Obesity is a condition in which body weight and body fat are greater than what is considered healthy for a given height. Body mass index (BMI)—calculated as a person's weight (in kilograms) divided by height squared (in square meters)—is a quick and simple way to estimate the amount of body fat, although it is not very accurate. Obesity is commonly defined as having a BMI greater than 30.

Having a high BMI can be a serious health hazard; obesity is associated with high blood pressure, osteoarthritis, diabetes, heart disease, respiratory problems, and some cancers. Unfortunately, the prevalence of obesity has risen to epidemic proportions over the past couple of decades. In the United States, the majority of adults are overweight and over 30 percent are categorized as obese. Childhood obesity has also become more common—approximately 15 percent of children and adolescents are obese—which has led to an increase in the incidence of health problems that were once rare among children.

Both genetic and environmental factors play a role in the development of obesity. Individuals with varying genetic predispositions may respond differently to the same environmental conditions; for example, some people may be genetically disposed to overeat or to store body fat more easily. In other words, within a population of people who share similar environmental and behavioral conditions (such as food quality and physical activity), there will be a range of body types.

The influence of genes on obesity is strong; there is evidence that 70–80 percent of the variation among individuals in body fatness is due to genetic factors. Furthermore, specific genes have been discovered to be associated with obesity. For example, a small percentage of people (estimated to be about one in 1,000) have a genetic mutation that affects the melanocortin 4 receptor (MC4R)—a receptor in the brain that receives signals to regulate appetite via the hormone leptin. This genetic mutation changes the surface of MC4R so that it is unable to process the message to control hunger. As a result, people with an MC4R mutation are more susceptible to obesity because their brains do not receive the signal to suppress the urge to eat. With no signal to tell them that they are full, they continue to feel hungry even after their body no longer needs additional energy.

However, the genetic influence is only part of the story—lifestyle is also critically important in contributing to obesity. The recent increase in the percentage of people categorized as obese cannot be attributed to genetic changes that favor obesity (genetic changes of populations occur much more slowly); the increase in obesity is more likely a result of sedentary lifestyles and calorie-rich diets that cause an energy imbalance. Body weight is maintained by consuming as many calories as are used, and a balanced diet of healthy foods contributes to overall health. Current lifestyles often support excess calorie consumption and not enough physical activity, which result in weight gain and poorer health.

Make hormone-receptor models in this NOVA scienceNOW classroom activity.

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