For a subset of individuals with available data on immune activation and anemic status ( n = 3189 individuals, 6719 observations), inclusion of ESR (a general indicator of inflammation), hemoglobin (Hb) (indicator of anemia), and leukocyte count (indicator of immune activation) accounts for additional variation in BT but does not affect the estimated rate of BT decline (table S5: model 6). The magnitude of BT decline over time was robust to other model specifications (table S5), including nonlinearity in the pattern of BT decline, considering finer-grained seasonal effects (e.g., by month instead of wet versus dry season), and restricting the sample to adults age 40+ years (table S5). All covariates are set to population mean. Shaded region outlined by dashed line denotes 95% CI. The rate of BT decline in the baseline model is −0.55☌ per decade ( P < 0.001), with men showing a slightly steeper rate of decline than women (−0.60☌ per decade, P date × sex < 0.01) ( Table 1: model 1). ![]() To assess temporal changes, we model BT as a function of measurement date using multilevel models adjusting for age, sex, height, and weight and including random intercepts for individual, community, and attending physician (see Materials and Methods). By the latest study period from 2012 to 2018, BT dropped 0.45☌ for women and 0.49☌ for men (tables S2 and S4). During the earliest study period from 2002 to 2006, unadjusted mean BTs for adult women and men were 37.02☌ and 36.94☌ (95% CI: 36.90 to 36.97), respectively ( Fig. Following ( 1), our analysis focused on changes in “habitual” BT by excluding cases with extreme measurements (39☌). BT is measured as a routine part of clinical exams performed by project physicians in Tsimane villages roughly annually or every other year ( n = 17,958 exams for 5481 individuals in 110 villages over 16 years tables S1 to S3, and fig. The Tsimane Health and Life History Project (THLHP) has been studying Tsimane health and lifestyle since 2002 with continuous biomedical surveillance ( 8). The Tsimane setting thus presents a rare opportunity to test whether a rural tropical population shows higher BT than commonly reported in high-income countries, whether BT has declined over time, potentially due to improved conditions, and if so, whether BT decline over time is associated with a lower infectious burden. Government-sponsored vaccination campaigns and improved access to social security income and medical services (e.g., hospitals and pharmacies) are among some of the other changes accompanying epidemiological transition over the 16-year study period. Health conditions have been changing throughout rural Bolivia, including the Tsimane territory, over the past two decades with improved public health infrastructure and access to markets. At the same time, the rapid pace of improvements in health and lifestyle over recent decades may be lowering BT, as reported in the United States ( 1). We also predict higher BT among infected than noninfected Tsimane. ![]() These conditions lead us to predict higher normal BT among Tsimane than residents of high-income countries. Tsimane also exhibit endemic polyparasitism, including various helminths ( 6), and helminthic infection is associated with higher resting metabolism in this population ( 7). ![]() Infections, particularly respiratory infections, are responsible for roughly half of all Tsimane deaths, and life expectancy at birth was in the low 40s until the late 20th century, before increasing by over a decade by the turn of the century ( 5). Tsimane live a subsistence lifestyle without access to running water or sanitation and experience high exposure to diverse pathogens, indicated by elevated immune activation biomarkers throughout life. Here, we test this hypothesis in a population-representative cohort of Tsimane Amerindians of the Bolivian Amazon using 16 years of mixed longitudinal data. BT is an easy-to-measure vital sign of human health, and thus, understanding factors affecting BT variation is of global public health interest. Yet, it is not known whether BT <37.0☌ is normal outside of high-income countries, nor have the contributions of infection and immune activation to temporal change in BT been extensively tested. ![]() One hypothesis is that historical reductions in infectious disease lowered both systemic inflammation and energetic costs of immune responses during the 19th and 20th centuries, resulting in lower BT ( 1). This effect was robust to potential confounders such as demographics, ambient temperature, time of day, and comorbidities. cohorts spanning almost two centuries revealed a 0.03☌ decline per decade in normal body temperature (BT) of adults, leading the authors to infer that “humans in high-income countries currently have a mean body temperature 1.6% lower than in the pre-industrial era” ( 1).
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