Commentary—fat but fit…and cold? Potential evolutionary and environmental drivers of metabolically healthy obesity

Abstract

As global obesity rates continue to rise, it is important to understand the origin, role, and range of human variation of the body mass index (BMI) in assessing health and healthcare. A growing body of evidence suggests that BMI is a poor indicator of health across populations, and that there may be a metabolically healthy obese phenotype. Here we review the reasons why BMI is an inadequate tool for assessing cardiometabolic health. We then suggest that cold climate adaptations may also render BMI an uninformative metric. Underlying evolutionary and environmental drivers may allow for heat conserving larger body sizes without necessarily increasing metabolic health risks. However, there may also be a potential mismatch between modern obesogenic environments and physical adaptations to cold climates, highlighting the need to further investigate the potential of metabolically healthy obese phenotypes among circumpolar and other populations and the broader meaning for metabolic health. Lay Abstract Global obesity rates are on the rise; however, body mass index (BMI) is a poor indicator of metabolic health. Here we propose that cold climate populations may exhibit high BMIs, but relatively healthy metabolic profiles (metabolically healthy obesity) due to environmental and evolutionary pressures associated with inhabiting a cold climate.

Human adaptation to cold and warm climatic conditions: A comparison between two geographically diverse Indigenous populations

“…The presence of metabolically healthy obese phenotypes exerts a positive selective pressure on the Monpa, which protects them from developing hypertension, irrespective of their greater adiposity and body fat. Therefore, the Monpa are aptly covered by the phrase "fat but fit" (Ocobock & Niclou, 2022), like several circumpolar populations, who have obesity but cardiovascularly fit, as also evident from scholarly work on people from the United States of America (Duncan & Bogart, 2009).…”

“…BAT is a type of mitochondria dense fat, which has received a great deal of recent attention both for its ability to increase heat production during cold exposure and for how it may alter metabolic rate and circulating blood glucose and fatty acid levels (cf. Ocobock & Niclou, 2022). BAT is a strong thermogenic effector organ in homeothermic animals exposed to cold.…”

“…According to this mechanism, a large body mass is advantageous in cold conditions not only because it has a higher number of cells to produce more heat than a smaller body, but also because the relationship between volume and surface area is such that a larger body loses less heat per unit volume than a smaller body (Foster & Collard, 2013). In addition, the combination of larger body size and greater adiposity for retaining heat, higher resting metabolic rates (RMR), and higher BAT activity with the ability to modulate cardio‐metabolic health suggest that metabolically healthy obese phenotypes may be more frequent among cold climate populations (Ocobock & Niclou, 2022). More precisely, expansion of adipose depots can be driven either by the increase in adipocyte size (hypertrophy) or by the formation of new adipocytes through the process of adipogenesis (hyperplasia).…”

“…It has been observed that adipocyte expansion through adipogenesis can offset the negative metabolic effects of obesity and hence leads to metabolically healthy obesity, whereas hypertrophy precipitates into metabolically unhealthy obesity (Ghaben & Scherer, 2019). Thus, greater body size and body adiposity among Indigenous circumpolar populations like the Evenki, Buryat, and Yakut (Levy et al, 2018; Snodgrass et al, 2008), of north‐eastern Siberia and Inuit reindeer herders (Rode & Shephard, 1994) of North America, are considered metabolically healthy obese phenotypes, which is a crucial evolutionary adaptation to cold climates—as these features would increase metabolic heat production while also reducing heat loss to a cold environment (Ocobock & Niclou, 2022).…”

Extreme climatic events and human biology and health: A primer and opportunities for future research

“…It is the well‐being and health of indigenous populations living in these regions who will be most affected. Anthropologists have previously shed light on how changes in cold weather and climate change are interlinked with changing food systems, technology, and physical activity levels, which have implications for food insecurity, metabolic health, chronic health, and birth outcomes (Egeland et al, 2011; Leonard et al, 2002; Ocobock & Niclou, 2022; Snodgrass, 2013; Snodgrass et al, 2005; Sorensen et al, 2009; Young & Mäkinen, 2010).…”