FRIDAY, Oct. 27 (HealthDay News) -- The testosterone-fueled American male may be losing his punch.

Over the past two decades, levels of the sex hormone in U.S. men have been falling steadily, a new study finds.

For example, average total testosterone levels in men aged 65 to 69 fell from 503 nanograms/decileter (ng/dL) in 1988 to 423 ng/dL in 2003.

The reasons for this trend are unclear, said researchers at the New England Research Institutes in Waterdown, Mass. They noted that neither aging nor certain other health factors, such as smoking or obesity, can fully explain the decline.

"Male serum testosterone levels appear to vary by generation, even after age is taken into account," study lead author Thomas G. Travison said in a prepared statement.

Testosterone is the primary male sex hormone and plays an important role in maintaining bone and muscle mass. Low testosterone levels have been linked to health problems, including lowered libido and diabetes.

It's normal for men's testosterone levels to peak in their late 20s and then start to gradually decline, experts say. But this study found that overall testosterone levels are lower than they were 20 years ago.

"In 1988, men who were 50 years and older had higher serum testosterone concentrations than did comparable 50-year-old men in 1996. This suggests that some factor other than age may be contributing to the observed declines in testosterone over time," Travison said.

He and his colleagues analyzed blood samples -- along with health and other information -- from about 1,500 men in the greater Boston area who took part in the Massachusetts Male Aging Study. That study collected data in 1987-89, 1995-97, and 2002-04.

"This analysis deals with men who were born between 1915 and 1945, but our baseline data were not obtained until the late 1980s, when the elder subjects were about 70 years old, and the youngest about 45," Travison said.

"Events occurring in earlier decades could certainly help explain our results, if their effects persisted into recent years," he noted.

The findings were published in the Journal of Clinical Endocrinology and Metabolism.

More information

The U.S. National Library of Medicine has more about testosterone.

-- Robert Preidt

SOURCE: Endocrine Society, news release, Oct. 24, 2006

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