There are many ways to stay warm on a cold day. We can seek shelter, turn up the thermostat, and huddle close together. Obvious physical adaptations to cold include fur, feathers, and clothing in the case of humans…and some pampered dogs.
Blood vessels near the skin may also constrict to prevent heat loss and some animals develop a layer fat under the skin that acts like insulation. We may also shiver to generate heat. But did you know that many animals can create their own heat by burning fuels they eat through a process called thermogenesis? This topic was the subject of a recent review published in Physiology.
Mitochondria are tiny organelles within each of our cells that are responsible for converting the foods we eat into ATP, the energy our cells need to function. With cold exposure, however, this process may become uncoupled resulting in the production of heat instead of ATP.
The thermoneutral zone for humans (without clothing and other comforts) is thought to be about 30-32°C. In this environmental temperature range, we do not need to make or lose heat for our body temperature to remain constant. Social norms, however, keep most of us clothed and feats of engineering have enabled humans to spend much of their time in controlled temperature environments around 22°C, which is much cooler than our thermoneutral zone. We compensate by wearing clothing. Despite their fur coat, laboratory mice also spend much of their time in environmental temperatures that are much cooler than their thermoneutral zone (28-34°C). Mice also have up to 10 times more brown adipose tissue than humans (2-5% vs 0.1-0.5%). Brown adipose tissue contains a lot of mitochondria that are uncoupled so these mice can generate much more of their own heat. Studies have also shown that these mice burn twice as much energy at 23°C than at 33°C. This may explain why nude mice weigh less than standard laboratory mice and why it is difficult to get them to gain excess weight on a diet that promotes obesity in their furry cousins.
Piglets are naturally very sensitive to cold temperatures as they do not appear to generate heat through mitochondrial uncoupling. This is because they are missing an uncoupling protein that turns on the heat generating process in other animals. The lack of this protein helps the animals stay warm by creating more insulation. Contrary to what you might be thinking, this protein was lost prior to the domestication of pigs. Similar to pigs, deletion of this protein also promotes obesity in mice when they are in their thermoneutral zone. If the mice are kept at cooler temperatures, however, this obesity-promoting effect is lost as they simply burn the excess calories.
What if we could harness the power of mitochondrial uncoupling to create heat as a way for humans to burn excess calories and lose weight? Could simply lowering the thermostat (and not putting on a sweater) help?
AJ Kowaltowski. Cold exposure and the metabolism of mice, men, and other wonderful creatures. Physiology. 37(4), 187-196, 2022.