How Testosterone Levels Affect Muscle growth and fat Loss


Testosterone is the primary male sex hormone, and it is responsible for much of a male’s reproductive growth and sexual development. Testosterone acts by stimulating androgen receptors and activating a host of biological changes and hormonal responses. Testosterone is also heavily involved in DNA transcription and the synthesis of various proteins. Given the importance of protein synthesis in the development of muscle tissue; how do serum testosterone levels affect muscle growth and consequently fat loss?




Testosterone plays a crucial role in the growth and development of the human male and does so primarily through two distinct methods of action. Some of the specific effects of testosterone action include:

Androgenic effects of testosterone – The maturation of sexual organs such as the penis and testes are mediated by testosterone action in the womb and later during puberty, the growth of hair and a deepening of the voice.

Anabolic effects of testosterone – Testosterone functions as a potent anabolic steroid. Testosterone promotes the growth of bone tissue, muscle mass, and the development of strength.

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Anabolism refers to a set of metabolic and genetic pathways that promote the assembly of molecular units from smaller subunits. Whereas catabolism involves the breakdown of molecules for endergonic function, anabolism is a process of “building up” tissues. Anabolism is heavily involved in the growth and differentiation of cells and the synthesis of complex molecular structures.

Therefore, the anabolic nature of testosterone makes it an essential component in men’s ability to grow and develop muscle mass.




Let us take a quick look at the way hormonal action regulates muscle growth.

For biochemical endocrinologists, the hormonal control of growth and differentiation of skeletal muscle is of the most challenging issues to study. In many cases, the practical aspect of DNA and RNA analysis of muscle cells proves to be challenging, and often it is difficult to distinguish the effects on muscle cells from those of other tissues intermixed with muscle tissue. However, great strides have been made into the mechanisms of action induced by hormones such as testosterone, HGH, and Insulin-like Growth Factor. More information on effects of Testosterone and Muscle size and strength in normal men.

For example, the existence of a cytoplasmic receptor for testosterone has been confirmed in skeletal muscle homogenates. Strikingly, this receptor prefers binding to testosterone over DHT unlike sex-linked tissues such as the prostate; which proves that testosterone is active in stimulating the labeling of muscle cell nuclei, muscle growth, and total protein content. Testosterone has even been confirmed to stimulate a transition from Type IIA fibers to Type IIB fibers in temporalis muscle cells. It has also been suggested that testosterone’s androgenic nature is capable of enhancing somatomedin secretion.



However, there are important considerations to keep in mind. For testosterone induced protein synthesis to occur, along with its subsequent muscle growth, a number of prerequisites must be met. Firstly of all, the pre-existing muscle tissue must have experienced exercise-induced micro-injury. In other words, stresses generated through the muscle’s own contractile action must have generated micro-tears in the skeletal muscle myofibers. Additionally, sufficient dietary protein must have been ingested by the individual in order to fuel the muscle fiber growth. In other words, without a proper exercise routine that involves sufficient resistance training, testosterone will not induce spontaneous muscle growth.

Nevertheless, there is evidence to back the claim that a testosterone surge such as one induced through the injection of anabolic steroids, can induce some degree of muscle growth without the presence of exercise-induced injury.

In a study conducted out of the Department of Medicine at Charles R. Drew University of Medicine and Science into the efficacy of androgenic anabolic substances, it was determined that men in a no-exercise group but who received testosterone dosages experienced more significant increases in muscle mass and strength than those given a placebo. However, the men who were assigned to the testosterone and exercise group experienced even more significant increases in muscle mass, size, and strength than those who were assigned to either of the no-exercise groups. The study concluded that doses of testosterone are able to increase fat-free mass and muscle strength in healthy men measurably. As previously state, the effect was more pronounced when combined with strength training exercises.





Given the explicit link that exists between the amount of developed musculature and total body fat, it would make sense for there to be a quantifiable link between Testosterone and belly fat. While some studies suggest that having low levels of serum testosterone causes excess belly to build up, others have claimed that having excess body fat has an inhibiting effect on the synthesis of testosterone.

Which precedes which, is perhaps the question.

Waist circumference can be used to predict low levels of testosterone in men because testosterone is, as mentioned earlier, heavily involved in the development of lean muscle mass. Given that lower testosterone levels are responsible, in part, for a loss of muscle mass, and statistically a smaller musculature is associated with elevated levels of body fat, it would seem the link is causal. In fact, the gradual decrease in serum testosterone that occurs with age also correlates with the fact that aging men are more likely to develop an excess of belly fat.

This proposition has repeatedly been substantiated in clinical trials. In a recent study, researchers observed an astonishing increase in body fat of 36% in individuals whose testosterone levels were systematically decreased below the natural baseline.

The role that Testosterone plays in the development of lean muscle mass, strength and the accumulation of body fat in mean cannot be denied. Besides exogenous supplementation, the natural synthesis of this powerful hormone can be stimulated through high-intensity resistance training. In women, this effect is practically non-existent due to the lower rate at which the female body synthesizes it. For this reason, women should not be afraid to weight-lift or practice resistance training for fear of developing massive muscles; that is the purview of men.




  • Millward, D. J., et al. “The relative importance of muscle protein synthesis and breakdown in the regulation of muscle mass.” Biochemical Journal 156.1 (1976): 185-188.
  • Florini, James R. “Hormonal control of muscle growth.” Muscle & nerve 10.7 (1987): 577-598.
  • Tipton, Kevin D., and Robert R. Wolfe. “Exercise, protein metabolism, and muscle growth.” International journal of sport nutrition and exercise metabolism 11.1 (2001): 109-132.
  • Bhasin, Shalender, et al. “The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men.” New England Journal of Medicine 335.1 (1996): 1-7.







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