Age-related changes in testosterone and SHBG among Turkana males

To determine age-related changes in biologically available testosterone (T) among men in a subsistence society and their relationship to energetic status, T, sex hormone binding globulin (SHBG) and anthropometric measures were compared among nomadic and settled Turkana pastoralists of northern Kenya. Hormonal measures were available for 104 nomadic men and 72 settled men, estimated ages 20-90 years. Comparison of the two subpopulations revealed significantly higher blood T (32.7+/- 15.1 vs. 23.4+/-15.2 nM) and SHBG (53.8+/- 19.5 vs. 39.7+/- 20.nM) but not free testosterone index (FTI) (65.6+/- 39.3 vs.66.3+/- 45.9) among the nomads. Total blood T did not exhibit a significant linear decline with age in either subgroup, while SHBG values showed a significant linear increase among the nomads. When controlled for energetic status, FTI showed a significant decrease with age among the nomads, but not the settled males. Total blood T was negatively associated with waist circumference among the nomads, but not the settled males. FTI showed a marginally significant negative association with waist circumference, suprailiac skinfold, and % body fat among the nomads but no associations with body composition among the settled group. These results add additional evidence that T is related to energetic status under conditions of negative energy balance and suggest that cross-population variation in the slope of age-related declines in free serum T and salivary T may be related to energetic status through the effects of SHBG.