Brown Fat

We are told that if you are overweight it is your own fault for eating too much!  The logic is like a tub of water where the water stands for food energy.  To live and move, a certain amount of water drains out of the tub, so you eat to fill the tub again.  The energy level drops an inch, so we eat, it goes up an inch, and our weight remains the same.  Very simple, but only partly true.

For one thing the numbers look screwy if you think about a year’s worth of eating.  If you add up the total energy flowing like water through the tub, and the amount of energy in a pound of flesh, it turns out that a ten pound weight gain is a tiny fraction of the whole, under a hundredth.  So if you eat one extra French fry a day you will gain ten pounds.  That cannot be right, and seems pretty unfair.

The big gap in the ‘bathtub’ theory of weight is how fast the water drains out of the tub.  Of course exercise uses energy; the more you exercise the more energy you use and the more you you need to eat.  But for most people less than half their energy fuels exercise – in fact it is close to 20%!  Most energy used is simply the energy to stay alive while resting – the beating heart, the ever-working liver, and a big item, the brain (which uses the same amount of energy all the time, waking or sleeping).  Scientists call this constant energy expenditure basal metabolism.

Regular, moderate exercise raises basal metabolism.   So if you exercise, and then figure the number of fat calories you have burned, the weight of fat ‘burned’ is surprisingly small.  The reason moderate exercise can cause weight loss is that your basal metabolism is elevated so you burn a little more energy all the time.

This brings us to today’s paper on brown fat.  Where does this extra basal metabolism energy go?  It has long been known that human fat comes in two types that can be distinguished just by looking, white fat and brown fat.  All the fat you know about is white, under the skin, the belly and thighs, and so forth; it has metabolic functions including fat energy storage.  Children have brown fat under the neck skin to keep them warm.  When you use extra energy, the energy becomes heat, like an electric heater in your  bedroom on a winter night.

(The reason brown fat is brown, is the presence of lots of mitochondria, the energy generator organelle found in most cells.  In mitochondria food oxidation (‘burning’) is turned into useful chemical energy with a proton pump.  In brown fat this pump can be short-circuited so that food is converted to heat without doing any useful work.)

Scientists found that children’s brown fat shrinks as they grow and is gone in adults.  That is the origin of the notion that adults don’t have the capacity to turn extra food into heat, but will instead gain weight.  Well, that turns out to be wrong.  Adults do have brown fat, but it is in a different place than anyone expected to find it.

This work is a triumph of imaging technology by scientists is Sapporo, Japan. In medical science, if you can label it you can image it, whether it be an active section of the brain or the location of tumor metastases. The scientists made a label for brown fat and put adults in a chilly room, and looked for what lights up in the image.

In the figure A is a young man with his brown fat activated and in B the brown fat is inactive. There is no brown fat in the back of the neck, as expected, but it is found in the shoulders and along the spine. So the elusive brown fat is positioned to warm up the central nervous system!

Aside from enjoying these beautiful images, this work suggests how advances in medical science can provide us with better information to manage weight control.  Statistics show that 4 out of 5 people who are obese have type 2 diabetes and that 4 out of 5 people who have  type 2 diabetes are obese. This new study shows that the old way of thinking about weight gain no longer “holds water”.  So now the challenge is to take a look at finding a new model to discuss how to manage weight control.

Is there a way to make the brown fat of adults act like the brown fat of the young?

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