Female Hormone Plays Role In Male Sex Drive

New research suggests oestrogen, not just testosterone, plays a role in growing bellies and lower sex drive in middle-aged men.

Oestrogen, the female sex hormone, may play a bigger role than previously thought in explaining how middle-aged men see their bellies grow and sex drive decrease.

So far these symptoms, associated with midlife crisis, have been blamed almost entirely on declining levels of testosterone, the male sex hormone.

But a new study suggests a role for oestrogen - a finding that may guide development of sex-drive treatments.

"A lot of things we think are due to testosterone deficiency are actually related to the oestrogen deficiency that accompanies it," said Dr Joel Finkelstein of Massachusetts General Hospital.

He led the study, published in the New England Journal of Medicine.

Men's bodies convert some of the testosterone they produce into oestrogen, and levels of both decline with age.

The study concludes that both hormones are needed for libido and that lower oestrogen may be responsible for middle aged men's expanding waistlines.

The research involved 400 healthy male volunteers, aged 20 to 50.

They were given drugs to temporarily reduce their testosterone production to pre-puberty levels.

Then they were given various doses of testosterone gel or placebo, but half the men were also given a drug to prevent testosterone's conversion into oestrogen.

This allowed researchers to assess the different effects of the hormones and of having testosterone but no oestrogen.

After 16 weeks, researchers saw that muscle size and strength depended on testosterone, while body-fat mass depended on oestrogen.

Both hormones were needed to maintain normal sex drive and performance.

Doctors say the finding is surprising but stress that more research is needed.

The new study, for example, was too short to see long-term benefits or risks, such as the effect of testosterone supplements on the heart, mental sharpness, prostate enlargement or cancer.