High resting metabolic rate among Amazonian forager‐horticulturalists experiencing high pathogen burden

Abstract

Objectives Resting metabolic rate (RMR) reflects energetic costs of homeostasis and accounts for 60-75% of total energy expenditure (TEE). Lean mass and physical activity account for much RMR variability, but the impact of prolonged immune activation from infection on human RMR is unclear in naturalistic settings. We evaluate the effects of infection on mass-corrected RMR among Bolivian forager-horticulturalists, and assess whether RMR declines more slowly with age than in hygienic sedentary populations, as might be expected if older adults experience high pathogen burden. Materials and Methods RMR was measured by indirect calorimetry (Fitmate MED, Cosmed) in 1,300 adults aged 20-90 and TEE was measured using doubly labeled water (n= 40). Immune biomarkers, clinical diagnoses and anthropometrics were collected by the Tsimane Health and Life History Project. Results Tsimane have higher RMR and TEE than people in sedentary industrialized populations. Tsimane RMR is 18-47% (women) and 22-40% (men) higher than expected using six standard prediction equations. Tsimane mass-corrected TEE is similarly elevated compared to Westerners. Elevated leukocytes and helminths are associated with excess RMR in multivariate regressions, and jointly result in a predicted excess RMR of 10-15%. After age 40, RMR declines by 69 kcal/decade (p<0.0001). Controlling for lean mass and height accounts for 71% of age-related RMR decline, and adding indicators of infection minimally affects the age slope. The residual level of age-related decline from age 40 is 1.2% per decade. Conclusion High pathogen burden may lead to higher metabolic costs, which may be offset by smaller body mass or other energy-sparing mechanisms.

Immune function in Amazonian horticulturalists

“…Tsimane also have high resting metabolic rates and total daily energy expenditures, as measured by doubly labeled water and respirometry (Gurven et al, 2016). All of these characteristics are likely to relate to the allocation of resources towards immune function, as additional energy is used and stored for maintenance rather than growth.…”

Growth references for Tsimane forager‐horticulturalists of the Bolivian Amazon

“…On the one hand, a clinical model might argue that any growth deficit relative to standards is evidence of suffering, and a failure of a child to “adapt” to disease. On the other, an adaptationist perspective might suggest that small stature can be adaptive due to reduced energy demands, improved thermoregulation, or faster maturation (Stinson, 1990; Walker et al, 2006; Migliano et al, 2007), or that overall smaller stature in Amazonians may represent an adaptation to high pathogen load, and the need to prioritize energy for immune defenses (Blackwell et al, 2016; Gurven et al, 2016b; Stieglitz et al, 2016; Urlacher et al, 2016a)…”

Cardiovascular disease and type 2 diabetes in evolutionary perspective: A critical role for helminths?

“…In addition to interfering with host nutrition through altering consumption and absorption, helminths can also affect blood glucose and lipid levels directly. Many pathogens rely on blood glucose for energy [128, 129] and pathogen-induced immune activation is costly, requiring significant increases in resting metabolic rate and glucose utilization [38, 130, 131]. Increasing evidence suggests that host lipids are manipulated by, and allocated to pathogens.…”

“…Hookworm infection and higher eosinophil counts are also associated with lower systolic BP. Resting metabolic rate is also higher among adults with active helminth infection [38]. …”

“…Intestinal helminths are lipophilic and absorb lipids either from their host’s gut contents or blood stream, either of which could reduce their host’s circulating lipids and thereby minimize accumulation of plaques in vasculature [36]. Additionally, endo-parasites like helminths represent a significant energetic cost from immune activation that increase resting metabolic rates, further limiting adipose tissue deposition [37, 38]. These helminths were likely a prevalent source of infection over human evolutionary history.…”