The Biology of Obesity

Obesity in the United States has reached staggering proportions where 34% of the population is considered to be obese. The measure of obesity is the so-called “body mass index (BMI) that represents the ratio between a person’s weight in pounds multiplied by 703 (a constant) over the height in inches taken to the second power – Weight (in pounds) X 703/ (height in inches) 2. For example an individual weighing 190 pounds and with a height of 66 inches would have a BMI of (190 * 703)/ (66 inches) 2 which is equal to 30.6. A BMI greater than 30 is defined as obese and a value of over 25 is considered overweight.

The BMI was originally developed by the Belgium statistician Adolphe Quetelet (1796-1874). It is important to note that this value does not measure body fat directly, but it is a useful indicator of obesity that is recognized throughout the world.

There are many factors that contribute to obesity in the American culture. This is, however, beyond the scope of this overview. We will examine, instead, what is currently understood regarding the biology of obesity. Current understanding seems to point to three different systems of the human body that contribute to obesity – namely, the nervous system –the brain, metabolism and the genetics. We will examine these in turn.

It has long been known that the areas of the brain including the hypothalamus and the brain stem are involved in regulating the feelings of hunger and fullness. In addition, current data provided by functional magnetic resonance imaging (FMRI) that allows an investigator to visualize metabolically active parts of the human brain have also implicated the so-called pleasure reward centers of the limbic system and the prefrontal cortex. It seems that overeating, in effect, mimics drug addiction. This understanding may eventually prove useful in the treatment of obesity.

It seems that some adult men and women have retained stores of brown fat from childhood. Unlike white fat, brown fat is associated with leanness and its main function seems to be the generation of heat; whereas, white fat is involved primarily in storing energy. In some ways brown fat seems to be closely related to muscle. It remains to be seen what the factors are that predispose certain individuals to retain brown fat.

Variations in twenty separate genes have been implicated in regards to obesity; however, their cumulative contribution would be too small to account for the marked prevalence of obesity in the general population. This does not preclude the contribution of environmental factors in activating or suppressing certain genes and contributing to obesity in this way. Such genetic switches have been discovered in mice, suggesting that analogous mechanisms operate in humans.

Understanding the biology of obesity is an essential step in determining the best ways to treat this growing epidemic that has become strongly associated with modern life and culture.