The Brown Fat Advantage

What would classify itself as the brown fat advantage? Brown fat is different from white fat in several ways. It has a much richer blood supply, and the fat cells are packed with dark mitochondria which give the brown fat its distinctive color. Mitochondria are the small, bean-shaped structures inside cells where energy from the food we eat is 'burned' or oxidized to produce ATP, the energy molecule we use inside our cells, and heat. Every cell in our bodies contains mitochondria, but the mitochondria in brown fat are unique. Whereas mitochondria in other tissues are very efficient in producing ATP and generate heat almost as a by-product, the mitochondria in brown fat are set up to burn calories, and produce heat instead of producing ATP.

So how do we get our brown fat 'revved up'? Do we have any of this highly metabolically active tissue? And where in our bodies is it lurking? Answers to these questions were provided by Stephane Krief, Daniel Ricquier and their colleagues at the University of Paris.3 Their team showed that in adults and children, small numbers of brown fat cells can be identified nestling deep inside deposits of white fat. In other words we all have a few, but they are in the main quiescent. Even more interestingly, the scientists found that when white fat cells are stimulated in certain ways they transform into brown fat cells, becoming more vascular and increasing their numbers of mitochondria.

Krief and Ricquier likened this to the changes that occur in muscle during training; it too becomes more vascular, and grows more mitochondria, so that it can work harder. But instead of physical exercise, they used caffeine or nicotine to trigger the shift from white to brown fat. This gives a new insight into the tense, chain-smoking and coffee-drinking types who stay skinny, but this is not a reason to get out the cigarettes or even, unless you like very strong coffee, the coffee grinder. There are other ways...

Quite early on it became apparent that brown fat was innerv­ated; that is, there is a special group of nerves that supplies the brown fat cells, and has the ability to switch them on. These are sympathetic nerves, and are part of the sympathetic nervous network whose main role is as a sort of emergency system.

Whenever we are threatened or stressed, the sympathetic nervous system plays a crucial role. It is an essential part of the fight or flight response. When confronted with a dangerous or stressful situation the brain, albeit indirectly, fires up the sympa­thetic nerves. They speed up the heartbeat and increase the amount of blood it pumps; they close the arteries leading to the gut and open those feeding the muscles, so that they will be given more oxygen and be able to work more effectively; they stimu­late the liver to produce glucose, so that the muscles will have the fuel they need. The platelets in the blood become more sticky, so that if you happen to be wounded in the ensuing conflict your on-board puncture repair kit will be in top form. And your sense of pain diminishes, so that in the heat of battle you will not be distracted by the discomfort of an in-growing toenail or disabled by a non-fatal wound.

Now it seems that the sympathetic nervous system was also involved, back in the times when humans were evolving, in readying our bodies for winter. It reacted to the stress of cooling temperatures late in the year, and activated our brown fat so that when the cold came, we could still function. And this makes perfect evolutionary sense of Pirkko Huttunens findings that we only grow significant amounts of brown fat in winter, and only when we are exposed to the cold.