Testosterone is one of the most important tools in your body to grow, repair, and thrive. It’s also an increasingly popular medical treatment for men looking to pause —or even reverse —the clock. If you’re trying to naturally raise your testosterone levels or are considering taking a testosterone-boosting supplement, don’t go in blindly! This is your complete education in the king of anabolic hormones.
What Does Testosterone Do?
Testosterone can enter cells passively—like a ghost traveling through a wall—and bind to androgen receptors that act directly upon the nucleus of a cell. It can also serve as a prohormone in sex glands and tissues such as your skin, hair follicles, and fat. For example, when the enzyme 5-alpha-reductase is present, such as in the prostate or skin, testosterone is converted to the more potent androgen, dihydrotestosterone (DHT).
In fat tissue, the aromatase enzyme converts testosterone to the estrogen, estradiol. In other words, the more fat tissue you have, the more estrogens your body can form from testosterone. In some cases, both occur: testosterone acts directly upon a tissue, but also as a prohormone by being converted to its androgen or estrogen metabolite.
In adults, testosterone has effects across the body:
- Muscles: Increases protein synthesis, and increases muscle mass and strength.
- Body Fat: Blocks the uptake of fat and storage of fat, and increases the number of fat burning beta-adrenergic receptors.
- Brain: Improves cognition, memory, sex drive, and affects feelings.
- Heart: Increases blood flow and cardiac output.
- Bone: Increases red blood cell production and bone growth, and maintains bone density.
- Male Sex Organs: Supports sperm production and viability, and promotes penis growth and erectile function.
- Skin: Supports collagen production and produces hair.
- Kidneys: Produces erythropoietin (EPO), which stimulates red blood cell production.
Additionally, testosterone increases insulin-like growth factor (IGF-1), and healthy testosterone levels are associated with good health and immune function, lower mortality rates, and lower body fat and increased metabolic rate.
In men with low testosterone, supplementing with the hormone has likewise been shown to improve energy, muscle mass, libido, sexual performance, and bone mineral density, and reduce body fat, depression, and problems within the urinary tract. There has also been research indicating that medical testosterone transdermal gel applied once a day by fibromyalgia patients can be an effective therapeutic against chronic pain.
How Does Testosterone Affect Body Fat, Weight, And Body Composition?
Low testosterone in men increases fat and weight gain, reduces caloric expenditure, increases the prevalence of blood glucose disorders and insulin insensitivity, and otherwise negatively affects metabolic control. In women, unusually high testosterone has many of the same negative effects.
Collectively and in both sexes, testosterone acts directly within your central nervous system (brain and spinal cord) to regulate metabolism, affecting how well your body burns calories and maintains healthy blood glucose levels.
For example, when 200 milligrams of prescription testosterone enanthate was injected just once per week for six months, in men ages 21-37 that were healthy but didn’t exercise, testosterone levels increased by 91.1 percent. Lean mass concurrently increased by 9.6 percent, and body fat reduced by 16.2 percent.
The results of 59 studies involving more than 5,000 subjects concluded that younger adults realize the greatest decrease in blood glucose in response to using prescription testosterone, as do adults with metabolic disorders. The studies also concluded that supplementing with testosterone significantly increases lean muscle mass, reduces body fat, and makes the body more sensitive to insulin. Data from these same studies suggested that with no change to exercise or diet, just adding supplemental testosterone can reduce body weight by up to 11.5 pounds in two years.
Studies involving over 1,000 healthy adults have reported similar effects, with a typical 6.2 percent decrease in body fat and 2.7 percent increase in lean body mass in response to taking prescription testosterone. The greatest effects occurring as the result of increased fat and glucose metabolism, and an increase in muscle mass.
How Does Testosterone Affect Muscle Performance?
Though many of the benefits of the testosterone produced in your body occur as a result of its conversion to its more potent androgen or estrogen, that’s not the case in muscle. Testosterone acts directly to stimulate muscle growth, which is why it’s called an anabolic steroid —even when your body is what’s making the steroid.
How Testosterone Makes Muscles Bigger: The prevailing theory is that testosterone activates and increases the number of muscle fiber precursor cells, called “satellite cells.” Once activated, these precursor cells can become incorporated into existing muscle fibers to make them bigger (hypertrophy), or the satellite cells can fuse together and themselves form new muscle fibers (hyperplasia).
Additionally, testosterone increases the number of control centers—nucleuses (myonuclei)—present within a muscle fiber, thereby also increasing the number of available androgen receptors that testosterone can bind to within muscle. When combined with training, which increases the sensitivity of androgen receptors, and the consumption of essential amino acids necessary to support protein synthesis, the effects of testosterone on muscle and performance is significantly amplified.
Testosterone is also anti-catabolic because it blocks the ability of catabolic hormones like cortisol to bind to their primary receptors. Thus, testosterone is both an anabolic and anti-catabolic steroid. This makes it fundamental for building and maintaining muscle mass, and for rapid exercise recovery.
How Testosterone Makes Muscles Stronger: Until recently, it was thought that testosterone increased strength and power simply by increasing muscle size. However, testosterone has also been shown to increase the amount of calcium that’s released within the cell, which can boost the force of muscular contractions. Similarly, in a recent rodent study, DHT has been shown to directly stimulate muscle contraction force output by up to 24-30 percent in both power and endurance muscle fibers.
How Testosterone Boosts Endurance: People aren’t just interested in testosterone’s muscle building and recovery benefits. Testosterone increases EPO, which stimulates red blood cell development. More red blood cells means more oxygen carrying capacity within the blood and to working muscles. Injecting adult rodents with testosterone has even been shown to increase the number of fat-burning and energy producing factories (mitochondria) present within cells, and improve mitochondrial function. If the same occurs in humans, that could have a profound effect on performance and is a major area of interest for anti-aging research.
How Testosterone Boosts Athletic Performance: As long as testosterone levels are within normal ranges, where they stand doesn’t seem to affect athletic performance in men. Blood collected from hundreds of male and female Olympic-level track and field athletes that competed in the 2011 and 2013 World Championships revealed that, in men, sprinting activities significantly elevated bioavailable testosterone, whereas throwing activities reduced it. In neither case did it predict how they actually performed.
In women, on the other hand, those with the highest free testosterone in the 400 meter sprint and hurdles, 800 meter sprint, the hammer throw, and pole vault outperformed their low-bioavailable testosterone counterparts by a margin of between 1.8 and 4.5 percent