Doctoral Dissertation Improvement: Conceptualizing the Adaptive Significance of Brown Adipose Tissue in Cold Climates

Background: Circumpolar regions present extreme environmental stresses. An elevated resting metabolic rate (RMR) and efficient non-shivering thermogenesis (NST) are central biological adaptations to cold climates and are characteristic of human circumpolar populations; however, the mechanisms that dictate these metabolic adaptations are poorly understood. Biological anthropology recognizes that adaptation occurs on multiple levels – homeostatic response, long-term acclimatization, developmental plasticity, and selection of genotypes. These adaptive modes are sensitive to different timescales of environmental change. The timescales of adaptation framework hypothesizes that each mode of adaptation represents a sequence of continuous evolutionary change where less durable, more plastic modes of adaption are eventually superseded by more durable and efficient mechanisms, such as genetic accommodation. Evidence suggests that adaptations to cold stress may be governed by each of these timescales. Within the past decade, researchers have discovered that brown adipose tissue (BAT), a form of fat that is highly thermogenic, is present among adults. This study investigates multiple levels of adaptation to cold stress by testing the following hypotheses regarding variation in adult BAT heat production. H1: BAT plays a role in metabolic adaptation via its function in homeostatic response, long-term acclimatization and developmental adaptation to cold stress. H2: As the duration of exposure to cold stress increases, the adaptive modes exhibited by individuals and populations will shift from transient and reversible adaptive modes toward more durable adaptive modes. Methods: In order to characterize multiple timescale of adaptation, the proposed study will compare groups living in Siberia that have been exposed to cold stress over various time depths. Data will be collected in the village of Berdygestiakh in order to compare individuals that are exposed to different daily amounts of cold stress. Also, data will be collected in the city of Yakutsk in order to compare individuals that were exposed to differing cold stress during infancy and childhood. Furthermore, the Yakutsk sample will compare participants that are ethnically Yakut, an indigenous Siberian group, to European-Russians in order to characterize the effect of differing time depths of ancestral exposure to cold stress. BAT activity will be quantified using infrared thermal imaging of the neck and shoulders. RMR will be measured using indirect calorimetry. Participants will wear temperature loggers for 72 hours in order to quantify average daily cold exposure. Anthropometric measurements will be taken using standardized techniques. Finally, a modified survey will be used to collect data on lifestyle, socioeconomic status and early-life cold exposure. Intellectual Merits: The “phenotype-first” model of evolution posits that phenotypic plasticity responds to a novel stressor on a short timescale, and thus facilitates the future evolution of more durable, intergenerational mechanisms, such as genetic adaptation, that act on a longer timescale. The proposed study tests this hypothesis by identifying and describing the relationship between multiple timescales of metabolic adaptation to cold stress via BAT heat production. Broader Impacts: The proposed project will promote teaching, training and learning by engaging university students in Siberia in biological anthropology concepts and methods. The study advances a new methodology for quantifying BAT activity that is inexpensive, portab