The Role of Skeletal Muscles in Metabolism During Cold Exposure

The Role of Skeletal Muscles in Metabolism During Cold Exposure

A recent study challenges the prevailing notion that fat tissue alone is responsible for the metabolic benefits associated with exposure to cold temperatures. Researchers from Canada have conducted a peer-reviewed comment article explaining how skeletal muscles are the primary heat generators and drive the metabolism of glucose and lipids when temperatures drop.

The interest in cold exposure as a drug-free strategy for managing obesity and its metabolic complications has been growing steadily. It has been observed that exercising in cold weather leads to increased fat burning, while cold exposure activates both skeletal muscle and brown adipose tissue (BAT) to enhance energy expenditure. However, the research team highlights data indicating that skeletal muscle, not fat tissue, is the primary thermogenic tissue in humans exposed to cold temperatures.

Although BAT in humans has been linked to lower body mass indices and a reduced risk of type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, it is only detectable in a small percentage of adults under typical indoor ambient conditions. This scarcity of BAT makes it challenging to assess its influence on metabolic health. On the other hand, nearly all adults have detectable BAT to varying degrees when exposed to chilly conditions. Moreover, BAT consumes three times more oxygen per gram than cold-stimulated skeletal muscle.

While it is inferred that regular cold exposure stimulates BAT and protects against metabolic complications, humans have limited amounts of BAT even in cold temperatures. Studies show that BAT thermogenesis contributes to less than 1 percent of energy expenditure in adults exposed to cold. In contrast, the shivering mechanism in skeletal muscles utilizes ATP and myosin ATPase activity to produce heat. Skeletal muscles also employ other methods and account for approximately 50 percent of the energy used during mild cold exposure. They have even evolved to generate heat while at rest.

Other Factors Influencing Metabolism in Cold Conditions

The remaining energy expenditure in response to cold likely involves multiple other body systems, such as the metabolic activity of the liver. Additionally, white adipose tissue (WAT), which acts as an insulator, utilizes energy through a process called triacylglycerol-fatty acid (TAG-FA) cycling. Cold exposure increases glucose use in skeletal muscle and lowers blood sugar levels in lean individuals and those with type 2 diabetes. In individuals with obesity and type 2 diabetes, insulin sensitivity increased by approximately 43 percent after 10 days of intermittent cold exposure, mainly due to skeletal muscle glucose use.

Comparing Humans and Mice

While studies on mice suggest that BAT plays a significant role in regulating lipid and glucose clearance, it should be noted that mice have substantially higher levels of BAT compared to humans, and BAT is present in rodents regardless of their climate. Therefore, the researchers argue that BAT is unlikely to directly influence systemic metabolism in humans exposed to cold temperatures. Nevertheless, BAT shows promise as a biomarker for assessing the health of adipose tissue and may provide opportunities for early disease diagnosis and treatment.

Conclusion and the Need for Further Research

This study challenges the prevailing notion that fat tissue alone is responsible for the metabolic benefits associated with exposure to cold temperatures. It emphasizes the role of skeletal muscles as the primary thermogenic tissue in humans exposed to cold temperatures, surpassing the contribution of brown adipose tissue (BAT). While it does not conclusively prove that cold exposure is universally beneficial for overall health, it highlights the potential of skeletal muscles and their involvement in glucose and lipid metabolism during cold exposure.

Further research is needed to fully understand the mechanisms and impacts of cold exposure on metabolism. This new perspective suggests exploring different approaches to studying the effects of cold exposure on human health. By understanding the role of skeletal muscles and other factors influencing metabolism in cold conditions, we can develop targeted strategies for managing obesity, type 2 diabetes, and other metabolic complications. Cold exposure may continue to hold promise as a drug-free option for enhancing energy expenditure and improving metabolic health.

Health

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