Seasonal and circadian variation in salivary testosterone in rural Bolivian men

✍ Scribed by Virginia J. Vitzthum; Carol M. Worthman; Cynthia M. Beall; Jonathan Thornburg; Enrique Vargas; Mercedes Villena; Rudy Soria; Esperanza Caceres; Hilde Spielvogel

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 244 KB
• Volume: 21
• Category: Article
• ISSN: 1042-0533
• DOI: 10.1002/ajhb.20927

✦ Synopsis

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 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 5 late winter, OTHER 5 other seasons). In Study A, of 115 T PM -T AM pairs, 51% 5 C CLASSIC , 39% 5 C REVERSE , 10% 5 C FLAT ; in Study B, of 184 T AM -T PM pairs, 55% 5 C CLASSIC , 33% 5 C REVERSE , 12% 5 C FLAT . Based on fitting linear mixed models, in both studies T OTHER-AM > T OTHER-PM (A: P 5 0.035, B: P 5 0.0005) and T OTHER-AM > T LW-AM (A: P 5 0.054, B: P 5 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 agerelated change in the male gonadal axis.