The lower your body fat percentage, the more effective your testosterone-ester injections will be. Endocrinologists in Muenster, Germany, discovered this when doing research on hormone preparations for male contraceptives. Their article has been published in the Journal of Andrology.
The aim of male contraception is to get the testes to stop producing sperm. The classic way of achieving this is to halt the production of the hormones LH and FSH by the pituitary gland – and you do that by administering sex hormones. FSH and LH stimulate the testes to produce sperm and testosterone. If you add exogenous androgens, the brain thinks that there are already so many hormones circulating in the body that the testes don’t need to do anything, and then stops the production of the stimulating hormones – and thus the production of sperm.
So far, most attention has been focused on slow-release testosterone-esters, and the German research is no exception. The researchers gave forty male test subjects slow-release testosterone undecanoate. [Structural formula shown above.]
The researchers gave the test subjects a thousand milligrams of testosterone-ester in two doses, six weeks apart. The more body fat the men had, the lower the increase in concentration of testosterone in the blood from the injections. In the figure below the grey curve represents the testosterone level of men with less than ten kilograms of body fat. The black curve represents men with more than eighteen kilograms of fat.
The units not shown on the horizontal axis represent time spans of two weeks.
It would seem that body fat inhibits the rise in the blood testosterone level after injections are given. You might argue that fat men tend to be heavier than thin men, but according to the researchers, body weight plays only a minor role in the inhibitory effect of the fat reserves. You see the effect more clearly if you look at the free testosterone levels.
That fat men react less well to testosterone can also be seen from the effect of the injections on the LH levels. These decreased by less in the fatter men.
The researchers do not know how body fat makes testosterone less effective. A popular theory is that fat cells are capable of converting testosterone into estradiol. The Germans found no confirmation of this, however, as the fat men had similar estradiol levels in their blood to those of the slim men.
If fat men are to use testosterone-esters as a form of contraception, they will need higher doses, the researchers speculate. The testosterone injections may also take longer to become effective in fatter males. And the same will apply to chemical athletes.