Testosterone has a powerful effect on muscle mass, strength and fat reduction. ​

In a nutshell testosterone: ​

• Improves how the body processes proteins by inhibiting the breakdown of protein
• Activating satellite cells which are a type of stem cells that play a role in muscle development
• Stimulating anabolic hormones, which promote muscle growth and protein synthesis
• Enhancing tissue growth

Myotrophic and Hypertrophic Effects of Testosterone on Muscles

Important mechanisms behind the myotrophic (increasing muscle weight) effects of testosterone were uncovered both in athletes using steroids for several years and in short-term controlled studies.

Both long-term and short-term testosterone usage accentuated the degree of fibre hypertrophy in human skeletal muscle by enhancing protein synthesis.

Hypertrophy is the process of organ or tissue enlargement by increasing the size of cells.

Testosterone increases the size of the muscle by activation of satellite cells and promoting myonuclear accretion when existing myonuclei cannot continue protein synthesis.

Interestingly, long-term steroid usage also enhances the frequency of fibres with centrally located myonuclei, which implies the occurrence of a high regenerative activity.

A mechanism by which testosterone facilitates the enlargement of muscle fibres is the activation of satellite cells and the promotion of myonuclear accretion when existing myonuclei become unable to sustain further enhancement of protein synthesis.

The effects of testosterone on skeletal muscle are thought to be mediated via androgen receptors expressed in myonuclei and satellite cells. Some evidence also suggests the existence of an androgen-receptor-independent pathway.

Under the influence of testosterone, some daughter cells generated by satellite cell proliferation may escape differentiation and return to quiescence or stay dormant, which help to replenish the satellite cell reserve pool.

*Myonuclei refers to the nuclei of a muscle fiber or cell. The number of myonuclei may be increased in a muscle cell by fusion of a satellite cell with a muscle fiber.

*Satellite cells are single nucleus cells found between the basement membrane and plasma membrane of the muscle fiber, and act as stem cells responsible for the growth & development of skeletal muscle.

Satellite cells are precursors to skeletal muscle cells and are responsible for the ability of muscle tissue to regenerate. ie These embryonic cells remain in the adult and can replace damaged muscle fibers to some degree.

*Daughter cells are genetically identical to the parent cell because they contain the same number and type of chromosomes.

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Reduction of Body Fat by Testosterone

Testosterone supplementation increases skeletal muscle mass and decreases fat mass. Body composition is moulded by committing mesenchymal pluripotent cells into myogenic lineage or muscle mass while suppressing their evolution into adipogenic lineage or fat mass.​

Think of testosterone like the classic step-mother who does everything in her power to enhance her daughter (muscle mass) while discouraging the step-daughter (fat mass).​

To understand the muscle enhancing fat losing mechanism of testosterone, mouse pluripotent cells were treated with testosterone and dihydrotestosterone for 14 days. Respective myogenic markers or signs of muscle growth and fat forming or adipogenic differentiation assessed. ​

• The number of myogenic cells, myotubes and protein levels increased in proportion to dose of testosterone & dihydrotestosterone.
• Androgen receptor mRNA and protein levels were low at baseline(when study started) but increased after testosterone or dihydrotestosterone treatment.
• Both testosterone and dihydrotestosterone decreased the number of adipocytes(fat mass) and down-regulated the expression of PPAR gamma 2 mRNA and PPAR gamma 2 protein and CCAAT/enhancer binding protein alpha.
• The effects of testosterone and dihydrotestosterone on myogenesis and adipogenesis were blocked by bicalutamide.

Therefore, testosterone and DHT regulate lineage or whether cells become muscle or fat in *mesenchymal pluripotent cells by favoring a myogenic lineage(muscle) and inhibiting their differentiation into the adipogenic lineage(fat) through an androgen receptor-mediated pathway.

“The observation that differentiation of pluripotent cells is androgen dependent provides a unifying explanation for the reciprocal effects of androgens on muscle and fat mass in men."

*Mesenchymal cells are a type of master cell which have pluripotency or have the potential/quality to grow into different types of cells or tissues.