Ageing, surprisingly, may have little to do with a gradual drop in testosterone levels, but is more likely to result from a man's behavioural and health changes, says a new study.

"It is critical that doctors understand that declining testosterone levels are not a natural part of aging and that they are most likely due to health-related behaviours or health status itself," said study co-author Gary Wittert, professor of medicine at the University of Adelaide, Australia.

"Men who had declines in testosterone were more likely to be those who became obese, had stopped smoking or were depressed at either clinic visit," Wittert said.

"While stopping smoking may be a cause of a slight decrease in testosterone, the benefit of quitting smoking is huge," added Wittert. This hormone is important for many bodily functions, including maintaining a healthy body composition, fertility and sex drive.

Few population-based studies have tracked changes in testosterone levels among the same men over time, as their study did, Wittert said, according to an Aadelaide statement.

Wittert and his co-authors analysed testosterone measurements in more than 1,500 men recorded at two clinic visits five years apart. All blood testosterone samples underwent testing at the same time for each time point, said Wittert.

Researchers included 1,382 men in the data analysis, aged between 35 and 80 years, averaging 54 years, after screening out those who were taking medicines or had medical conditions known to affect hormones.

On average, testosterone levels did not decline significantly over five years; rather, they decreased less than one percent each year, the authors reported. However, when the investigators analysed the data by subgroups, they found that certain factors were linked to lower testosterone levels at five years than at the beginning of the study.

Unmarried men in the study had greater testosterone reductions than did married men. Wittert attributed this finding to past research showing that married men tend to be healthier and happier than unmarried men. "Also, regular sexual activity tends to increase testosterone," he explained.