Estrogen is a regulator of fluid retention in both men and women. This effect appears to be mediated in part by changes in hypothalamic arginine vasopressin (AVP), the primary hormone involved in controlling water reabsorption in the kidneys. Increased levels of estrogen tend to increase AVP levels, which can promote the increased storage of water. Estrogen also appears to act on the renal tubes in the kidneys in an aldosterone- independent manner to increase the reabsorption of sodium. Sodium is the major electrolyte in the extracellular environment, and helps to regulate the osmotic balance of cells. Higher levels can significantly increase water in the extracellular compartment. Anabolic/androgenic steroids that either convert to estrogen, or possess inherent estrogenic activity, are, likewise, those steroids that are associated with increased extracellular water retention.
Estrogenic anabolic/androgenic steroids are generally favored for mass gaining (bulking) purposes. A steroid user may ignore water retention during this phase of training, occasionally even finding the sheer increases in size to be a welcome benefit. Estrogenic steroids such as testosterone and oxymetholone are also regarded as the strongest mass- and strength-building agents, which may be caused in part by anabolic benefits of elevated estrogenic activity. The excess water stored in the muscles, joints, and connective tissues is also commonly believed to increase an individual’s resistance to injury. With the use of many strongly estrogenic anabolic/androgenic steroids, water retention can account for a large portion (35% or more) of the initial body weight gain during steroid treatment. This weight is quickly lost once the steroids are discontinued or estrogenic activity is reduced.
Non-aromatizing steroids such as oxandrolone and stanozolol have also been shown to promote increased water retention, so this effect is not entirely exclusive to aromatizable or estrogenic substances. Anabolic steroids with low or no estrogenic action tend to produce modest increases in whole body water and intracellular fluid retention, however, and not in the visible extracellular compartment. These steroids are considered to be more cosmetically appealing, and are generally favored by bodybuilders and athletes when looking to improve lean mass and muscle definition. Popular anabolic/androgenic steroids that are associated with low visible water retention include fluoxymesterone, methenolone, nandrolone, oxandrolone, stanozolol, and trenbolone.
Excess water retention may be addressed with the use of ancillary medications such as the anti-estrogen tamoxifen citrate, or an aromatase inhibitor such as anastrozole. By minimizing the activity of estrogens, these drugs can effectively reduce the level of stored water. In most cases where an aromatizable steroid is used, aromatase inhibitors prove to be significantly more effective at achieving this goal. A common practice among bodybuilders during competition is to also use a diuretic, which can shed excess water by directly increasing renal water excretion. This is regarded as the most effective method for rapidly improving muscle definition, but it can also be one of the most acutely risky practices as well. Water retention is not a persistent side effect of steroid use. Excess water is quickly eliminated, and normal water balance returned, once anabolic/androgenic steroid administration is halted.
Wlliam Llewellyn (2011) - Anabolics