Elevated resting metabolic rates among female, but not male, reindeer herders from sub-arctic Finland

Objective: Elevated resting metabolic rates (RMR, kcal/day) are a well-established mechanism for maintaining core body temperature among cold climate populations. A high degree of interindividual variation has recently been noted among circumpolar populations. To further examine RMR variability, we investigated anthropometric and metabolic differences among reindeer herders from subarctic Finland. Methods: Resting metabolic rates, body mass, body composition, height, age, and sex were measured among 20 reindeer herders (5 females, 15 males, 20-64 years) from seven herding districts surrounding the Arctic Circle of Finland in January of 2019. Results: Females had a mean RMR of 1798 ± 216 kcal/day and males 1753 ± 503 kcal/day. When controlling for body mass and fat-free mass, females had significantly higher RMRs relative to males (P <.01). Contrary to previous cold climate population studies, measured RMR among males was not significantly different from predictive equation estimates (P >.05). However, predictive equations significantly underestimated female RMR by a mean of 25.2% ± 5.9% (P <.01). Conclusion: These results mirror earlier findings, though in ways previously unseen. In this population: (a) There is a high degree of RMR interindividual variability, but only among males, and (b) there is evidence for elevated RMR, but only among females. Though the sample size is small, preliminary results suggest the presence of sex-based differences in metabolic adaptations to cold climates within this population subset. Potential reasons for this sex-based difference are discussed including a presentation of a hypothesis about the dual role of thyroid hormone in both reproductive and metabolic processes.