Salivary Testosterone, Age, and Adiposity Associations Among Shuar Males in Amazonian Ecuador Challenge Assumptions...

Adult male testosterone concentrations in high income countries often decrease with age and adiposity, a pattern typically viewed as "normal." However, testosterone is expected to be adaptively regulated within the range of resource constrained, high pathogen, natural fertility conditions across which it evolved to function. We therefore examine associations among testosterone diurnal variation, age, and adiposity among Indigenous Shuar males of Amazonian Ecuador.

Morning and evening saliva was sampled over three consecutive days to capture diurnal testosterone variation (n = 104, ages 12-67), with one-time measures of adiposity (body fat, BMI). Multilevel models tested predicted associations. Average morning and evening testosterone ratio was calculated to assess diurnal variation, and regression analyses tested the association between this ratio and age.

Variation in testosterone concentrations at waking was apparent by age, with young males exhibiting the highest concentrations. Diurnal testosterone variation decreased with age (β = -0.006, p = 0.001). Significant age-by-BMI or percent body fat interactions were documented (p < 0.05). At lower adiposity levels, mean testosterone concentrations across the day were lowest at younger ages, highest in middle-aged participants, and slightly lower at older ages. At higher adiposity levels this pattern was reversed (for BMI) or attenuated (for percent body fat).

"Normal" testosterone levels are largely based on studies from high-income populations that do not account for diverse ecological conditions known to influence human physiology. This study complements others highlighting the complex relationships that exist among age, adiposity, and diurnal testosterone patterns in subsistence populations, suggesting socio-ecological regulation of testosterone.