Changes in BMI modulate age-associated changes in sex hormone binding globulin and total testosterone, but not bioavailable testosterone in young adult men: the CARDIA Male Hormone Study.

OBJECTIVES

To compare age-associated 8-year changes in total testosterone, calculated bioavailable testosterone and sex hormone binding globulin (SHBG) across five groups of men stratified according to change in body mass index (BMI) (i.e., BMI stable (+/-0.69 kg/m(2)), decreased (-0.7 kg/m(2)), increased minimally (0.7-1.74 kg/m(2)), increased moderately (1.75-3.19 kg/m(2)) and increased most (> or =3.20 kg/m(2))).

DESIGN

Eight-year longitudinal cohort study.

SUBJECTS

Four hundred and seventy-four black and 695 white men, aged 24-31 years at the time of the first hormone measurement.

MEASUREMENTS

Aging-related changes in serum SHBG, total testosterone and bioavailable testosterone.

RESULTS

SHBG significantly increased with age for men whose BMI decreased, and there were progressively smaller increases for men whose BMI was stable, or whose BMI increased minimally or moderately (range 1.1-0.3 nM per year, P< or =0.03, respectively). There was no age relationship with SHBG among men whose BMI increased most. Total testosterone did not change with age for men whose BMI decreased, was stable or increased minimally, but for men whose BMI increased moderately and most there was a graded decrease in total testosterone with age (beta=-0.2 and -0.4 nM per year, respectively, P< or =0.005). However, bioavailable testosterone decreased with age to a similar extent across all groups.

CONCLUSIONS

These results suggest that changes in BMI during young adulthood modulate age-related changes in SHBG and total testosterone, but not bioavailable testosterone.