Not all body fat is created equal, however. Fat tends to collect in two primary areas of the body: the abdomen (creating the “apple” shape more common in men), and the hips, buttocks, and thighs (creating the “pear” shape more common in women). Although fat on the lower extremities may be unsightly to some, it does not pose the same health risks as abdominal fat.
Deep abdominal fat that surrounds the organs is called visceral fat and is the fat most likely to cause the health risks of obesity, probably because of its close proximity to the liver and other organs. Because abdominal girth is a good predictor of visceral fat, circumference measurements around the torso (at the level of the belly button) can be used to assess abdominal fat. Abdominal girth of 40 inches or more for men and 35 inches or more for women have been associated with higher levels of insulin resistance, cardiovascular disease, cancer, diabetes, and other conditions.
What causes obesity?
Obesity is a complex disease that involves the interaction of genetics, physiology, metabolism, and lifestyle. While genetics play an important part, the degree of impact varies from person to person. Recent studies indicate that for some people, genetic factors may be 25% responsible for an individual’s body mass and body fat, while for others, genetics may be 70% of the story.
Genes carry the biological code that determines everything from the color of your eyes to the length of your toes. There are genes for diseases as well. Scientists believe that there may be a cluster of genes for obesity. If you are born with any of the genes for obesity, you have a predisposition for the disease. But having a predisposition for obesity doesn’t mean you will definitely be obese. That depends on many factors, perhaps the most important being your lifestyle. For example, if you are sedentary, consume more calories than you expend, and have a predisposition for obesity, the likelihood of becoming overweight or obese is high. However, even if you have obesity genes, if you burn lots of calories with regular physical activity, the genes may not express themselves.
This idea is best exemplified in Native Americans, a population that has a genetically high predisposition for diabetes (and perhaps for obesity as well). When Native Americans live traditional hunter-gatherer lifestyles, they stay slim and rarely develop diabetes. But when they live sedentary lives and their diets include lots of fat, refined sugar, and excess calories, they gain weight and develop diabetes and heart disease, sometimes at alarming rates.
The same phenomenon has been observed among the Aborigines of Australia. When they lived in the outback and hunted, fished, and gathered plant foods, Aboriginal people had very low rates of diabetes. But when they moved to cities, their collective rate of diabetes soared to three times that of Australian Caucasians. Aborigines who have returned to traditional lifestyles have seen their level of insulin resistance (a reliable indicator of diabetes) decrease by 33%.
Genes may also affect the hormones that control satiety (feeling full), appetite, metabolism, and fat distribution. We all know someone who seems to eat as much as he likes and never gains a pound. That’s at least partly genetics, but there may be other factors at work here, too. In one study, researchers fed a group of people 1,000 extra calories per day for 80 days to get them to gain weight. The outcome was that everyone gained a different amount of weight (the range was 9-32 pounds), even though they all consumed the same number of calories and did similar levels of planned activity.
How is this possible? The researchers discovered that some people were more active at rest than others. They actually fidgeted more, and this accounted for a good deal of the variation in weight gain. The other factor probably has to do with the presence or absence of so-called “thrifty” genes. Thrifty genes are programmed to store fat efficiently—a vestige of the Stone Age, when the body stored fat if food was plentiful in the event that the next season might bring famine. Some of us, it seems, have retained these ancient genes designed for storing fat efficiently. The problem is that we live in an era and an area of the world where there is almost always abundance and never famine, so our stored fat never gets used.