Introduction: Understanding The Role Of Testosterone In Muscle Growth

Testosterone is a hormone that plays a critical role in muscle growth and development. It is responsible for promoting muscle protein synthesis, which increases muscle mass and helps increase strength. Testosterone also enhances recovery from exercise, making it a key factor in muscle regeneration. While testosterone is commonly associated with males, females also produce testosterone, although in much smaller amounts. 

In this blog, we will explore the science of testosterone, the normal ranges of testosterone, the recent testosterone based changes in the population, and the  impact of testosterone levels on muscle growth. 

The Science Of Testosterone and Muscle

Testosterone has a lot of roles in the body, but we’re going to focus on how it affects skeletal muscle. Testosterone binds to androgen receptors on muscle cells. After binding, there is a conformational change that occurs, then translocation from the cytoplasm to the nucleus, where it binds DNA. This triggers activates genes involved in muscle protein synthesis and inhibits genes involved in muscle protein breakdown, among other things. Most researchers believe that the majority of testosterone’s effects are mediated by its interaction with the androgen receptor (AR) and that these effects are directly proportional to the total circulating concentrations of testosterone. An increase in skeletal muscle mass induced by testosterone is linked to hypertrophy of both type I and type II fibers, as well as an elevation in the number of satellite cells and myonuclei.

Muscle protein synthesis is the process of creating new muscle proteins from amino acids, while muscle protein breakdown is the process of breaking down muscle proteins into amino acids. Testosterone promotes muscle protein synthesis by increasing the transcription of genes involved in this process, such as myogenin and IGF-1. It also inhibits muscle protein breakdown by decreasing the transcription of genes involved in this process, such as atrogin-1 and MuRF-1. When muscle protein synthesis outpaces muscle protein breakdown for long periods, it causes an increase in muscle mass. 

Satellite cells are muscle stem cells that are responsible for muscle growth and repair. Testosterone promotes satellite cell activation by increasing the expression of satellite cell markers such as Pax7 and MyoD. This leads to an increase in the number of satellite cells, which can differentiate into new muscle fibers and repair damaged muscle tissue. Testosterone also promotes the fusion of satellite cells with existing muscle fibers, leading to an increase in muscle fiber size and strength.

In addition to its effects on muscle growth, testosterone also affects muscle metabolism. It is thought to increase glucose uptake by muscle cells via the GLUT4 receptor, especially in those who have low testosterone levels. Testosterone also increases the expression of genes involved in mitochondrial biogenesis, leading to an increase in the number and size of mitochondria in aging men with subnormal testosterone levels. This translates to an improvement in muscle endurance and fatigue resistance. 

What Are Normal Levels Of Testosterone?

Testosterone levels can vary depending on age, sex, and other factors. Testosterone levels typically peak during puberty and early adulthood, and then gradually decline with age. This decline is a natural part of the aging process and can result in a range of symptoms, including decreased libido, reduced muscle mass and strength, and increased fat mass. However, the rate of decline can vary widely between individuals, and some may experience a more rapid decline in testosterone levels than others.

In adult males, normal testosterone levels typically range from 300 to 1,000 ng/dL (nanograms per deciliter). However, the range of “normal” levels can vary depending on the laboratory that performs the test.

In females, normal testosterone levels are typically much lower than in males, ranging from 15 to 70 ng/dL. However, testosterone levels can vary throughout a woman’s menstrual cycle, with the highest levels occurring during ovulation. 

Several factors can contribute to a decline in testosterone levels with age, including decreased activity of the testes, increased levels of sex hormone-binding globulin (SHBG), and alterations in the hypothalamic-pituitary-gonadal axis. Other factors that can contribute to low testosterone levels in older men include obesity, chronic illness, and certain medications. Testosterone levels can also decline with age in women, leading to symptoms such as decreased muscle mass and strength, decreased libido, and decreased bone density. Low testosterone levels in women can be caused by a range of factors, including menopause, hysterectomy, and certain medical conditions, such as adrenal or pituitary gland disorders.

Women with polycystic ovary syndrome (PCOS) may have higher than normal testosterone levels, which can lead to symptoms such as acne, excess hair growth, and irregular periods. It is important for individuals to discuss their testosterone levels with a healthcare provider to determine if they fall within a normal range and to address any potential concerns.

Are Testosterone Levels Increasing Or Decreasing In The Population?

There is evidence to suggest that testosterone levels may be decreasing in the general population. A recent study in the United States found that testosterone levels have been declining in adolescent and young adult males over the past two decades. The study analyzed data from a national database and found that total testosterone levels were lower in the later cycles (2011-2016) than in the earlier cycles (1999-2000), even among men with normal body mass index (BMI). The decline in testosterone levels may be due to several factors, including environmental factors and changes in lifestyle. Another study found that nearly one quarter of men aged 20–59 years have low testosterone and metabolic syndrome, finding that a large waistline increases the risk of low testosterone six-fold among young men. However, further studies are needed to confirm these findings. 

There are several possible factors that may be contributing to this decline in testosterone levels. One possible explanation is the increase in obesity rates in recent decades. Obesity is known to be associated with lower testosterone levels, as excess body fat can convert testosterone to estrogen.

While there is evidence to suggest that testosterone levels may be decreasing in the general population, it is important to note that testosterone levels can vary widely between individuals and that many factors, including genetics and lifestyle factors, can influence testosterone levels. It is also important to note that testosterone levels can be affected by medications and medical conditions, such as hypogonadism.

The Importance Of Testosterone Levels For Muscle Development

Testosterone has been traditionally viewed as a primary driver of muscle growth, especially in men. However, recent studies have challenged this assumption, suggesting that the role of testosterone in muscle development may not be as significant as once thought.

For example, a study was conducted to investigate the relationship between hormone responses after exercise and the impact on muscle development. The study involved 56 young men who underwent a 12-week resistance exercise training program. The researchers measured the hormone responses, including testosterone after an intense leg resistance exercise routine. The results showed that there was no significant correlation between testosterone and gains in lean body mass or leg press strength.

Another study examined the hormonal response of college men who completed 12 weeks of high-volume resistance training. The study found that there were no differences in resting hormone levels between the trained and untrained group, except for cortisol which decreased in both groups. During exercise, all hormone levels increased, but after correction for plasma volume, only cortisol and growth hormone showed significant differences. The study found that the degree of muscle fiber hypertrophy was correlated with the increase in growth hormone during exercise, but not testosterone. This study suggests that resistance training does not significantly affect resting hormone levels, but exercise-induced responses are similar throughout the training period, and growth hormone plays a role in muscle development. 

Another study was conducted on resistance-trained men to determine the factors that contribute to the heterogeneity in muscle growth following resistance exercise training (RET). The study measured circulating hormones, intramuscular hormones, and intramuscular hormone-related variables before and after 12 weeks of RET. The study found that neither circulating nor intramuscular hormones were consistently significant or selected for the change in muscle mass. However, the study did find that intramuscular androgen receptor content, but not hormones or enzymes, influences muscle hypertrophy following RET in previously trained young men. 

One more study, conducted by Bhasin et al., in HIV-infected men with low-testosterone (<349 mg/dL) provides insight. The participants were randomly assigned to four groups: placebo, resistance training only, testosterone injections only, or combined testosterone and training. The study found that baseline testosterone levels did not correlate with changes in fat-free mass (FFM), and there were no significant differences in muscle gain between men with testosterone levels <275 ng/dL and men with levels between 275-350 ng/dL.

What Are The Factors That Can Affect Testosterone Levels?

Dieting can lead to weight loss, which has been shown to increase testosterone levels in overweight and obese men. Losing excess weight can help to reduce the amount of fat tissue in the body, which in turn can lead to a decrease in estrogen levels and an increase in testosterone levels. 

However, extreme calorie restriction and rapid weight loss can have negative effects on testosterone levels. This can be seen best in physique athletes who reach extremely low levels of body fat. Severely limiting calorie intake can lead to a decrease in testosterone levels, as the body tries to conserve energy. Additionally, extremely low-fat diets or diets that are deficient in certain nutrients, such as zinc or vitamin D, can also lead to a decrease in testosterone levels.

The Link Between Testosterone Levels and Muscle Mass

Testosterone is found in much higher concentrations in men than in women, which has led to the assumption that men should have greater muscle gains than women. However, studies have shown that this may not necessarily be the case.

In fact, my research has demonstrated that although men do tend to have greater absolute gains in muscle mass than women, the relative gains are similar between the sexes. This suggests that other factors, such as differences in muscle fiber type or nervous system activation, may be more important than testosterone levels in determining muscle growth.

Can Decreasing Testosterone Levels Decrease Muscle Growth?

The most suitable approach to answer this question is through a series of studies where testosterone is lowered by drugs, but this isn’t a common occurrence in most individuals, and may not reflect what happens in normal people. 

In one study by Mauras and colleagues, where young men were given Lupron, a medication that suppresses natural testosterone production. After 10 weeks, testosterone levels in the participants dropped significantly from an average of 535 ng/dL to 31 ng/dL, which is similar to testosterone levels typically found in women. The study found that fat-free mass decreased by 2.1 kg, and whole body protein synthesis decreased by 13% in the participants receiving Lupron. This would indicate large drops in testosterone decrease muscle mass, but it’s unlikely that this extreme drop would occur in people (unless they took this drug).

Another study provides more insight. Kvorning and colleagues conducted a study in which young men were given either goserelin, a drug that suppresses natural testosterone production, or a placebo. These men, who had little experience with resistance training, participated in an 8-week strength training program. The study found that testosterone levels dropped significantly from an average of 651 ng/dL to 31 ng/dL in the group receiving goserelin. Despite the suppression of testosterone, there was no significant difference in improvements in 10-rep maximum (RM) performance compared to the placebo group. However, improvements in leg lean mass were significantly less for the goserelin group, and total lean body mass also tended to be lower (with a p-value of 0.07, where 0.05 is considered significant). The placebo group gained an average of 1 kg more lean body mass than the goserelin group. These findings suggest that suppressing natural testosterone production may have negative effects on muscle mass and strength gains in young men undergoing resistance training.

Can Increasing Testosterone Levels Increase Muscle Growth?

One of the most interesting effects of testosterone are found when it is manipulated using several drugs both inside and outside the normal physiological range (Ranges: females 15 to 70 ng/dL, males 300 to 1,000 ng/dL). This can best be seen by a series of studies by Dr. Bhasin. 

First, he aimed to determine the effects of graded doses of testosterone on various physiological processes such as body composition, muscle size, strength, power, and cognitive functions. They administered monthly injections of a long-acting gonadotropin-releasing hormone (GnRH) agonist, which suppresses endogenous testosterone secretion, and weekly injections of 25, 50, 125, 300, or 600 mg of testosterone enanthate for 20 weeks to men aged 18-35 years. Results showed that changes in fat-free mass, muscle size, strength, and power were dose-dependent and correlated with testosterone concentration. The study revealed that there is a dose-dependent relationship between testosterone and muscle size, even when testosterone levels are within the physiological range. For instance, transitioning from the lower end of physiological testosterone levels (306 ng/dL) to the mid-range (542 ng/dL) led to a 2.8 kg increase in fat-free mass.

Dr. Bhasin repeated this study in men aged 60-75y, finding similar results. For example, those receiving a small dose of testosterone (50mg per week) increased fat free mass by 1.7kg and those who were given 125mg per week increased by 4.2kg over 20 weeks. This occurred without any exercise intervention and demonstrates that gains in muscle mass are dose-dependent with testosterone levels. 

In hypogonadal men, testosterone replacement therapy is known to increase lean mass and decrease fat mass. In young eugonadal men, high doses of testosterone are required to decrease total body fat mass, while similar doses increase lean mass. Current male hormonal contraceptive regimens include a second agent, such as a progestin, to improve efficacy. To study the effect of testosterone and levonorgestrel (LNG) combinations on body composition, a group of young, healthy, eugonadal men were randomized into four treatment groups. The results showed that testosterone alone rapidly increases lean mass and decreases fat mass in 4-8 weeks. However, the combination of testosterone and LNG also rapidly increases lean mass, but has no effect on fat mass, while LNG alone increases fat mass. These findings suggest that androgen-based male hormonal contraceptives might have favorable effects on body composition, although further study is required to determine the impact of these changes on cardiovascular risk in normal men.

Another study by Herbst et al., analyzed the effects of exogenous testosterone (100 mg testosterone enanthate per week) on men with normal testosterone levels with no exercise intervention. Testosterone levels increased from 570 ng/dL to 734 ng/dL, with a peak of 1196 ng/dL. As a result, the peak level was at the upper limit of the normal range, and the lowest point was within the upper portion of normal range. The study found that lean mass increased by ~3kg after 8 weeks of treatment plus a four week recovery period.

Taken together, these studies suggest that increasing testosterone levels within normal physiological ranges may contribute to increases in muscle mass. However, most of these studies use DXA, which is good but not ideal for measuring actual changes in muscle size. 

The Bottom Line: The Impact Of Testosterone On Muscle Growth and What It Means For You

Testosterone plays an important role in muscle growth, with the majority of its effects mediated by its interaction with the androgen receptor. Testosterone promotes muscle protein synthesis, inhibits muscle protein breakdown, and promotes satellite cell activation, leading to an increase in muscle fiber size. Normal testosterone levels range from 300 to 1,200 ng/dL in males and 15 to 70 ng/dL in females, but can vary depending on age, sex, and other factors. 

Recent reports indicate that testosterone is decreasing inside and outside of the USA. This is likely due to increases in obesity and changes in lifestyle (e.g., lower physical activity). Currently, it appears that resistance training (without losing weight) does not increase testosterone levels, but still increases muscle mass. Cross-sectional data indicate that the level of testosterone and muscle mass are not related; however, experimental data suggest that increasing testosterone within normal physiological ranges will increase muscle mass. 

Sound confusing and a little contradicting? Well, unfortunately it is.

And so what is it that YOU should do now? We suggest to prioritize naturally optimizing your testosterone levels if you want a higher quality of life, better recovery, less body fat, and more than likely, more muscle mass. There is enough research to suggest that it at least helps a little and even some that suggest it helps a lot with muscle growth! And at worst, it’s neutral — which means it’s definitely not going to hurt having high levels of testosterone.

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