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 approximately 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 T(OTHER-AM) > T(OTHER-PM) (A: P = 0.035, B: P = 0.0005) and T(OTHER-AM) > T(LW-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.
Vitzthum
Virginia J
VJ
Anthropology Department and Kinsey Institute for Research in Sex, Gender & Reproduction, Indiana University, Bloomington, Indiana 47405, USA. Vitzthum@Indiana.edu
Worthman
Carol M
CM
Beall
Cynthia M
CM
Thornburg
Jonathan
J
Vargas
Enrique
E
Villena
Mercedes
M
Soria
Rudy
R
Caceres
Esperanza
E
Spielvogel
Hilde
H
D000222:Adaptation, Physiological; D000328:Adult; D001838:Bolivia; D002940:Circadian Rhythm / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012424:Rural Population; D012463:Saliva / Q000737:chemistry; D012621:Seasons; D013739:Testosterone / Q000097:blood / Q000378:metabolism*; D055815:Young Adult
D013739:Testosterone