[SCIENCE WORLD]Obesity - just keeping warm?

About 80 percent of the American population aged over 25 is overweight. So are one-fourth of Korean adults. Those who are fat have higher blood sugar, blood pressure and cholesterol levels than those of normal heft. As time passes, overweight persons are more prone to strokes or heart attacks than persons of normal weight. One gets fat by eating more than his bodily systems consume, so diet and exercise are keys to health.

Prevention is better then treatment. In the United States, governments sometimes subsidize exercise classes and gyms. The whole country is concerned about how to improve eating habits. Despite such frenetic efforts, obesity is on the increase, not declining. Why? Because it is very difficult to change one's habits. Worse, we have inherited strong survival instincts from our long-ago ancestors, and reducing weight goes against those instincts.

Small animals eat less than larger ones, but they burn off more energy per gram. For example, rats have a metabolic rate that is 25 times higher than that of elephants. So rats burn up one gram of food 25 times faster than elephants. The major reason for the faster metabolism of a rat is that small animals have to generate more heat per gram of their weight to maintain their body temperature at a level similar to that of large animals. Elephants do not have to produce as much heat as mice because it takes longer for elephants to send heat from the center of their bodies to the surface. Elephants also have a relatively smaller surface area of their bodies, which emit heat, in proportion to their weight. If elephants produced heat at the rate of mice, their central body parts would be cooked.

There is a correlation across species between size and metabolic rate. Max Kleiber made the first systematic measurements of the correlation in 1932. Last year, Geoffrey West, a particle physicist from Los Alamos National Laboratory, and Brian Enquist of the University of New Mexico discovered that a universal scaling law applies to all kinds of life forms.

They explain that the constant ratio between the size of an animal and its metabolic rate is necessary for cells to maintain a body temperature at which its metabolism is the most active. If we accept that law, we should regard obesity as the chemical and physical result of an animal's attempt to maintain its optimal body temperature.

When a natural law is discovered, things that used to be hard to understand are understood, and new things are explained. The appearance of human obesity throughout the world can be understood as a disruption of metabolic systems in generating heat. Heat is produced as food is burned off in cell functions. That suggests the possibility that the cause of obesity is the depression of that cell function. In addition, considering that the function is easily disrupted, it is understandable that lifespans of those animals that eat a lot and consume energy quickly are short. It also explains why the "yo-yo phenomenon" - weight lost through crash diets will soon return - is inevitable.

The writer is a professor of medicine at Seoul National University.

by Lee Hong-kyu