US researchers have discovered that adult humans also have brown fat (previously thought only to be present in babies and children) which burns calories and uses energy, unlike the more abundant white fat that mainly just stores energy. They suggest that if scientists could find a way to activate this “good” fat reserve, it could open the door to new treatments for obesity and type 2 diabetes.
The research was led by scientists at the Joslin Diabetes Center in Boston, Massachusetts, and is published online in the 9 April issue of the New England Journal of Medicine, NEJM.
The study uncovered two significant things: not only do adults have brown fat (which scientists thought disappeared as children grew into adults), but this “good” fat is metabolically active in adults: it burns calories.
Lead author Dr Aaron Cypess, a Research Associate and Staff Physician at Joslin, told the press:
“Not only did we find active brown fat in adult humans, we found important differences in the amount of brown fat based on a variety of factors such as age, glucose levels and, most importantly, level of obesity.”
Obesity is a major risk factor for type 2 diabetes. Cypess and colleagues suggested it might be possible to find a way to stimulate the growth of brown fat as a way to control weight and improve glucose metabolism.
Senior author Dr C. Ronald Kahn, who heads the Joslin Section on Obesity and Hormone Action and the Mary K. Iacocca Professor of Medicine at Harvard Medical School, said that:
“The fact that there is active brown fat in adult humans means this is now a new and important target for the treatment of obesity and type 2 diabetes.”
For the research, Cypess, Kahn and colleagues looked at data on nearly 2,000 patients who for various reasons had undergone PET/CT (positron emission tomography/computed tomography) scans over a three year period.
The results showed that 7.5 per cent of the female patients and over 3 per cent of the male patients had substantial brown fat deposits.
Kahn explained this is likely to be an underestimate because PET and CT scans can only pick out clumps of brown fat cells over a certain size and when they are active, so smaller and less active deposits were probably overlooked.
The researchers also identified 33 other patients who had undergone tissue tests that found brown fat in their necks and in places where the PET and CT scans had detected the largest concentrations. When they tested the tissue of two of these patients they found it contained a protein that is only present in brown fat: UCP-1, which plays a key role in generating heat.
Cypess said these findings suggest that there is “previously unrecognized, heat-generating brown fat in many adults”.
When they analysed fat deposits by age, they found that younger patients were more likely to have more brown fat.
The brown fat was also likely to be more active during cold weather, which is in keeping with its known heat generating role. (The more active the brown fat, the more brightly it shines on the scans, making it more difficult to see what is happening, which is why patients are sometimes given beta blockers to reduce brown fat activity before a scan.)
They also found that brown fat was more common in lean adults with normal glucose levels.
Kahn said they were particularly interested in the finding that patients who were overweight or obese (as measured by their BMI, body mass index) were less likely to have lots of brown fat.
“Likewise, patients taking beta-blockers and patients who were older were also less likely to have active brown fat. For example, individuals both over age 64 and with high BMI scores were six times less likely to have substantial amounts of brown fat,” explained Kahn.
The researchers said this points to a possible role for brown fat in controlling weight metabolism and that having more brown fat could protect people against age-related obesity.
Kahn said it was likely that brown fat is more common among the general population than suggested in the findings, probably more spread out on the body and not so easy to find using most scanning technologies.
The researchers hope that improving the detection and measurement of the amount and activity of brown fat in humans could improve our knowledge of of what it does in the body and how to use it as a target for treating obesity and other metabolic disorders.
In an accompanying editorial, Dr Francesco Celi, of the Clinical Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, and who was not involved in the research, agreed:
“This study, by demonstrating the presence and physiological activity of brown fat in adult humans, shows that this tissue may provide a novel and valuable target for interventions, pharmacological and environmental, to modulate energy expenditure.”
Celi commented that scientists have long debated whether brown fat existed in humans, and whether it did anything important. But this new research shows not only that it is present, but is important for body weight and glucose metabolism.
“We hope this opens up a new therapeutic area for obesity and type 2 diabetes by modifying the activity of brown fat,” wrote Celi.
In previous studies, Kahn and others found that a bone growth protein, BMP-7, influenced the development of brown fat in rodents, and they also found clusters of brown fat dispersed between muscle fibers in mice that were resistant to obesity and diabetes.
“Identification and Importance of Brown Adipose Tissue in Adult Humans.”
Cypess, Aaron M., Lehman, Sanaz, Williams, Gethin, Tal, Ilan, Rodman, Dean, Goldfine, Allison B., Kuo, Frank C., Palmer, Edwin L., Tseng, Yu- Hua, Doria, Alessandro, Kolodny, Gerald M., Kahn, C. Ronald.
Sources: Journal abstract, Joslin Diabetes Center.
Written by: Catharine Paddock, PhD