Biological Causes of Obesity

Eating too much high-fat or high-calorie food while remaining sedentary is certainly one biological cause of obesity. Furthermore, a woman’s metabolism slows down by 25 percent after menopause, which means that unless she either decreases her calories by 25 percent or increases her activity level by 25 percent to compensate, she will probably gain weight. There are also other hormonal problems that can contribute to obesity, such as an underactive thyroid gland

(called hypothyroidism), which is very common in women over age fifty. Since diet and lifestyle changes are so difficult, there is an interest in finding genetic causes for obesity. If we’ve inherited obesity, that would mean it is beyond our control,which would probably be comforting for many people. Now that the Human Genome Project is underway, the goal of which is to map every gene in the human body, efforts are proceeding to find the “obesity gene” or “fat gene.” Few scientists believe that obesity is simply genetic, however. In other words, there are so many environmental and social factors that can “trip” the obesity “switch,” finding a specific gene for obesity is about as worthwhile as finding the “anger gene” or “crime gene.” An important theory about why we get fat concerns insulin resistance. It’s believed that when the body produces too much insulin, we will eat more to try to maintain a balance. This is why weight gain is often the first symptom of Type 2 diabetes. But then we have to ask, What causes insulin resistance to begin with? Many researchers believe it is triggered by obesity. So it becomes a “chicken or egg” puzzle. There are also many theories surrounding the function of fat cells. Are some people genetically programmed to have more, or “fatter,” fat cells than others? There are no answers here, as yet.

What about the brain and obesity? Some propose that obesity is all in the head and has something to do with the hypothalamus (a part of the brain that controls messages to other parts of the body) somehow malfunctioning when it comes to sending the body the message “I’m full.” It’s believed that the hypothalamus may control satiation messages. To other researchers, the problem has to do with some sort of defect that prevents the body from recognizing hunger cues or satiation cues; the studies in this area are not conclusive, however. A study reported in a 1997 issue of Nature Medicine showed that people with low levels of the hormone leptin may be prone to weight gain. In this study, people who gained an average of 50 pounds over three years started out with lower leptin levels than people who maintained their weight over the same period. Therefore, this study may form the basis for treating obesity with leptin. Experts speculate that 10 percent of all obesity may be due to leptin resistance. Leptin is made by fat cells and apparently sends messages to the brain about how much fat our bodies are carrying. As with other hormones, it’s thought that leptin has a stimulating action that acts as a thermostat of sorts. In mice, adequate amounts of leptin somehow signaled the mouse to become more active and eat less, while too little leptin signaled the mouse to eat more while becoming less active.

Interestingly, Pima Indians in the United States, who are prone to obesity, were shown to have roughly one-third less leptin in blood analyses. Human studies of injecting leptin to treat obesity are in the works right now, but to date have not been shown to be effective.