Measures of bioavailable serum testosterone and estradiol and their relationships with muscle strength, bone density, and body composition in elderly men
In the present cross-sectional study of 403 independently living elderly
men, we tested the hypothesis that the decreases in bone mass, body
composition, and muscle strength with age are related to the fall in
circulating endogenous testosterone (T) and estrogen concentrations. We
compared various measures of the level of bioactive androgen and estrogen
to which tissues are exposed. After exclusion of subjects with severe
mobility problems and signs of dementia, 403 healthy men (age, 73-94 yr)
were randomly selected from a population-based sample. Total T (TT), free
T (FT), estrone (E1), estradiol (E2), and sex hormone-binding globulin
(SHBG) were determined by RIA. Levels of non-SHBG-bound T (non-SHBG-T), FT
(calc-FT), the TT/SHBG ratio, non-SHBG-bound E2, and free E2 were
calculated. Physical characteristics of aging included muscle strength
measured using dynamometry, total body bone mineral density (BMD), hip
BMD, and body composition, including lean mass and fat mass, measured by
dual-energy x-ray absorptiometry. In this population of healthy elderly
men, calc-FT, non-SHBG-T, E1, and E2 (total, free, and non-SHBG bound)
decreased significantly with age. T (total and non-SHBG-T) was positively
related with muscle strength and total body BMD (for non-SHBG-T,
respectively, beta = 1.93 +/- 0.52, P < 0.001 and beta = 0.011 +/- 0.002,
P < 0.001). An inverse association existed between T and fat mass (beta =
-0.53 +/- 0.15, P < 0.001). Non-SHBG-T and calc-FT were more strongly
related to muscle strength, BMD, and fat mass than TT and were also
significantly related to hip BMD. E1 and E2 were both positively,
independently associated with BMD (for E2, beta = 0.21 +/- 0.08, P <
0.01). Non-SHBG-bound E2 was slightly strongly related to BMD than total
E2. The positive relation between T and BMD was independent of E2. E1 and
E2 were not related with muscle strength or body composition. In summary,
bioavailable T, E1, total E2, and bioavailable E2 all decrease with age in
healthy old men. In this cross-sectional study in healthy elderly men,
non-SHBG-bound T seems to be the best parameter for serum levels of
bioactive T, which seems to play a direct role in the various
physiological changes that occur during aging. A positive relation with
muscle strength and BMD and a negative relation with fat mass was found.
In addition, both serum E1 and E2 seem to play a role in the age-related
bone loss in elderly men, although the cross-sectional nature of the study
precludes a definitive conclusion. Non-SHBG-bound E2 seems to be the best
parameter of serum bioactive E2 in describing its positive relation with
BMD