Prohormones are precursors to hormones and are most typically sold to bodybuilders as a precursor to the natural hormone testosterone. This conversion requires naturally occurring enzymes in the body. Side effects are not uncommon, as prohormones can also convert further into DHT and estrogen. To deal with this, many supplements also have aromatase inhibitors and DHT blockers such as chrysin and 4-androstene-3,6,17-trione. To date most prohormone products have not been thoroughly studied, and the health effects of prolonged use are unknown. Although initially available over the counter, their purchase was made illegal without a prescription in the US in 2004, and they hold similar status in many other countries. They remain legal, however, in the United Kingdom and the wider European Union. Their use is prohibited by most sporting bodies.
Osteoblast cells are known to express creatine kinase. Bone growth factors such as IGF-1, PTH, and even Vitamin D seem to induce bone growth alongside increases in creatine kinase activity. Vitamin D has been noted to work indirectly by increasing the cellular energy state (these hormones increase creatine kinase in order to do so) in order to make bone cells more responsive to estrogen. This evidence, paired with enhanced growth rates of osteoblasts in the presence of higher than normal (10-20mM) concentrations of creatine suggest a role of creatine in promoting osteoblastic and bone growth, secondary to increasing energy availability.
Naturally produced in the kidneys, pancreas and liver, creatine is transported to muscle tissue where it is transformed into creatine phosphate, from which the energy molecule ATP is produced to regenerate the muscles' ability to contract and generate power during short-burst (anaerobic) activity. This translates to more productive workouts and faster muscle growth.
That being said, men aren’t the only ones who suffer from low testosterone levels. Women can also suffer from testosterone deficiency which can affect their overall well-being in addition to their sex drive. If you’re a woman or man concerned about your testosterone levels, in addition to using supplements like the ones below, you should contact your doctor who will be able to diagnose any deficiencies and recommend additional treatments.
However, don’t despair over the poor reviews. There are other ways to achieve a testosterone booster for muscle gain. One of them is simply Vitamin D. A deficiency in this vitamin can lower your levels of testosterone. Furthermore, you can get a very natural boost simply by weightlifting and engaging in HIIT (High Intensity Interval Training) exercise. In addition, you should avoid some foods like soy and alcohol which can lower testosterone levels (11). Through these natural solutions and lifestyle changes, you can influence your hormone profile, creating a balance that’s more favorable for muscle growth.
Creatine is a hydrophilic polar molecule that consists of a negatively charged carboxyl group and a positively charged functional group . The hydrophilic nature of creatine limits its bioavailability . In an attempt to increase creatines bioavailability creatine has been esterified to reduce the hydrophilicity; this product is known as creatine ethyl ester. Manufacturers of creatine ethyl ester promote their product as being able to by-pass the creatine transporter due to improved sarcolemmal permeability toward creatine . Spillane et al  analyzed the effects of a 5 days loading protocol (0.30 g/kg lean mass) followed by a 42 days maintenance phase (0.075 g/kg lean mass) of CM or ethyl ester both combined with a resistance training program in 30 novice males with no previous resistance training experience. The results of this study  showed that ethyl ester was not as effective as CM to enhance serum and muscle creatine stores. Furthermore creatine ethyl ester offered no additional benefit for improving body composition, muscle mass, strength, and power. This research did not support the claims of the creatine ethyl ester manufacturers.
One of the studies noting a reduction in fatigue in healthy subjects given creatine (8g) for five days prior to a mathematical test noted a relative decrease in oxygenation hemoglobin in the brain and an increase in deoxygenated hemoglobin, which normally indicates a reduction in cerebral oxygenation. The authors made note of how cytoplasmic phosphocreatine can increase oxygen uptake into cells (noted in vitro in a concentration dependent manner between 0-25mM) and suggested that either cells were taking up more oxygen from hemoglobin, or that increased mitochondrial efficiency resulted in less of a need for oxygen.
At the end of the day, you have to focus on how you feel. “Listen to your body,” says Davis. “It tells you when it needs a day off.” As a rule of thumb, take a rest day if your perceived pain is above a seven on a scale of 10, Davis advises. Or, focus on a different body part (say, if your legs are sore, focus on upper-body moves). Can't stop, won't stop—at least, till your next rest day.
In regard to the blood brain barrier (BBB), which is a tightly woven mesh of non-fenestrated microcapillary endothelial cells (MCECs) that prevents passive diffusion of many water-soluble or large compounds into the brain, creatine can be taken into the brain via the SLC6A8 transporter. In contrast, the creatine precursor (guanidinoacetate, or GAA) only appears to enter this transporter during creatine deficiency. More creatine is taken up than effluxed, and more GAA is effluxed rather than taken up, suggesting that creatine utilization in the brain from blood-borne sources is the major source of neural creatine. However, “capable of passage” differs from “unregulated passage” and creatine appears to have tightly regulated entry into the brain in vivo. After injecting rats with a large dose of creatine, creatine levels increased and plateaued at 70uM above baseline levels. These baseline levels are about 10mM, so this equates to an 0.7% increase when superloaded. These kinetics may be a reason for the relative lack of neural effects of creatine supplementation in creatine sufficient populations.