stress. It also plays a vital role in various physiological processes, including immune function, glucose metabolism, and blood pressure regulation. Although these hormones have different functions, they are interconnected, and their relationship can impact overall health and hormonal balance.

Several studies have demonstrated an inverse relationship between cortisol and testosterone levels, meaning that as cortisol levels rise, testosterone levels tend to decrease, and vice versa. This negative relationship has been observed in various contexts, including during acute stress, chronic stress, and sleep deprivation (Sapolsky, 1986; Leproult et al., 1997). The underlying mechanisms behind this relationship involve the hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-pituitary-gonadal (HPG) axis, which regulate the production of cortisol and testosterone, respectively. Both axes are sensitive to stress, and when stress levels increase, the HPA axis is activated, leading to increased cortisol production. This activation can subsequently suppress the HPG axis, resulting in reduced testosterone synthesis (Rivier & Rivest, 1991).

Prolonged exposure to elevated cortisol levels due to chronic stress can have detrimental effects on testosterone production and overall health. A study by Cumming et al. (1983) found that men with higher cortisol levels had lower testosterone levels, which in turn were associated with reduced muscle mass, increased body fat, and decreased bone density. Additionally, high cortisol levels have been linked to mood disturbances, reduced cognitive function, and increased risk of cardiovascular disease, highlighting the importance of managing stress to maintain hormonal balance and overall health (Chida & Steptoe, 2009).

Several strategies can help mitigate the impact of cortisol on testosterone levels and promote hormonal balance. Regular exercise is one such approach, as it has been shown to both reduce cortisol levels and boost testosterone production (Hill et al., 2008). A well-rounded exercise program, including resistance training, aerobic exercise, and a well-rounded diet, can help manage stress and maintain optimal hormone levels. Additionally, adopting stress management techniques, such as mindfulness meditation, yoga, or deep breathing exercises, may help reduce cortisol levels and support healthy testosterone production (Pascoe et al., 2017).

Do Testosterone Boosters (Supplements) Actually Work?

The market for testosterone-boosting supplements has grown rapidly in recent years, fueled by the increasing awareness of the importance of testosterone for overall health and fitness. While some of these products have been supported by scientific research, others lack evidence for their efficacy. Below I broke down the top 5 supplements often said to increase testosterone.

1. D-Aspartic Acid (D-AA): D-AA is an amino acid that has been claimed to boost testosterone by stimulating the release of luteinizing hormone (LH), which in turn signals the testes to produce more testosterone. However, research has found inconsistent results regarding the effects of D-AA on testosterone levels, with some studies showing no significant increase in testosterone levels (Topo et al., 2009; Melville et al., 2017). Furthermore, one study found that long-term supplementation with D-AA may actually lead to a decrease in testosterone levels (Willoughby et al., 2013).
2. Tribulus Terrestris: Tribulus Terrestris is an herbal supplement that has been marketed as a testosterone booster. However, research has shown that it does not have a significant effect on testosterone levels in humans (Rogerson et al., 2007; Neychev and Mitev, 2016).
3. Fenugreek: Fenugreek is an herb that has been claimed to boost testosterone levels by blocking the conversion of testosterone into estrogen. However, studies have found mixed results, with some studies showing an increase in testosterone levels (Steels et al., 2011) and others showing no significant effect on total testosterone (Wankhede et al., 2016).
4. Zinc: Zinc is a mineral that is necessary for testosterone production, and some studies have found that supplementation with zinc can increase testosterone levels in deficient individuals (Prasad et al., 1996; Netter et al., 2016). However, in individuals with normal zinc levels, there is no evidence that supplementation with zinc will lead to an increase in testosterone levels (Kilic et al., 2006; Osterberg et al., 2010).
5. DHEA: DHEA is a hormone that is produced by the adrenal glands and has been marketed as a testosterone booster. However, research has shown that supplementation with DHEA does not lead to a significant increase in testosterone levels in healthy individuals (Brown et al., 2002; Welle et al., 2001).

Overall, while some supplements have been marketed as testosterone boosters, the evidence supporting their efficacy is often weak or inconsistent. The International Society of Sports Nutrition has a list of supplements and all of the above show little to limited evidence, so it’s likely not worth taking any of them to boost testosterone. Another great website to check for supplement science is Examine.

How To Monitor Your Testosterone Levels

The most common method for assessing testosterone levels is through a blood test, also known as serum testosterone testing. This test measures the total testosterone in the bloodstream, which includes both free testosterone (the biologically active form) and testosterone bound to proteins such as sex hormone-binding globulin (SHBG) and albumin. 

Total testosterone levels can provide a useful overview of an individual’s hormonal status (Bhasin et al., 2018). However, some experts argue that measuring free testosterone may be more informative, as it reflects the portion of testosterone that is readily available to exert its biological effects (Rosner et al., 2007). Free testosterone can be measured directly through equilibrium dialysis or calculated using equations that incorporate total testosterone, SHBG, and albumin levels (Vermeulen et al., 1999).

Salivary testosterone testing is another method used to assess free testosterone levels. This non-invasive test involves collecting a saliva sample, which is then analyzed for testosterone concentration. Salivary testosterone has been shown to correlate well with serum free testosterone levels, making it a viable alternative for individuals who prefer to avoid blood tests (Wang et al., 1981; Taieb et al., 2003). However, it is important to note that salivary testosterone levels can be influenced by factors such as salivary flow rate and oral health, which may affect the accuracy of the test (Celec et al., 2015).

Urine testing for testosterone is another method, but it is less commonly used due to its relatively low sensitivity and potential for contamination. This test measures the urinary excretion of testosterone and its metabolites, which can provide information about the body’s overall production and metabolism of the hormone (Cawood et al., 2009). Urine testing may be more appropriate for specific clinical situations, such as monitoring individuals on testosterone replacement therapy or detecting the use of anabolic steroids in sports.

When interpreting testosterone test results, it is crucial to consider factors that may influence hormone levels, such as age, time of day, and recent physical activity. Testosterone levels typically peak in the early morning and decline throughout the day, making it essential to account for diurnal variations when comparing test results (Brambilla et al., 2009). Additionally, acute exercise has been shown to transiently increase testosterone levels, which may affect test results if blood or saliva samples are collected shortly after physical activity (Vingren et al., 2010).

In conclusion, monitoring testosterone levels can provide valuable insights into an individual’s hormonal status and help guide decisions related to lifestyle modifications, supplementation, and potential medical interventions. Blood tests, saliva tests, and urine tests are all available methods for assessing testosterone levels, each with its advantages and disadvantages. When interpreting test results, it is essential to consider factors that may influence hormone levels, such as age, time of day, and recent physical activity. 

Summary

Optimizing testosterone levels can have many benefits for both men and women, including improved muscle mass, bone density, mood, and libido. While there are many supplements on the market that claim to boost testosterone levels, the scientific evidence for most of these is weak or non-existent. Instead, lifestyle factors such as regular exercise, weight management, sleep quantity and quality, as well as stress reduction have been shown to have a significant impact on testosterone levels. In addition, ensuring a well-rounded diet with adequate intake of key nutrients such as vitamin D and zinc can also support healthy testosterone levels. It’s important to remember that testosterone levels can vary widely among individuals, and what may be considered normal for one person may not be the same for another. Finally, testosterone levels generally decrease after age 50, so monitoring them over the long-term may be beneficial to ensure they remain within a healthy range.

References

1. Bhasin, S., Brito, J. P., Cunningham, G. R., Hayes, F. J., Hodis, H. N., Matsumoto, A. M., … & Yialamas, M. A. (2018). Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism, 103(5), 1715-1744.
2. Davison, S. L., Bell, R., Donath, S., Montalto, J. G., & Davis, S. R. (2005). Androgen levels in adult females: changes with age, menopause, and oophorectomy. The Journal of Clinical Endocrinology & Metabolism, 90(7), 3847-3853.
3. Harman, S. M., Metter, E. J., Tobin, J. D., Pearson, J., & Blackman, M. R. (2001). Longitudinal effects of aging on serum total and free testosterone levels in healthy men. The Journal of Clinical Endocrinology & Metabolism, 86(2), 724-731.
4. Lunenfeld, B., & Gooren, L. (2002). Andropause: is androgen replacement therapy indicated for the aging male? An International Journal of Medicine, 95(3), 579-597.
5. Bhasin, S., Cunningham, G. R., Hayes, F. J., Matsumoto, A. M., Snyder, P. J., Swerdloff, R. S., & Montori, V. M. (2010). Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism, 95(6), 2536-2559.
6. Dobs, A. S., Meikle, A. W., Arver, S., Sanders, S. W., Caramelli, K. E., & Mazer, N. A. (2013). Pharmacokinetics, efficacy, and safety of a permeation-enhanced testosterone transdermal system in comparison with bi-weekly injections of testosterone enanthate for the treatment of hypogonadal men. The Journal of Clinical Endocrinology & Metabolism, 84(10), 3469-3478.
7. Fui, M. N., Dupuis, P., & Grossmann, M. (2014). Lowered testosterone in male obesity: mechanisms, morbidity, and management. Asian Journal of Andrology, 16(2), 223-231.
8. Morgentaler, A. (2006). Testosterone and prostate cancer: an historical perspective on a modern myth. European Urology, 50(5), 935-939.
9. Xu, L., Freeman, G., Cowling, B. J., & Schooling, C. M. (2013). Testosterone therapy and cardiovascular events among men: a systematic review and meta-analysis of placebo-controlled randomized trials. BMC Medicine, 11(1), 108.
10. Andersen, M. L., Alvarenga, T. F., Mazaro-Costa, R., Hachul, H. C., & Tufik, S. (2011). The association of testosterone, sleep, and sexual function in men and women. Sleep Medicine Reviews, 15(5), 323-331.
11. Axelsson, J., Ingre, M., Akerstedt, T., & Holmbäck, U. (2005). Effects of acutely displaced sleep on testosterone. The Journal of Clinical Endocrinology & Metabolism, 90(8), 4530-4535.
12. Hirshkowitz, M., Whiton, K., Albert, S. M., Alessi, C., Bruni, O., DonCarlos, L., … & Neubauer, D. N. (2015). National Sleep Foundation’s sleep time duration recommendations: methodology and results summary. Sleep Health, 1(1), 40-43.
13. Leproult, R., & Van Cauter, E. (2011). Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA, 305(21), 2173-2174.
14. Leproult, R., Copinschi, G., Buxton, O., & Van Cauter, E. (1997). Sleep loss results in an elevation of cortisol levels the next evening. Sleep, 20(10), 865-870.
15. Luboshitzky, R., Aviv, A., Hefetz, A., Herer, P., Shen-Orr, Z., Lavie, L., & Lavie, P. (2002). Decreased pituitary-gonadal secretion in men with obstructive sleep apnea. The Journal of Clinical Endocrinology & Metabolism, 87(7), 3394-3398.
16. Chida, Y., & Steptoe, A. (2009). Cortisol awakening response and psychosocial factors: a systematic review and meta-analysis. Biological Psychology, 80(3), 265-278.
17. Cumming, D. C., QuigleyM. E., & Yen, S. S. (1983). Acute suppression of circulating testosterone levels by cortisol in men. The Journal of Clinical Endocrinology & Metabolism, 57(3), 671-673.
18. Hill, E. E., Zack, E., Battaglini, C., Viru, M., Viru, A., & Hackney, A. C. (2008). Exercise and circulating cortisol levels: the intensity threshold effect. Journal of Endocrinological Investigation, 31(7), 587-591.
19. Leproult, R., Copinschi, G., Buxton, O., & Van Cauter, E. (1997). Sleep loss results in an elevation of cortisol levels the next evening. Sleep, 20(10), 865-870.
20. Pascoe, M. C., Thompson, D. R., & Ski, C. F. (2017). Yoga, mindfulness-based stress reduction and stress-related physiological measures: A meta-analysis. Psychoneuroendocrinology, 86, 152-168.
21. Peters, E. M., Anderson, R., Nieman, D. C., Fickl, H., & Jogessar, V. (2001). Vitamin C supplementation attenuates the increases in circulating cortisol, adrenaline and anti-inflammatory polypeptides following ultramarathon running. International Journal of Sports Medicine, 22(7), 537-543.
22. Pischon, T., Hankinson, S. E., Hotamisligil, G. S., Rifai, N., & Rimm, E. B. (2003). Habitual dietary intake of n-3 and n-6 fatty acids in relation to inflammatory markers among US men and women. Circulation, 108(2), 155-160.
23. Rivier, C., & Rivest, S. (1991). Effect of stress on the activity of the hypothalamic-pituitary-gonadal axis: peripheral and central mechanisms. Biology of Reproduction, 45(4), 523-532.
24. Sapolsky, R. M. (1986). Stress-induced elevation of testosterone concentration in high ranking baboons: role of catecholamines. Endocrinology, 118(4), 1630-1635.
25. Ahtiainen, J. P., Hulmi, J. J., Kraemer, W. J., Lehti, M., Nyman, K., Selänne, H., … & Häkkinen, K. (2015). Heavy resistance exercise training and skeletal muscle androgen receptor expression in younger and older men. Steroids, 103, 37-42.
26. Brambilla, D. J., Matsumoto, A. M., Araujo, A. B., & McKinlay, J. B. (2009). The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men. The Journal of Clinical Endocrinology & Metabolism, 94(3), 907-913.
27. Chandler, R. M., Byrne, H. K., Patterson, J. G., & Ivy, J. L. (1994). Dietary supplements affect the anabolic hormones after weight-training exercise. Journal of Applied Physiology, 76(2), 839-845.
28. Cook, C. J., Crewther, B. T., Kilduff, L. P., Drawer, S., & Gaviglio, C. M. (2012). Skill execution and sleep deprivation: effects of acute caffeine or creatine supplementation—a randomized placebo-controlled trial. Journal of the International Society of Sports Nutrition, 9(1), 1-9.
29. Hill, E. E., Zack, E., Battaglini, C., Viru, M., Viru, A., & Hackney, A. C. (2008). Exercise and circulating cortisol levels: the intensity threshold effect. Journal of Endocrinological Investigation, 31(7), 587-591.
30. Kraemer, W. J., Looney, D. P., Martin, G. J., Ratamess, N. A., Vingren, J. L., French, D. N., … & Hatfield, D. L. (2017). Changes in creatine kinase and cortisol in National Collegiate Athletic Association Division I American football players during a season. Journal of Strength and Conditioning Research, 31(1), 2-8.
31. Maggio, M., Ceda, G. P., Lauretani, F., Cattabiani, C., Avantaggiato, E., Morganti, S., … & Valenti, G. (2011). Magnesium and anabolic hormones in older men. International Journal of Andrology, 34(6pt2), e594-e600.
32. Prasad, A. S., Mantzoros, C. S., Beck, F. W., Hess, J. W., & Brewer, G. J. (1996). Zinc status and serum testosterone levels of healthy adults. Nutrition, 12(5), 344-348.
33. Vingren, J. L., Kraemer, W. J., Ratamess, N. A., Anderson, J. M., Volek, J. S., & Maresh, C. M. (2010). Testosterone physiology in resistance exercise and training: the up-stream regulatory elements. Sports Medicine, 40(12), 1037-1053.
34. Cawood, M. L., Field, H. P., & Ford, C. G. (2009). Testosterone measurement by isotope-dilution liquid chromatography–tandem mass spectrometry: validation of a method for routine clinical practice. Clinical Chemistry, 55(8), 1472-1479.
35. Celec, P., Ostatníková, D., & Caganová, M. (2015). On the effects of testosterone on brain behavioral functions. Frontiers in Neuroscience, 9, 12.
36. Rosner, W., Auchus, R. J., Azziz, R., Sluss, P. M., & Raff, H. (2007). Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. The Journal of Clinical Endocrinology & Metabolism, 92(2), 405-413.
37. Taieb, J., Mathian, B., Millot, F., Patricot, M. C., Mathieu, E., Queyrel, N., … & Boudou, P. (2003). Testosterone measured by 10 immunoassays and by isotope-dilution gas chromatography-mass spectrometry in sera from 116 men, women, and children. Clinical Chemistry, 49(8), 1381-1395.
38. Vermeulen, A., Verdonck, L., & Kaufman, J. M. (1999). A critical evaluation of simple methods for the estimation of free testosterone in serum. The Journal of Clinical Endocrinology & Metabolism, 84(10), 3666-3672.
39. Vingren, J. L., Kraemer, W. J., Ratamess, N. A., Anderson, J. M., Volek, J. S., & Maresh, C. M. (2010). Testosterone physiology in resistance exercise and training: the up-stream regulatory elements. Sports Medicine, 40(12), 1037-1053.
40. Wang, C., Catlin, D. H., Demers, L. M., Starcevic, B., & Swerdloff, R. S. (2004). Measurement of total serum testosterone in adult men: comparison of current laboratory methods versus liquid chromatography-tandem mass spectrometry. The Journal of Clinical Endocrinology & Metabolism, 89(2), 534-543.
41. Wankhede S, Langade D, Joshi K, Sinha SR, Bhattacharyya S. Examining the effect of Withania somnifera supplementation on muscle strength and recovery: a randomized controlled trial. J Int Soc Sports Nutr. 2015 Nov 25;12:43. doi: 10.1186/s12970-015-0104-9. PMID: 26609282; PMCID: PMC4658772.
42. Topo E, Soricelli A, D’Aniello A, Ronsini S, D’Aniello G. The role and molecular mechanism of D-aspartic acid in the release and synthesis of LH and testosterone in humans and rats. Reprod Biol Endocrinol. 2009 Oct 27;7:120. doi: 10.1186/1477-7827-7-120. PMID: 19860889; PMCID: PMC2774316.
43. Melville GW, Siegler JC, Marshall PW. Three and six grams supplementation of d-aspartic acid in resistance trained men. J Int Soc Sports Nutr. 2015 Nov 16;12:15. doi: 10.1186/s12970-015-0078-7. PMID: 26593457; PMCID: PMC4650143.
44. Brown GA, Vukovich MD, Reifenrath TA, Uhl NL, Parsons KA, Sharp RL, King DS. Effects of anabolic precursors on serum testosterone concentrations and adaptations to resistance training in young men. Int J Sport Nutr Exerc Metab. 2000 Sep;10(3):340-59. doi: 10.1123/ijsnem.10.3.340. PMID: 10997957.
45. Willoughby DS, Stout JR, Wilborn CD. Effects of resistance training and protein plus amino acid supplementation on testosterone and growth hormone levels in young males. Int J Sport Nutr Exerc Metab. 2007 Aug;17(4):392-402. doi: 10.1123/ijsnem.17.4.392. PMID: 17986903.
46. Rogerson, S., Riches, C. J., Jennings, C., Weatherby, R. P., Meir, R. A., & Marshall-Gradisnik, S. M. (2007). The effect of five weeks of Tribulus terrestris supplementation on muscle strength and body composition during preseason training in elite rugby league players. Journal of strength and conditioning research, 21(2), 348-353.
47. Neychev, V. K., & Mitev, V. I. (2016). The aphrodisiac herb Tribulus terrestris does not influence the androgen production in young men. Journal of ethnopharmacology, 189, 175-181.
48. Brown, G. A., Vukovich, M. D., Sharp, R. L., Reifenrath, T. A., Parsons, K. A., & King, D. S. (2002). Effect of oral DHEA on serum testosterone and adaptations to resistance training in young men. Journal of Applied Physiology, 92(5), 2268-2275.
49. Welle, S., Jozefowicz, R., Statt, M., & Andrews, R. (2001). Relationship between muscle growth and ultimate muscle strength: some physiological and methodological considerations. European Journal of Applied Physiology, 84(6), 521-527.
50. Kilic, M., Baltaci, A. K., Gunay, M., Gökbel, H., Okudan, N., & Cicioglu, I. (2006). The effect of exhaustion exercise on thyroid hormones and testosterone levels of elite athletes receiving oral zinc. Neuro endocrinology letters, 27(1-2), 247-252.
51. Osterberg, E. C., & Bernie, A. M. (2010). Complementary and alternative medicine for the athlete: current trends and future directions. Current sports medicine reports, 9(4), 187-193.
52. Prasad, A. S., Mantzoros, C. S., Beck, F. W., Hess, J. W., & Brewer, G. J. (1996). Zinc status and serum testosterone levels of healthy adults. Nutrition, 12(5), 344-348.
53. Netter, A., Nahoul, K., & Hartoma, R. (2016). Effect of zinc administration on plasma testosterone, dihydrotestosterone, and sperm count. Archives of andrology, 7(1), 69-73.
54. Al-Dujaili, E. A. S., & Smail, N. F. (2012). Pomegranate juice intake enhances salivary testosterone levels and improves mood and well-being in healthy men and women. Endocrine Abstracts, 28, P313.
55. Derouiche, A., Jafri, A., Driouch, I., El Khasmi, M., Adlouni, A., Benajiba, N., … & Aarab, L. (2013). Effect of argan and olive oil consumption on the hormonal profile
56. Hawkes, W. C., & Hornbostel, L. (1996). Effects of dietary selenium on mood in healthy men living in a metabolic research unit. Biological Psychiatry, 39(2), 121-128.
57. Michnovicz, J. J., Adlercreutz, H., & Bradlow, H. L. (1997). Changes in levels of urinary estrogen metabolites after oral indole-3-carbinol treatment in humans. Journal of the National Cancer Institute, 89(10), 718-723.
58. Ouattara, B., Simard, R., Holvoet, C., Dodin, S., & Légaré, F. (2010). Efficacy of Maca (Lepidium meyenii) used as a potential energizer and aphrodisiac. Food Quality and Preference, 21(2), 198-202.
59. Pischon, T., Hankinson, S. E., Hotamisligil, G. S., Rifai, N., & Rimm, E. B. (2003). Habitual dietary intake of n-3 and n-6 fatty acids in relation to inflammatory markers among US men and women. Circulation, 108(2), 155-160.
60. Prasad, A. S., Mantzoros, C. S., Beck, F. W., Hess, J. W., & Brewer, G. J. (1996). Zinc status and serum testosterone levels of healthy adults. Nutrition, 12(5), 344-348.