Sex hormones control weight and shape

October 6, 2013

John DiTraglia MD

Contributing Columnist

Testosterone (T) and estrogen are closely related steroid hormones. One makes you a boy and the other makes you a girl. Boys and girls grow up to be shaped differently. So it’s complicated.

It’s long been known that T levels in men go down as they get older and T levels have been decreasing in all men for some as yet unknown environmental reasons in addition to and separate from the aging reductions. Studies have shown that if your T levels are low, muscle mass and strength are reduced and fat is increased. Men with low levels of T report less sexual thinking and activity, and they have erectile dysfunction. If you give men with low levels of T replacement hormone you can fix all those problems. T treatment is prescribed for millions of men and the number is increasing rapidly. But estradiol, an estrogen, is also decreased if T is decreased because in men estradiol is mostly made from T. So maybe it’s not all about T.

To probe this question investigators from the endocrine department at the Massachusetts General Hospital in Boston did a study on 400 normal young men, 20 to 50 years of age, who had normal levels of T and agreed to let their sex hormones be manipulated for awhile.(1) They gave them all a hormone that suppressed their bodies’ production of T. They also gave about half of them an enzyme that prevented the conversion of T to estradiol. Then they replaced the T and estradiol at various doses in subgroups. So the groups were those who had low T, 2 levels of T replacement, and some with T replaced but low estradiol because they got the enzyme blocker. Then they watched what happened.

They found that there was a wide variation of sensitivity to T. In general fat increased at mildly low levels of T and muscle mass and strength decreased at more severe levels of deficiency. Sexual desire and erectile function were varied. However the effect on fat but not on muscle was caused by the manipulation of estradiol. Also they found that both T and estradiol were equal contributers to sexual function. Also there was some variation among individual men to the effects of these hormones. This story was reported on all the major news wires, especially the part about estrogen and belly fat.

So it’s complicated. And women are probably even more complicated. This is not about diet and exercise. Although giving extra T might increase your exercising and eating the converse is probably not true.

1. Finkelstein JS et al. Gonadal steroids and body composition, strength, and sexual function in men. N Engl J Med 2013;369:1011-22.