Cold exposure has attracted much attention as a natural and drug-free method of treating obesity and its metabolic complications. It is commonly believed that brown adipose tissue (BAT) is responsible for the metabolic benefits of cold exposure, but a new study calls this view into question. A team of Canadian researchers from McMaster University emphasizes the role of skeletal muscle as a major heat generator and driver of glucose and lipid metabolism in response to cold exposure.
The role of skeletal muscle:
Contrary to popular belief, researchers have argued that skeletal muscles, not BAT, are the primary thermogenic tissue in people exposed to cold. Skeletal muscles play a critical role in increasing energy expenditure when exposed to cold. Not only do they burn more fat during exercise in cold weather, but they also release heat at rest. In fact, skeletal muscles account for about 50% of the energy expended during moderate cold exposure.
Limited BAT presence:
Although the presence of BAT has been associated with lower body mass indexes and reduced risk of metabolic diseases such as type 2 diabetes and cardiovascular disease, its presence in the human body is limited. Only about 5% of adults detect BAT in typical indoor environmental conditions. This scarcity makes it difficult to interpret the effects of BAT on metabolic health.
Importance of cold exposure:
Cold exposure is necessary for adequate BAT stimulation. Regular cold exposure can protect against or reverse metabolic complications. However, even at low temperatures, BAT is minimized in humans compared to rodents, and studies show that thermogenesis by BAT provides less than 1% of energy expenditure in cold-exposed adults. Therefore, a direct effect of BAT on systemic metabolism in humans is unlikely.
Other factors affecting energy expenditure:
The remaining energy expenditure in response to cold is likely related to other body systems. Metabolic activity of the liver and the energy expended by white adipose tissue (WAT) in the process of fat breakdown and recovery known as triacylglycerol-fatty acid (TAG-FA) cycling also contribute to total energy expenditure.
Effect on glucose metabolism:
Exposure to cold increases glucose consumption in skeletal muscles and lowers blood sugar levels in lean people and type 2 diabetics. In obese people with type 2 diabetes, intermittent cold exposure for 10 days increased insulin sensitivity by about 43%, mainly due to increased glucose utilization by skeletal muscles.
Although BAT is a promising biomarker of adipose tissue health and may aid in the diagnosis and treatment of disease, its direct effects on systemic metabolism in cold-exposed humans are limited. In contrast, skeletal muscle plays an important role in generating heat and stimulating glucose and lipid metabolism during cold exposure. Further studies are needed to fully understand the complex mechanisms underlying the metabolic benefits of cold exposure.