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Hormones play a crucial role in regulating various aspects of muscle development and maintenance. For example, testosterone, a male sex hormone, is a primary driver of muscle growth and individuals with lower testosterone levels may find it challenging to build and maintain muscle tone. Similarly, oestrogen promotes muscle repair and growth in women, and its reduction during menopause can accelerate muscle loss. Additionally, cortisol, a stress hormone, can inhibit muscle growth and lead to muscle loss when present in excess. On the other hand, Insulin-like Growth Factors (IGFs) stimulate muscle growth and increase lean body mass. Anabolic-androgenic steroids (AAS) and growth hormones have been shown to increase muscle mass in patients with muscle atrophy, but their use is controversial due to associated side effects. Hormone replacement therapy can be considered to counteract degenerative changes in skeletal muscle, but more research is needed to understand the direct and indirect effects of female hormones.
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What You'll Learn
Testosterone and muscle growth
Testosterone is a male sex hormone that is vital for building and maintaining muscle mass. It is produced primarily in the testicles of men and the ovaries of women, although men tend to produce more of it. Testosterone plays a crucial role in overall health and is responsible for maintaining bone density, muscle mass, and red blood cell production.
Testosterone levels are positively correlated with muscle protein synthesis, which means that higher testosterone levels lead to increased muscle growth. This occurs through the stimulation of muscle cells to produce more protein, which promotes muscle growth and repair. Additionally, testosterone increases the number of receptors on muscle cells that bind to other anabolic hormones, such as insulin-like growth factor 1 (IGF-1), further enhancing its muscle-building effects.
Testosterone also affects the type of muscle fibres in the body. There are two types of muscle fibres: slow-twitch (Type 1) and fast-twitch (Type 2). Slow-twitch fibres are better for endurance exercise, while fast-twitch fibres are more suited for explosive movements and weightlifting as they generate more force. Studies have shown that testosterone levels are positively correlated with the number of fast-twitch muscle fibres in the body.
In men, testosterone production peaks during adolescence and early adulthood, gradually declining after the age of 30. However, the rate of decline varies from person to person, with some men experiencing a more significant decrease in testosterone levels. Low testosterone levels can lead to decreased muscle mass, increased body fat, and other health issues. Testosterone replacement therapy may be an option for those with low testosterone levels, but it is important to consult a doctor before starting any hormone treatment as side effects may occur.
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Oestrogen and muscle loss in post-menopausal women
Women typically experience the menopausal transition in their mid-40s, with menopause being defined by the permanent cessation of menstrual periods in women in their late 40s or early 50s. Following menopause, oestrogen levels are reduced to a negligible amount in most women, which has implications for muscle mass and strength.
The loss of muscle mass and muscle force in women accelerates relative to similarly-aged men after menopause. The ability of muscle to generate force declines significantly following menopause. This is supported by studies that have found that males and females have similar adductor pollicis muscle strength (relative to muscle mass) until the age of female menopause. However, after menopause, the loss of muscle force relative to muscle mass accelerates much faster in women than in similarly-aged men.
The weight of evidence from human and animal studies demonstrates that oestrogen-based hormone replacement therapy (HRT) has significant beneficial effects on skeletal muscle mass, strength, and protection from damage in older women. The benefits of HRT in maintaining muscle function and mass may help offset age-related loss of muscle mass and strength and may help prolong the ability of ageing women to maintain a functioning, independent lifestyle.
Some studies have found evidence of enhanced muscle function and size in 50–57-year-old post-menopausal women taking HRT relative to placebo controls in randomised control studies. Using a one-year HRT intervention, studies have reported that those subjects taking HRT increased muscle size, vertical jump height, and running speed over the course of the study period, while those given a placebo either only maintained or had reductions in all of these measures over the same time period.
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Cortisol and muscle breakdown
Cortisol is a catabolic hormone that inhibits muscle growth by breaking down molecules to release energy. It is the main glucocorticoid released from the adrenal cortex and is commonly known as the body's stress hormone. Cortisol is also produced in response to abnormal conditions such as infection, cancers, diabetes, and chronic kidney disease.
Prolonged exposure to high levels of cortisol can lead to various disorders, including Cushing's syndrome, which is characterised by weight gain, fatty deposits, diabetes, hypertension, and proximal muscle weakness. Cortisol excess is a critical factor contributing to muscle atrophy, which is the loss of skeletal muscle mass and function. This can be caused by ageing, prolonged fasting, a sedentary lifestyle, diabetes, cachexia, sepsis, burns, and glucocorticoid excess.
Exercises that stimulate the release of cortisol include those that use heavy weights and large muscle groups, such as lengthy cardio sessions. However, short-term durations of these exercises can be beneficial as muscles need to break down slightly to grow. To minimise the release of cortisol, it is recommended to keep workout durations shorter and ensure that muscles are not excessively stressed.
In addition to exercise, certain foods can affect cortisol levels. Eating fewer carbs can increase growth hormones, while eating carbs before or during a workout can minimise cortisol levels.
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Insulin-like growth factors (IGFs) and muscle growth
Insulin-like growth factors (IGFs) are proteins with a high sequence similarity to insulin. They are produced in the liver in response to growth hormones, and their levels increase when growth hormone (GH) levels rise. IGFs play a key role in promoting cell growth and differentiation, continuing to have an anabolic effect in adults.
IGF-1, or Insulin-like Growth Factor 1, is the main mediator of human growth hormone and is part of a wide network of growth factors, receptors, and binding proteins involved in mediating cellular proliferation, differentiation, and apoptosis. IGF-1 is important for the regulation of normal physiology, as well as pathological states, including cancer. It is also a popular doping agent in sports, and its deficiency can lead to growth retardation and intellectual deficits.
IGF-2, or Insulin-like Growth Factor 2, is thought to be a primary growth factor required for early development. While IGF-2 may be primarily fetal in action, it is also essential for the development and function of organs such as the brain, liver, and kidney.
In the context of skeletal muscle homeostasis, IGF-I is believed to mediate most of the growth-promoting effects of circulating GH. It supports muscle protein synthesis and muscle growth and may promote tissue recovery after injuries. IGF-I has been studied for its potential to overcome the loss of muscle in the elderly, but the results have been negligible. Increasing circulating IGF-I levels in elderly subjects did not affect the rate of protein synthesis in muscles or lead to increased strength.
Exogenous augmentation of IGF-I is not an effective method of increasing muscle mass due to its potentially adverse effects, including disruption of the insulin system and cancer. However, IGF-I is still abused by athletes striving to reach peak athletic form.
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Hormone therapy for transgender women and muscle strength
Hormones play a vital role in regulating metabolism, which in turn influences muscle gain and loss. Anabolic hormones, which use energy, and catabolic hormones, which release energy, both affect muscle growth. For example, cortisol, the main catabolic hormone, inhibits muscle growth by breaking down molecules to release energy.
Transgender women undergoing hormone therapy experience a decrease in muscle strength, lean body mass, and muscle area. However, these values remain above those of cisgender women, even after 36 months of therapy. This suggests that transgender women may retain some strength advantages over their cisgender counterparts.
Longitudinal studies have shown that testosterone suppression and estrogen supplementation in transgender women lead to a modest decrease in thigh volume and quadriceps cross-sectional area after 12 months of treatment. The decrease in testosterone levels required for transgender women to compete in sports is insufficient to remove or reduce the male advantage in muscle mass and strength.
While there is strong evidence that transgender women will retain some strength and muscle mass advantages after cross-hormone therapy, the current research is limited to untrained individuals. It is uncertain how transgender women athletes undergoing advanced training regimens to counteract muscle loss during therapy would respond. More comprehensive performance tests and studies on the training capacity of transgender women undergoing hormone therapy are needed to fully understand the impact on muscle strength and athletic performance.
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Frequently asked questions
Anabolic hormones, such as testosterone, promote muscle growth and maintenance. Higher levels of testosterone are associated with increased muscle mass and strength. Catabolic hormones, such as cortisol, inhibit muscle growth by breaking down molecules to release energy.
In transgender women, hormone therapy decreases muscle strength, lean body mass, and muscle area. However, their strength is still higher than that of cisgender women, even after 36 months of therapy.
Insulin-like growth factors (IGFs) are produced in the liver in response to growth hormones. IGFs stimulate muscle growth, increase lean body mass, and accelerate recovery time.
