Bodybuilders would find apigenin useful because it's a natural aromatase inhibitor. Aromatase is the enzyme that converts androgens, including testosterone, into estrogen. But the most interesting aspect of apigenin in relation to testosterone is shown in an upcoming study. Testosterone levels drop in most men beginning at about age 40. While there are various theories as to the cause of this hormonal decline, one plausible theory relates the drop in testosterone to a decrease of a protein synthesized in the testes, and wherever else steroid hormones are made in the body, including the brain and adrenal glands. This protein is called the steroidogenic acute regulatory protein or StAR. What StAR does is transport cholesterol, which is the raw material from which all steroid hormones are produced from, including testosterone, estrogen, and cortisol, from the outer mitochondrial membrane in cells to the inner membrane, where production of the hormones begins. Cholesterol requires the StAR transport protein because it cannot penetrate the inner mitochondrial cell membrane by itself; it must be ferried or transported into the cell. The production of StAR is stimulated by lutenizing hormone (LH) secreted from the anterior pituitary gland. StAR is considered the rate-limiting step in steroid hormone synthesis, since without the raw material provided by cholesterol, no hormones can be made.