As the prevalence of obesity in America approaches epidemic proportions, researchers are scrambling to learn more about how the body stores fat, how fat tissues affect body systems and the role body fat plays in determining one's risk of chronic diseases.
What researchers have learned is that fat is much more than an extra layer of insulation. It actually works as an endocrine organ that secretes dozens of proteins that affect the inner workings of the body.
One such protein is leptin. Discovered in 1994, leptin is released by fat cells. One of its functions is to send signals to the brain to boost or curb appetite. As a person gains weight, their fat cells release more leptin, which in theory should cause a decrease in appetite and an increase in the number of calories burned. However, researchers have found that obese people often are not sensitive to leptin; thus, it has little effect on their appetite and calorie usage. What's more, leptin may work against people trying to lose weight because, as a person loses weight, the body tends to release less leptin, potentially contributing to a boost in appetite and reduction in the number of calories burned at a given activity level.
Another key protein secreted by fat cells is adiponectin. Adiponectin helps insulin move sugar from the bloodstream into the body's cells where it can be used either for fuel or stored. People who are overweight, and especially those who are insulin resistant, have low levels of adiponectin, which may raise the risk for diabetes and heart disease.
Larger fat cells also cause the release of a variety of inflammatory proteins that result in low-level inflammation throughout the body. While the inflammation does not cause symptoms such as fever or pain, it can reduce the ability of insulin to move glucose out of the blood, thus leading to the development of plaque in the arteries and increased risk for heart attack or stroke.
In addition to their biochemical influences, the location of fat cells seems to be important. People who gain weight around the middle (so-called apples) have been shown to be at higher risk for heart disease and diabetes than those who gain their weight in their hips and thighs (so-called pear shapes). Men are more likely to be apple-shaped, and women pear-shaped, at least in their younger years. Older women become more apple shaped as their weight shifts following menopause. Fat stored under the skin (subcutaneous fat) seems to have less influence on the development of chronic diseases than fat stored deep in the abdomen or near the heart and liver.
While scientists have learned much about fat over the last decade, there is still more research to be done. Discovering that fat cells are capable of sending signals to the body and learning how the body stores fat are just the tip of the iceberg. Researchers now are working to determine how diet, exercise and other changes within the body affect the storage and regulation of fat tissue. In so doing, researchers hope to uncover more effective ways to treat obesity.