Seasonal and circadian variation in salivary testosterone in rural Bolivian men.

Testosterone (T) plays a key role in the increase and maintenance of muscle mass and bone density in adult men. Life history theory predicts that environmental stress may prompt a reallocation of such investments to those functions critical to survival. We tested this hypothesis in two studies of rural Bolivian adult men by comparing free T levels and circadian rhythms during late winter, which is especially severe, to those in less arduous seasons. For each pair of salivary T-AM/T-PM samples (collected in a similar to 12-h period), circadian rhythm was considered classic (C-CLASSIC) if T-AM > 110%T-PM, reverse (C-REVERSE) if T-PM > 110%T-AM, and flat (C-FLAT) otherwise. We tested the hypotheses that mean T-AM > mean T-PM and that mean T-LW < mean T-OTHER (LW = late winter, OTHER = other seasons). In Study A, of 115 T-PM-T-AM pairs, 51% = C-CLASSIC, 39% = C-REVERSE, 10% = C-FLAT; in Study B, of 184 T-AM-T-PM pairs, 55% = C-CLASSIC, 33% = C-REVERSE, 12% = C-FLAT. Based on fitting linear mixed models, in both studies TOTHER-AM > TOTHER-PM (A: P = 0.035, B: P = 0.0005) and TOTHER-AM > TLW-AM (A: P = 0.054, B: P = 0.007); T-PM did not vary seasonally, and T diurnality was not significant during late winter. T diurnality varied substantially between days within an individual, between individuals and between seasons, but neither T levels nor diurnality varied with age. These patterns may reflect the seasonally varying but unscheduled, life-long, strenuous physical labor that typifies many non-industrialized economies. These results also suggest that single morning samples may substantially underestimate peak circulating T for an individual and, most importantly, that exogenous signals may moderate diurnality and the trajectory of age-related change in the male gonadal axis. Am. J. Hum. Biol. 21:762-768, 2009. (C) 2009 Wiley-Liss, Inc.