Biologic and Medical Causes
Obesity results when a person consumes more calories than energy. Several different factors may influence weight gain.
About 90% of people who lose weight through dieting gain every pound back regardless of their weight-loss method.
Some evidence suggests that every person has an inherited weight range that varies by only about 10% either up or down from some set point. For instance, a man whose "genetically-determined" weight is 200 pounds would tend to swing from 180 to 220 pounds. He would be unlikely to lose or gain more than this.
Genetic factors that influence fat metabolism and regulate the hormones and proteins that control appetite may play some part in 70 - 80% of obesity cases.
The Biologic Pathway to Appetite
Appetite is determined by processes that occur both in the brain and gastrointestinal tract. Eating patterns are controlled by areas in the hypothalamus and pituitary glands. The body produces a number of molecules that stimulate or suppress appetite. In some cases, the following factors may produce imbalances in this process:
- Insulin. Insulin is a hormone that helps change blood sugar (glucose) into energy. During digestion, carbohydrates from our diet break down into different types of sugar molecules (including glucose). Proteins from ones diet break down into smaller molecules called amino acids. Immediately after eating, blood glucose levels rise. This triggers the release of insulin, which pours into the bloodstream. Insulin pushes the glucose and amino acids into cells and muscles. Insulin and other hormones determine which nutrients will be burned for energy or stored for future use. The inability to use insulin efficiently (insulin resistance) has been associated with both obesity and diabetes.
- Leptin. Leptin is a hormone that is released by fat cells. Some think this hormone may also be released by cells in the stomach. Leptin appears to play an important role in insulin resistance and fat storage in the body, but its role in obesity is unclear. The most likely scenario is that leptin levels rise as more fat is stored in the cells. This increase curbs appetite. Falling levels of leptin make you feel hungry. In people who have genetically lower levels of leptin, however, the brain may be tricked into thinking that it is always starving because there is no leptin to suppress appetite. This can lead to gain weight.
- Resistin. Resistin is a hormone produced by fat cells that makes the body resistant to insulin activity. Some experts believe it may help explain the role of obesity in diabetes type 2.
- Intestinal Chemicals. Ghrelin is a chemical produced in the stomach. It appears to be important in triggering the desire to eat. Peptide YY3–36 (PYY) is a substance secreted in the intestines after a meal. The level of PYY is proportionate to the number of calories a person eats. PYY tells the brain that you feel full. Deficiencies in these substances may contribute to some cases of obesity. Researchers are hoping that blocking ghrelin or infusing PYY may be possible therapies for obesity.
- Other Chemicals. Many brain chemicals are being studied for their role in appetite stimulation and weight gain. Among them are neuropeptide Y, melanocortins, agouti-related protein, and melanocyte stimulating hormone. Pain-relieving chemicals called endorphins may be critical in reducing appetite and regulating energy use. Cholecystokinin, a hormone released in the upper intestine that stimulates digestive juices, may work to control meal size.
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