Every athlete knows that his/her urine will be tested for drug1 use when they enter an important competition. To avoid detection of steroids in their urine, athletes will stop using the drugs well before the competition. Yet, in many cases, steroids can still be detected long after the athlete stops using them (even weeks later!). The reason for this lies in the chemical structure of the anabolic steroid2. As discussed above, anabolic steroids are very lipophilic3 molecules. This property makes it very difficult for the drug to remain long enough in the liver to be metabolized (inactivated) or in the kidney to be excreted in the urine (Figure 9). The lipophilic drug moves with its concentration gradient from the liver or the kidney cells right back into the bloodstream. Thus, it doesn’t get eliminated very well. Instead, it seeks out fat cells that exist in the athlete (these are much smaller than those found in the average couch potato!). The lipophilic steroid likes to enter fat cells, and with repeated use, the steroid accumulates there. When the user stops taking the steroid, the blood levels decline rather quickly in the absence of the drug. But now, the steroid concentration inside the fat cell becomes greater than that in the blood, so the concentration gradient reverses in the direction of fat cell-to-blood capillary. The fat cells are like a storage depot, releasing small amounts of steroid into the blood over time (via passive diffusion4). Eventually the steroid gets metabolized and makes its way to the kidney to be excreted in the urine. This explains why it is possible to detect small amounts of the steroid in the urine at competition weeks after the athlete stops using it.
1 a substance that affects the structure or function of a cell or organism.
2 synthetic versions of testosterone designed to promote muscle growth without producing androgenic effects. The better term is anabolic-androgenic steroid.
3 high lipid solubility. Lipophilic compounds dissolve readily in oil or organic solvent. They exist in an uncharged or non-polar form and cross biological membranes very easily.
4 the movement of a solute in its uncharged form to cross a membrane along a concentration gradient. No energy is required.
Figure 9 Most drugs are metabolized in the liver and then excreted by the kidney. With repeated use, steroids accumulate in fat cells. Once off the steroid, it enters the blood from the fat and it is then excreted.