Unlike popular belief, not only the youth, but also older men with lower sex drives are likely to feel morose and depressed, according to a new study.

Depression affects between two and five per cent of the population at any given time, the study said. Women are more likely to be depressed until the age of 65, when physical differences between the stronger and the weaker sex all but disappear. Studies suggest sex hormones might be responsible for this phenomenon.

Osvaldo P. Almeida of the University of Western Australia and his colleagues based their study on a survey of 3,987 men aged 71 to 89. Between 2001 and 2004, the men completed reported on their demographics and health history.

They underwent testing for depression and thinking, learning and memory difficulties. Information about physical health conditions was obtained from a short survey and an Australian health database.

Researchers also collected blood samples from the participants and recorded levels of total testosterone and free testosterone, which are not bound to proteins.

A total of 203 of the participants (5.1 per cent) met the criteria for depression. They had significantly lower total and free testosterone levels then men who were not depressed.

After controlling for other factors - education level, body mass index and cognitive scores - men in the lowest quintile (20 percent) of free testosterone concentration had three times the odds of having depression compared to men in the highest quintile.

The mechanism by which low hormone levels might affect depression risk has not been identified, but might involve changes in the levels of neurotransmitters or hormones in the brain, the authors note.

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