Obesity 101: The Physiology of Fatness

"The other day my exercise students were asking about the health risks of obesity, and how it contributes to heart disease and other health problems. I have always heard that excess body fat 'strains the heart,' but I realized that I don't really know what that means. I needed to learn more about why obesity causes health problems so I could discuss this topic more knowledgeably with my clients." Everyone knows by now that most countries are experiencing dramatic increases in obesity rates among adults and children. In the United States, the Centers for Disease Control and Prevention, Atlanta, Ga., states that approximately 30 percent of adults are obese, as measured by body mass index. Another one-third of the adult population is overweight, but not yet obese, and about 16 percent of kids ages six to 19 are overweight or obese. Most people also know that public health officials have expressed alarm concerning the predicted rise in obesity-associated health problems that may develop over the next decade. What are these health problems, and how does obesity contribute to their development?

The physiology of fatness

Body fat is a good thing, in moderation. Fat cushions and protects the organs. Fat under the skin helps keep us warm, and storage fat helps us through future food shortages. Our ability to make and store fat could have contributed to the survival of our species, as people who were adept at storing fat survived lean times and maintained their fertility enough to populate the planet. When you consume more calories than you expend, a majority of these excess calories are converted to triglyceride molecules, the body's primary form of fat storage. Fat cells, or adipocytes, dedicate a great part of their volume to triglyceride storage. Adipose tissue is comprised of many adipocytes, along with other structural elements such as blood vessels and connective tissue. Scientists used to regard adipose tissues as fairly inert storage depots that took in or released triglyceride depending on energy balance in the body. Excess calorie consumption was thought to lead to increased fat storage, while a calorie deficit would signal the adipocytes to release triglyceride for the body to use as fuel. Fat cells still do these things, but scientists are beginning to unravel some of the cellular biochemistry involved in fat storage and metabolism, and some of the physiological processes that occur when triglyceride supply overwhelms the body's immediate storage capacities.

Adipose tissue joins the endocrine and immune systems

Researchers have identified a number of chemical messengers that allow adipose tissue to help regulate fat storage, and allow it to communicate with other organs and systems in the body. Some of these messengers act as hormones, sending signals to other parts of the body. Leptin, for example, is a messenger produced by adipose tissue. Leptin concentration in the blood is thought to inform the brain about triglyceride storage levels. Researchers have hypothesized that when the brain finds out that storage levels are getting low (lower leptin levels), the brain turns on the hunger signal that tells you to go look for some food. Tissues that produce hormones qualify for inclusion in the body's endocrine system, a collection of hormone-producing organs that help to regulate body functions. Adipose tissue appears to be the site of a great deal of immune system activity, as well. Large numbers of a type of white blood cell known as macrophages have been observed in adipose tissue, especially in the fat of people who are obese. Macrophages engulf foreign invaders such as bacteria and viruses, and the body's own dead cells. The macrophages found in adipose tissue appear to be responding to damaged adipocytes. Adipocytes can grow larger, as more fat is stored, but they cannot expand indefinitely. It is possible that, with obesity, adipocytes cannot keep up with the body's demand to store triglyceride. Cells may leak or become damaged, signaling macrophages to move in to clean up the mess. Macrophages, in turn, release chemical messengers called cytokines, such as interleukins, that summon more white blood cells and lead to more inflammation. Some of these cytokines appear to interfere with normal blood sugar regulation and to contribute to the development of type 2 diabetes. Scientists still have a great deal to learn about the biochemistry of obesity, and how obesity contributes to many health problems. The more they learn, the more evidence we have about the benefits of a healthful lifestyle, as regular physical activity and good eating habits help to reverse the negative health effects of obesity.

References Bliss, R.M. Inflammatory news about fat cells: Molecules that sequester dying fat cells also spread inflammation. Agricultural Research 54 (3): 4-7, March 2006. Centers for Disease Control. Overweight and obesity. www.cdc.gov/nccdphp/ dnpa/obesity/. Accessed December 2006. Tilg, H., and H.R. Moschen. Adipocytokines: Mediators linking adipose tissue, inflammation and immunity. Nature Reviews Immunology 6: 772-783, October 2006.