Testosterone is a steroid hormone which means it is able to enter a cell without an additional carrier protein and it is naturally produced in the human body. It is synthesized from cholesterol molecules circulating in the body. It is mainly produced in males via the sex organs but small amounts are also secreted from the adrenal glands. It is regulated by a homeostatic system known as the hypothalamic-pituitary-testicular axis.
Testosterone is of the utmost importance to muscle development and is part of the hormonal influences that happen in young males upon puberty which results in growth muscle mass and increases the density of bones. Testosterone can be increased by obtaining the proper nutrients in the diet, such as cholesterol from animal sources, complete proteins, and even certain vegetables such as spinach.
That being said, by ingesting these foods, one most often only obtains adequate levels of testosterone and not enough to cause any significant improvements performance wise. Testosterone must be supplemented for older males beyond their teen years to see increased in muscle mass and bone density. It is obvious why testosterone isolates make for a highly valued substance among weight lifters and other athletes.
Supplementing testosterone is often done at night as certain supplements aid testosterone production via the sleep cycle. A healthy sleeping routine is one of the ways an individual can increase circulating levels of testosterone. Even a slight deficiency in production of this hormone can hurt gains in the gym.