What’s inside your fat cells?
What is the difference between white, brown and beige fat, and could some types actually help with weight loss? By Amy Olson and Hayley Birch
What types of fat are there?
There are two main 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.
In 2012, scientists discovered a third type of fat, something called beige fat. Beige fat cells are brown fat cells that occur in white fat tissue in response to certain triggers, like extreme cold.
What do we know about brown fat?
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.
Its high number of mitochondria means that brown adipose tissue is very efficient at respiring and therefore considered healthier than WAT.
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 or beige fat be used in weight loss?
Currently, anti-obesity measures focus on a reduction in energy or food intake. Could targeting brown or beige fat lead to new treatments for obesity?
Perhaps. Researchers are exploring the differences between brown fat and beige fat, and are working to understand what they do in the body. A 2013 review paper reports: “Regardless of its natural role, increasing the activity of brown fat, beige fat or both through drugs or other methods holds tremendous promise for the treatment of metabolic disease.”
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.Lead image:
David Gregory and Debbie Marshall/Wellcome Images
- Nature News: Boosting ‘good’ fat to burn off the bad
- NPR: Brown fat
- PRDM16 controls a brown fat/skeletal muscle switch (2008)