Brown fat: a good fat?
What types of fat are there?
There are two types of adipose tissue, or fat, found in mammals: white adipose tissue (WAT), or white fat, and brown adipose tissue (BAT), or brown fat. White fat serves three functions: as heat insulation, as mechanical cushioning and, most importantly, as an energy source. Brown fat is important for thermogenesis (making heat) in newborns, but precisely what purpose the cells serve in adults is still unclear.
Very young babies don't shiver to keep warm; they burn brown fat instead. In newborn infants, brown fat makes up about 25 per cent of the body mass and is located on the back, along the upper half of the spine and toward the shoulders.
In adult mammals, the presence, amount and distribution of each type of fat depends upon the species, but there is much more white fat than brown. Adults carry many kilograms of white fat but just a few grams of brown fat, concentrated in the front part of the neck and the upper chest.
Brown fat is made up of heat-producing cells full of mitochondria (energy-generating structures), and it is metabolically active, unlike white fat. The brown colour comes from iron that is attached to proteins in these mitochondria.
What do we know about brown fat?
Researchers are not sure what brown fat does in adults, but it is known that it burns a lot of energy. According to Ronald Kahn, a researcher at the Joslin Diabetes Center at Harvard Medical School in Boston, Massachusetts, 50 grams of brown fat could burn up to 20 per cent of a person's daily energy intake.
Researchers investigating a protein called PRDM16 (PR domain containing 16) found that this protein could trigger cells that usually make white fat cells to make brown fat cells instead. Kahn and colleagues extended this work, and they found that a protein called BMP7 (bone morphogenic protein 7) is vital for the generation of brown fat cells. The researchers artificially increased the amount of BMP7 made in some mice and used untreated mice as a control. After five days, treated mice had more brown fat, lower weight gain and a higher body temperature than the untreated animals.
Could brown fat be used in weight loss?
Currently, anti-obesity measures focus on a reduction in energy or food intake. Could this kind of work on brown fat lead to new treatments for obesity?
It is possible that drugs that mimic the activity or boost the levels of these proteins could promote the conversion of white fat cells to brown fat cells. Researchers are also looking at how to increase how much brown fat people carry and increase its metabolic activity to help it burn energy faster. Some even suggest that people with obesity might be able to receive a transplant of brown fat cells - the idea being that brown fat cells could be placed in the abdomen to stimulate energy burning in the white fat stored there.
What are the potential implications of these treatments?
Although addressing the growing problem of obesity is crucial, the obvious problem with seemingly 'quick fix' treatments like this is that they do not tackle the root causes of the condition.
Having such treatments available could well encourage more people to eat unhealthily and/or lead more sedentary lives with little or no exercise. Similarly, people who are obese may simply decide they are going to eat more and end up lessening the effects of the treatment or cancelling it out entirely.
It is never going to be healthy to eat lots of fatty foods, irrespective of what anti-obesity treatments may be available. A diet high in fat not only increases the chances of obesity but is also a major risk factor for heart disease and some forms of cancer - two conditions that definitely won't be solved by injecting brown fat into the abdomen or popping a pill to increase the amount of BMP7 or PRDM16.
Image: Scanning electron micrograph (SEM) of part of a lobule of adipose (fat) tissue. Credit: David Gregory and Debbie Marshall, Wellcome Images.
This article is part of the online content for ‘Big Picture: Food and Diet’.
Further reading
Cypess AM et al. Brown fat as a therapy for obesity and diabetes. Curr Opin Endocrinol Diabetes Obes 2010 Apr;17(2):143-9