Context: Age-specific estimates of mean testosterone (T) concentrations appear to vary by year of observation and by birth cohort, and estimates of longitudinal declines in T typically outstrip cross-sectional decreases. These observations motivate a hypothesis of a population-level decrease in T over calendar time, independent of chronological aging.
Objective: The goal of this study was to establish the magnitude of population-level changes in serum T concentrations and the degree to which they are explained by secular changes in relative weight and other factors.
Design: We describe a prospective cohort study of health and endocrine functioning in randomly selected men of age 45–79 yr. We provide three data collection waves: baseline (T1: 1987–1999) and two follow-ups (T2: 1995–1997, T3: 2002–2004).
Setting: This was an observational study of randomly selected men residing in greater Boston, Massachusetts.
Participants: Data obtained from 1374, 906, and 489 men at T1, T2, and T3, respectively, totaling 2769 observations taken on 1532 men.
Main Outcome Measures: The main outcome measures were serum total T and calculated bioavailable T.
Results: We observe a substantial age-independent decline in T that does not appear to be attributable to observed changes in explanatory factors, including health and lifestyle characteristics such as smoking and obesity. The estimated population-level declines are greater in magnitude than the cross-sectional declines in T typically associated with age.
Conclusions: These results indicate that recent years have seen a substantial, and as yet unrecognized, age-independent population-level decrease in T in American men, potentially attributable to birth cohort differences or to health or environmental effects not captured in observed data.
What’s behind this, if true? The authors say that smoking and obesity have been factored out. Some other possibilities: xenoestrogens, for instance in soy: use of pornography; endocrine disruptors in plastics; perhaps even feminism with the displacement of men in the workplace. It’s been shown that diets high in saturated fat are better for T levels, so maybe the urging to eat less sat fat has had an effect.
For what it’s worth, at least one urologist believes that declining sperm counts are a myth. Not the same as T levels of course, but how certain is our knowledge of declining T levels?