White fat is detested in modern societies as unaesthetic and a threat to health. Researchers are discovering ways that white fat gets changed into beige fat that burns energy rather than storing it.
Why Children Seem Not to Feel the Cold
Small mammals can be very good at surviving under extremely cold conditions. One case is provided by the harp seal, which is born onto polar ice and prospers nonetheless. This feat is accomplished thanks to brown fat, a tissue that is specialized for rapidly generating heat. This tissue is found close to vital organs. It uses a metabolic trick to produce heat faster than other tissues.
Brown fat is functional in all young mammals, including humans. That is why children can spend long periods playing in the snow without complaining about the cold. So, although fat is normally thought of as a store of energy, brown fat is specialized for burning fat and releasing heat (thermogenesis).
Brown Fat and Retaining a Slender Body
From the time that they are independent of their mothers, young mammals maintain a slender body that helps them to move quickly and elude predators. This resistance to weight gain is due to having active brown fat. Brown fat is activated by cold, which stimulates the sympathetic nervous system that innervates the tissue.
The sympathetic nervous system is also activated by playing with other youngsters, which often involves faked attacks, escapes, and locomotor rotational movements. Each of these sudden movements seems designed to stimulate the sympathetic nervous system (1).
Researchers are finding that brown fat is functional in human adults and that it plays an important role in defending them from obesity. Brown fat is also protective against heart disease and secondary diabetes. Not all adults have functioning brown fat, but recent work suggests that ordinary white fat can be transformed into a heat-burning tissue known as “beige fat.”
How Beige Fat is Activated by Exercise
There are two behavioral mechanisms through which white fat is changed into thermogenic beige fat. One is physical activity, or exercise. When a person is physically active, their sympathetic nervous system is aroused and this produces changes in fat cells that involve expression of more mitochondria and increased generation of heat.
The other method involves exposure to cold, which also stimulates the sympathetic nervous system and increases heat generation in beige fat. Experimental participants were immersed in cold water.
These results are very exciting because they suggest behavioral interventions that can convert white fat to beige fat. In so doing, it could help the overweight to retain a healthier level of body fat. As always, the question is whether such measures can produce a large enough effect to help those who are very overweight. This question is particularly salient given the emergence of a new class of weight-loss drugs that are producing greater weight loss than previously achieved.
Turning Down the Thermostat
There is nothing new in the conclusion that exercise is an effective ingredient in any weight-loss attempt. What is new is the mechanism of weight loss through brown or beige fat. These mechanisms provide a possible explanation for the long-baffling phenomenon of some individuals resisting weight gain in our over-nourished societies whereas others steadily accumulate fat. It could be that the former have thermogenically active fat that prevents them from gaining weight.
Just as physically active populations, like farm laborers of a bygone era, retained a slender body build despite eating more than contemporary people, it is possible that people had greater exposure to cold in earlier times and that this both preserved the function of specialized brown fat from childhood and activated heat production in ordinary white fat, changing it to beige fat.
This raises a question about whether the warm conditions maintained in most modern homes and workplaces are optimal for our health. Perhaps turning down the thermostat would stimulate brown fat and help us to cope with the obesity epidemic that is arguably the biggest health problem facing developed countries. This is true whether one studies healthcare costs or analyzes the years of healthy life lost to heart disease, secondary diabetes, kidney disease, and all the other ailments falling under the rubric of obesity-related diseases.
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