To investigate the contribution of different metabolic tasks to child TEE in Shuar and industrialized populations, we modeled energy used in REE, diurnal REE fluctuation, physical activity, immune function, growth, and digestion.

Resting energy expenditure. Human REE is strongly related to FFM, specifically the size and tissue-specific metabolic rates of the internal organs (41). As the FFM of Shuar and U.S./U.K. children were indistinguishable (Table 1), we used the observed REE value for the U.S./U.K. sample (1042 kcal/day; Table 1) for both the Shuar and industrial TEE models.

Diurnal REE fluctuation. REE fluctuates throughout the day in a circadian rhythm, with its nadir near 06:00 and its peak near 16:00 (25). The mean diurnal difference between minimum and maximal adult REE is ~10% (25). We modeled an additional 10% increment of REE for the industrial TEE model (diurnal REE fluctuation increment = 0.1REE or 104 kcal/day). We hypothesize that diurnal fluctuation in REE is related to the diurnal production of regulatory metabolic hormones (e.g., cortisol and testosterone). In subsistence populations such as the Shuar and similarly in physically active populations, waking and diurnal salivary cortisol levels reduce as much as ~80% compared to industrialized populations (42). Thus, we modeled only a 2% increment in REE for the Shuar TEE model (diurnal REE fluctuation increment = 0.02REE or 21 kcal/day).

Immune function. Given the positive relationship between blood markers of immune activity and REE in both the present Shuar sample (Fig. 3) and in other Amazonian forager-horticulturalist populations (20), we assumed that the elevation of Shuar children’s REE relative to U.S./U.K. values was entirely due to greater infectious disease burden and resultant immune activity. This approach yields an immune function cost for Shuar children of 192 kcal/day. This estimate is similar to that reported for adult Tsimane, an Amazonian population that is ecologically and immunologically similar to the Shuar (7, 20).

Growth. Growth costs for the industrial TEE model were estimated by multiplying the mean rate of growth for U.S. children age 3 to 10 years old [7.2 g/day; (26, 43)] by the childhood-specific cost of synthesizing all new tissue [1.8 kcal/g; (12)]. This cost reflects the energy needed to synthesize new tissue and does not include the energy content of the new tissue itself, which is not captured in DLW measures of TEE. The cost of synthesizing new tissue for growth is thus 13 kcal/day for the industrial TEE model. On the basis of ~20% slower growth velocities among Shuar children (26), we estimated the growth cost for the Shuar TEE model as 10 kcal/day.

Physical activity. We estimated the energy cost of physical activity using the ratio of AEE to accelerometer-measured body movement among the Shuar sample. This approach assumes that AEE among the Shuar is entirely (or almost entirely) reflective of musculoskeletal activity, whereas AEE for the U.S./U.K. cohort is inflated by greater diurnal fluctuation in REE. For Shuar children, the ratio of AEE/mean accelerometer counts per minute (CPM; Table 1) yields 0.58 kcal/CPM. Mean body mass in the U.S./U.K. sample is 14% greater than the Shuar sample (Table 1), indicating that that the cost of movement for U.S./U.K. children should be 14% greater or 0.66 kcal/CPM. Last, we assumed that the efficiency of movement might be up to 5% greater for the Shuar due to their greater habitual physical activity (24), which yields a final U.S./U.K. cost of movement of 0.70 kcal/CPM. Multiplying this ratio by observed mean CPM for the industrial sample (379 CPM) yields a cost of physical activity of 264 kcal/day for the industrial TEE model.

Digestion. The energy cost of digestion (TEF) is closely related to TEE and was estimated as 0.1TEE (12) in both the Shuar and industrial TEE models.

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