Leydig cell function in normal men: effect of age, life-style, residence, diet, and activity.

The decrease of plasma testosterone (T) in old age may be inherent to the aging process or secondary to environmental factors such as stress, minor illness, physical inactivity, etc. We determined plasma T, apparent free T, and gonadotropin levels in healthy men (n = 71), aged 26-90 yr living under identical conditions, i.e. as monks in a monastery. We found a highly significant age-dependent decrease in morning and mean 24-h T and apparent free T levels and a moderate increase in gonadotropin levels; the nycthemeral variation in plasma levels was, however, decreased in the elderly. In a larger group of healthy normal men (n = 302), smokers had higher T levels than nonsmokers in all age groups; values were similar whether older men lived in a geriatric institution or in their home. Diet had no discernable effect on T levels, whereas in young men plasma T levels decreased more in response to stress such as hypoglycemia or myocardial infarction than in elderly subjects. Moderate physical training in men, convalescing from myocardial infarction, did not alter consistently plasma T levels. We conclude that the age-associated decrease in plasma T levels is inherent to the aging process.