Aromatase is highly expressed in adipose tissue and the brain, and is also expressed significantly in skeletal muscle. 3α-HSD is highly expressed in skeletal muscle as well.Natural AAS like testosterone and DHT and synthetic AAS are analogues and are very similar structurally. For this reason, they have the capacity to bind to and be metabolized by the same steroid-metabolizing enzymes. According to the intracellular metabolism explanation, the androgenic-to-anabolic ratio of a given AR agonist is related to its capacity to be transformed by the aforementioned enzymes in conjunction with the AR activity of any resulting products. For instance, whereas the AR activity of testosterone is greatly potentiated by local conversion via 5α-reductase into DHT in tissues where 5α-reductase is expressed, an AAS that is not metabolized by 5α-reductase or is already 5α-reduced, such as DHT itself or a derivative (like mesterolone or drostanolone), would not undergo such potentiation in said tissues. Moreover, nandrolone is metabolized by 5α-reductase, but unlike the case of testosterone and DHT, the 5α-reduced metabolite of nandrolone has much lower affinity for the AR than does nandrolone itself, and this results in reduced AR activation in 5α-reductase-expressing tissues.
Anabolic steroids, also known more properly as anabolic–androgenic steroids (AAS), are steroidal androgens that include natural androgens like testosterone as well as synthetic androgens that are structurally related and have similar effects to testosterone. They increase protein within cells, especially in skeletal muscles, and also have varying degrees of virilizing effects, including induction of the development and maintenance of masculine secondary sexual characteristics such as the growth of facial and body hair. The word anabolic, referring to anabolism, comes from the Greek ἀναβολή anabole, "that which is thrown up, mound". Androgens or AAS are one of three types of sex hormone agonists, the others being estrogens like estradiol and progestogens like progesterone. AAS were synthesized in the 1930s, and are now used therapeutically in medicine to stimulate muscle growth and appetite, induce male puberty and treat chronic wasting conditions, such as cancer and AIDS. The American College of Sports Medicine acknowledges that AAS, in the presence of adequate diet, can contribute to increases in body weight, often as lean mass increases and that the gains in muscular strength achieved through high-intensity exercise and proper diet can be additionally increased by the use of AAS in some individuals.Health risks can be produced by long-term use or excessive doses of AAS. These effects include harmful changes in cholesterol levels (increased low-density lipoprotein and decreased high-density lipoprotein), acne, high blood pressure, liver damage (mainly with most oral AAS), and dangerous changes in the structure of the left ventricle of the heart.