Just as there’s good cholesterol and bad cholesterol, good carbs and bad carbs, there’s also good fat. Whereas white fat stores energy, padding our hips, thighs, arms and bellies, brown fat — studded with energy generators known as mitochondria – burns energy. Newborns have a ring of brown fat around their necks, helping them stay warm. By adulthood, it’s detectable in only 3 percent of men and 7.5 percent of women, with higher rates among younger and thinner people.
In a study released yesterday by the Journal of Pediatrics, positron emission tomography (PET) scanning reveals the presence of brown fat in nearly half of children, though its level of energy-burning activity varies.
PET, which uses nuclear medicine tracers, has long been used in cancer patients, picking up “hot spots” of energy consumption in cells that can indicate active tumor growth. But some hot spots turn out to be deposits of energy-burning brown fat — and PET offered an opportunity to study it.
Children’s Hospital Boston radiologist Laura Drubach, of Children’s program in Nuclear Medicine and Molecular Imaging, and Aaron Cypess, of Joslin Diabetes Center, looked at PET scans from 172 cancer patients at Children’s, and found that brown fat, rather than declining after the newborn period, actually increases in amount until it peaks at age 13-15, after which it begins declining. They also show that the leaner the child, the more metabolically active his or her brown fat is. Unlike the gender difference in adults, boys and girls and similar levels of brown fat activity.
“Increasing the amount of brown fat in children may be an effective approach at combating the ever increasing rate of obesity and diabetes in children,” Cypess says.
But here’s what Cypess calls the billion-dollar question: Do lean children have more active brown fat because they are lean, or does the presence of brown fat keep them lean? If the latter is true, then strategies that encourage more brown fat cells to develop – perhaps by manipulating genes or progenitor cells — or that make brown fat more active, could offer a whole new way of treating obesity.
Some experiments have tried manipulating genes or fat progenitor cells to create more brown adipocytes. Some drugs may ramp up their activity. A low-tech approach might be to simply turn down the thermostat – in other mammals, brown fat plays an important role in adapting to cold temperatures. In a study from the Netherlands, young men showed no brown fat activity on PET-CT scans until they spent two hours in a cold room. Similarly, Drubach and colleagues showed in 2009 that warming patients before scanning them reduced detectability of brown fat from 31 percent of children to 5 percent.
What’s cool about brown fat is that a little goes a long way, burning huge numbers of calories. But energy balance is a complicated affair and the body has many redundant mechanisms for adjusting energy storage and expenditure, as well as appetite – which is why it’s hard to maintain weight loss.
Children, with their relatively robust stores of brown fat, and the clear inverse relationship between brown-fat activity and body mass, may prove to be the missing link in understanding brown fat’s role in a healthy energy balance – before we start any misguided tinkering. “We believe that the ability to non-invasively evaluate brown fat activity in vivo with PET imaging provides a better understanding of its prominent role in pediatric physiology, and may possibly provide insights into the treatment of childhood obesity,” Drubach says.
Drubach LA, Palmer EL 3rd, Connolly LP, Baker A, Zurakowski D, & Cypess AM (2011). Pediatric Brown Adipose Tissue: Detection, Epidemiology, and Differences from Adults. The Journal of Pediatrics PMID: 21839465