It is well-documented that testosterone levels decline with age in men. After the age of 40 years, total T decreases on average -4 ng/dL ( -0.124 nmol/L) per year  or 1.6% per year , and bioavailable T by -2 to 3% per year.  In older men (over 60 years of age), the average decline in total testosterone levels has been found to be 110 ng/dL every decade.
However, the relative contributions of changes in health and lifestyle to that decline have not been adequately evaluated. A notable study was set out to investigate this…
The objective of this study was to establish the relative importance of aging, health, and lifestyle in contributing to the testosterone decline in aging men.
A prospective cohort study of health and endocrine functioning in randomly selected men with a baseline visit (T1, 1987-1989) and two follow-up visits (T2, 1995-1997; T3, 2002-2004) was conducted.
An observational study of men residing in greater Boston, Massachusetts, was conducted.
Participants included 1667 men aged 40 – 70 at baseline; follow-up was conducted on 947 (57%) and 584 (35%) at T2 and T3, respectively.
There were substantial declines in total serum T and FT levels associated with aging alone. However, many health and lifestyle changes were associated with accelerated decline.
When not taking health status into consideration, declines of -14.5% (range -16.3% to -12.6%) in total T and -27.0% (range -29.1% to -5.0%) free T per decade of aging.
The corresponding estimated trends including only subjects who were apparently healthy were less sharp; among such subjects, declines of -10.5% (range -14.0% to -7.0%) in total testosterone and -22.8% (range -26.9% to -18.7%) in free testosterone per decade of aging.
A 4- 5 kg/m2 increase in body mass index (BMI), for example a man 5’8 (1.75 m) gaining weight from 176 lb (80kg, BMI 26) to 202 lb (92 kg, BMI 30), was associated with a decline in total testosterone level comparable to that associated with approximately 10 years of aging.
This study, and many others [5-8], show that both chronological aging and changes in health and lifestyle factors are associated with declines in serum T. Co-morbidities and lifestyle influences may be as strongly associated with declining testosterone levels as is aging itself.
Another study confirmed these finding by showing that the effect of increasing body fat and waist size on lowering testosterone levels is more substantial, compared with that of age.  This later study also demonstrated that body fat/waist size is the most important determinant of the differences seen in total testosterone levels , regardless of age.  Accumulating research data underscores the major impact of body fat/waist size in determining testosterone levels.
The sharper decline in both testosterone levels in people with poor health, among whom obesity, metabolic syndrome, diabetes and cardiovascular disease is common, indicates that a substantial proportion of the apparent aging effect is attributable to changes in health status. Thus, the age-related testosterone decline can be partially prevented and/or slowed down by adherence to a healthy lifestyle incorporating a sound diet and regular exercise.
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