While normal white fat stubbornly stores excess calories on hips, bellies and thighs, over the last few years a picture has emerged of a different kind of fat â one which, paradoxically, might help us to lose weight. This is brown fat, which challenges all our assumptions about the fat in our bodies:Â it burns calories rather than storing them.Â
It was only six years ago we discovered that brown fat exists and is active in adults. Since then, it has become the focus of attention as a potential tool to help combat obesity and its related diseases. And the idea that there might be a way to burn through calories without the need to exercise is a tempting prospect for many of us.
âWe all know you only need a modest change in energy balance to put on weight â eating one or two extra biscuits a day is enough,â says Michael Symonds at the University of Nottingham, UK. âSo if you could activate brown fat, or increase its activity, you could potentially reduce your body weight.â
Symonds is one of a number of researchers working to develop behavioural, surgical and pharmaceutical therapies that might harness the power of brown fat, and some of these could be as simple as taking a cold dip in the pool or eating spicy food.Â
What makes brown fat so interesting is its ability to burn food directly to produce heat, whereas energy extracted from food is usually stored first, then released during activity such as exercise. It can produce 300 times more heat per gram than any other tissue in the body. This is because brown fat cells have a disproportionately high number of mitochondria â the small energy producing structures in cells â which also gives the stuff its eponymous colour. These mitochondria are slightly different from those in other cells, too, because they contain a protein called thermogenin, or UCP1, which enables brown fat to turn energy to heat directly.
This furnace-like ability is vital for regulating temperature in some mammals and in babies, who are unable to shiver to keep warm. But until recently it was thought to become defunct after infancy in humans. Then in 2009, several studies showed that brown fat was present and functional in adults in the neck, shoulders and around the spinal cord.
This discovery changed the question from whether adults have brown fat, to whether we can make use of it to help with weight control. âIt was a eureka moment,â says Symonds.
The amount of brown fat each of us has varies, though. Slimmer people tend to have more of it, which might help explain why some people seem to burn through everything they eat, while others pile on the pounds.
So the first step is to find out how much, if any, of this âgoodâ fat you have. Because brown fat is activated when the body is exposed to the cold, Symonds and his team have helped pioneer the use of a thermal imaging camera to detect it.
When animals are cold, they initially regulate their temperature by shivering. But after repeated exposure, shivering decreases while energy expenditure stays the same. Studies in rodents have shown that this is down to brown fat activity. If the same is true in humans, then regular cold exposure could help you adapt to the cold and burn calories in the process.Â
Evidence for this comes from an intriguing study conducted by the US army in the 1960s, which subjected 10 almost nude men to temperatures of 11 °C , for 8 hours a day for a month. Electrodes on their skin showed that, like rats, shivering decreased after about two weeks, suggesting that their bodies had somehow adapted to the cold. The team concluded that another metabolic process was at work, although it remained a mystery.
Fifty years later, Anouk van der Lans at Maastricht University in the Netherlands and colleagues wondered whether brown fat was responsible. So in 2012 they recreated the study using PET scans and fat and muscle biopsies to measure brown fat activity, as well as monitoring shivering. After 10 days, brown fat activity had increased and the subjects were better at producing heat without shivering, so they shivered less. They also found the cold easier to tolerate.
 Encouragingly, in this study, a temperature of about 16 °C was cold enough to switch on the tissue. âNobody thinks that getting so cold that youâre uncomfortable is necessary,â says Aaron Cypess of the US National Institute of Diabetes and Digestive and Kidney Diseases, an author of one of the 2009 papers.
How many calories can you expect to shed? Estimates vary hugely. One trial of Japanese men found that spending 2 hours a day in a
17 °C room for six weeks boosted brown fat activity by 50 per cent, and got rid of 5 per cent of their body fat. At the start of the experiment the men burned 108 calories during 2 hours in the cold, but this rose to 289 calories after doing it every day for six weeks.
That doesnât necessarily mean all those calories are burned by the brown fat itself â in studies that only involve short bursts of cold exposure, it could be down to other mechanisms like shivering. For example, one study of volunteers with an average of 50 grams of brown fat found they burned around 300 extra calories a day when exposed to moderate cold for 30 minutes â but brown fat only accounted for 20 calories of this.
Despite the mixed results, those figures are encouraging enough for some people to make cold exposure part of their daily routine. âThe mechanism of how it happens is important to understand, but for practical reasons, the result is what people care about,â says Wayne Hayes, a NASA scientist who has created the Cold Shoulder, a waistcoat filled with ice packs designed to activate brown fat.
Cypess and others believe that brown fat could make a contribution to weight loss strategies with regular cold exposure. But what if you donât like the cold? There could be a tastier alternative.
Capsaicin, a compound in chillies, seems to stimulate brown fat in a similar way. Mice fed capsaicin as part of a high-fat diet, for example, have increased metabolic activity and donât put on weight. This fits with a small study in which 10 men who took capsaicin pills daily had greater brown fat activity in the cold and burned more calories after six weeks.
âCapsaicin is promising as it is natural, and relatively safe and inexpensive,â says Cypess. âBut we are awaiting the definitive experiment showing that a dose of capsaicin directly leads to activation of brown fat.â
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