Heat production in skeletal muscle stimulates the metabolic benefits of exposure to cold temperatures, according to new research, challenging models that hold adipose tissue responsible.
In peer review Comment sectionA team of Canadian researchers explains how muscles are the primary heat generator, and drive glucose and lipid metabolism when the temperature drops.
There has been considerable interest in cold exposure, particularly as a drug-free option for managing obesity and its metabolic complications, the authors note. More fat is burned during exercise in cold weather, and cold exposure uses both skeletal and brown muscles adipose tissue (the ‘good’ kind of fat) to increase energy expenditure.
However, the team writes“We highlight data showing that skeletal muscle is the primary thermogenic tissue in cold-exposed humans.”
Brown adipose tissue (BAT) in humans is linked to lower body mass indices and lower prevalence of type 2 diabetes, cardiovascular disease, and non-alcoholic fatty liver disease.
Still himself according to McMaster University endocrinologist Logan Townsend and his colleagues “a few (about 5 percent) adults have spontaneously detectable BAT under normal indoor environmental conditions.”
Our choice to be cozy reduces our need for thermogenesis, but it certainly does interpret the impact of BAT on difficult metabolic health.
Under cold conditions, BAT is detectable to varying degrees in almost all adults, and BAT consumes three times more oxygen per gram than cold-stimulated skeletal muscle.
“Since cold exposure is necessary to adequately stimulate human BAT,” the team writes“it is concluded that stimulation of BAT by regular cold exposure protects or reverses metabolic complications.”
However, people are not much BAT even in the cold, and studies show that it contributes to thermogenesis less than 1 percent of energy expenditure in cold-exposed adults.
Our skeletal muscles also use other methods, which account for about 50 percent of the energy we use during mild cold exposure. They have even found a way to generate heat while resting.
Thompson and his colleagues consider that the remaining energy expenditure in response to cold is likely to be related to numerous other systems of the body, such as the metabolic activity of the liver. And our ‘insulation’ white adipose tissue (WAT) uses energy to break down and rebuild fats in a process called the triacylglycerol fatty acid (TAG-FA) cycle.
Cold increases the use of glucose in skeletal muscle and lowers blood sugar in thin people and those with type 2 diabetes. In individuals with obesity and type 2 diabetes, insulin sensitivity increased about 43 percent after 10 days periodic cold; scientists say these effects are largely attributed to skeletal muscle glucose utilization.
Studies on mice exposed to cold indicates that BAT plays a major role in regulating lipid and glucose clearance, but the scientists suggest that they have very different levels of BAT from humans and that BAT is present in rodents regardless of their climate.
The team writes“BAT is unlikely to directly affect systemic metabolism in cold-exposed humans.”
They emphasize that BAT has a commitment as biomarker of reproductive tissue health and may open the door to preemptive disease diagnosis and treatment.
The proposal does not rule out cold as the primary stimulus for human BAT thermogenesis, nor does it mean that cold exposure is definitely beneficial to overall health. More research is needed in this area, and new approaches are recommended.
“A concerted effort is needed to move towards a more integrated perspective,” the authors conclusion“which simultaneously examines short thermogenic tissues, WAT and liver and places special emphasis on the organ most involved in heat production and consumption of circulating substrates in cold skeletal muscles.”
The article is published in Nature Metabolism.