The mechanism of action of SARMs' tissue-specific effects continues to be debated as of 2020update. Because of the potentially better side effect profile of SARMs compared to testosterone, SARMs have been proposed for use in the treatment of hypogonadism and for androgen replacement therapy. Unlike most current forms of testosterone replacement, SARMs are orally bioavailable and largely eliminated via hepatic metabolism and metabolized through amide hydrolysis in the case of arylpropionamides and A-ring nitro reduction of andarine. SARMs can be agonists, antagonists, or partial agonists of the AR depending on the tissue, which can enable targeting specific medical conditions while minimizing side effects.
Benign Prostatic Hyperplasia (BPH), known more simply as prostate enlargement, is the noncancerous growth of the prostate. In contrast, the S4-treated rats only had their prostate and seminal vesicle weights increase by 16% and 17%, respectively. While the DHT-treated rats also experienced the same results, the weights of their prostate and seminal vesicles (which contribute to semen secretion) increased by more than 2x that of the control group. In other words, it binds to the AR with double to triple the strength that testosterone normally would.
Unlike bulking compounds that add mass and water weight, S4 creates a dense, dry, and defined look. However, its impact on vision receptors sets it apart from other SARMs—something we’ll explore in detail later. Because it doesn’t aromatize into estrogen, S4 does not cause water retention or gynecomastia, making it a preferred option for cutting or recomposition cycles. S4, also known as Andarine, is a Selective Androgen Receptor Modulator (SARM) developed by GTx, Inc. for treating muscle wasting, osteoporosis, and benign prostatic hyperplasia (BPH).
Conflicting results have been obtained concerning the importance of testosterone in maintaining cardiovascular health. Testosterone does not appear to increase the risk of developing prostate cancer. The brain is also affected by this sexual differentiation; the enzyme aromatase converts testosterone into estradiol that is responsible for masculinization of the brain in male mice. Testosterone also regulates the population of thromboxane A2 receptors on megakaryocytes and platelets and hence platelet aggregation in humans. The male brain is masculinized by the aromatization of testosterone into estradiol, which crosses the blood–brain barrier and enters the male brain, whereas female fetuses have α-fetoprotein, which binds the estrogen so that female brains are not affected. It has been theorized that brain masculinization is occurring since no significant changes have been identified in other parts of the body. The levels remain in a pubertal range for a few months, but usually reach the barely detectable levels of childhood by 4–7 months of age.
It is nonsteroidal, orally bioavailable, and is selective for anabolic actions in skeletal muscle and bone tissue. "Selective androgen receptor modulators like Andarine have demonstrated the ability to maintain lean mass during catabolic conditions without increasing androgenic side effects" (Kearbey et al., Endocrinology). Androgen receptors occur in many different vertebrate body system tissues, and both males and females respond similarly to similar levels. 5α-DHT binds to the same androgen receptor even more strongly than testosterone, so that its androgenic potency is about 5 times that of T. Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α-dihydrotestosterone (5α-DHT) by the cytoplasmic enzyme 5α-reductase. Androgens such as testosterone have also been found to bind to and activate membrane androgen receptors.
When it comes to SARMs and steroids, SARMs are extremely selective at certain dosages, but there is a ceiling where increasing the dosage only increases the potential for side effects in the body, with a greatly diminishing level of muscle growth potential. Although the mechanism by which Andarine exerts anabolic activity at the androgen receptor is the same as other SARMs, it ultimately leads to different effects on body composition than other SARMs. Personally, I believe the positive effect anabolic agents have on body fat is an indirect result of increased muscle mass which then increases metabolism and results in more fat being burned at rest and an increase in total daily energy expenditure. Both S1 and S4 demonstrated partial agonist activity in the androgenic tissues (prostate) and full agonist activity in muscle (levator ani muscle) in both castrated and intact rats R. Consequently, prostate weight, seminal vesicle weight and muscle weight all plummet in parallel to the crashing androgen levels (hence why low Testosterone in men causes loss of muscle tissue).

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