Weight control is the process of losing or avoiding excess body fat. Weight control depends on the relationship between the amount of food you eat and the energy your body uses to maintain itself or to exercise. This relationship is partly governed by heredity and other factors that people cannot control. But in general, the less you eat and the more you exercise, the less fat you will have.
Weight control is medically important because excess body fat is a major factor in many health problems. Excess fat that is severe enough to pose a medical risk is called obesity. People who carry this degree of excess fat are called obese. Obesity may also contribute to social and emotional difficulties.
Many people who are not medically obese still weigh more than they would like. People tend to regard unusually thin individuals, such as actors and fashion models, as ideals of personal appearance. But there is no single "best weight" for anyone. Bodies come in all shapes and sizes, and weight changes at different times of life. Many physicians use a measurement called body mass index (BMI) to determine if adults are obese. You can calculate your BMI by dividing your weight in kilograms by the square of your height in meters. For example, a woman weighing 130 pounds (59 kilograms) who is 5 feet 4 inches (1.6 meters) tall has a BMI of 59 divided by 2.56 (that is, 1.62), or about 23. Many doctors consider a BMI from 19 to 25 average, and a BMI above 26 or 27 obese.
Dangers of obesity
Obese people are more likely than thinner people to get certain diseases. Such diseases include appendicitis, cirrhosis, diabetes, coronary heart disease, and other diseases of the heart and blood vessels. Treatment of these diseases in people with obesity may be less likely to succeed. Obese patients with one of these diseases may have a better chance of recovery if they reduce.
Obese people have more falls and other accidents than thinner people because excess weight slows people down and hampers coordination. Obesity can also complicate surgery and recovery from injuries. Excess weight cuts down freedom of movement, especially in the elderly, and thus can lower general health because of lack of exercise. Decreased movement can also complicate treatment of obese patients with arthritis.
In many parts of the world, obese people experience discrimination. Fat people are often treated unkindly by others, including their classmates in school. They may have less social success than thinner people and find it harder to get jobs.
Causes of obesity
Eating too many calories. Food energy is measured in units called calories. Each food calorie technically equals 1,000 of the calories used to measure other kinds of heat energy (see CALORIE). If you eat more calories than you burn, most of the excess will turn into body fat. If you eat fewer calories than you use, your body will burn its stored fat for energy. Whenever you eat about 3,500 more calories than you use, you will gain 1 pound (0.5 kilogram). You will lose a pound of body fat if you consume 3,500 fewer calories than you use. Some countries that use the metric system measure the heat energy from food in joules instead of calories. One food calorie equals about 4,182 joules.
The amount of food you eat can play a much more important role in weight control than the kinds of food. People who are overweight, of average weight, or underweight may all eat the same kinds of food. Their weight differences often result from the amount they eat in relation to the amount of energy they use.
Certain centers in the brain control the sensations associated with appetite, hunger, and satisfaction. These centers normally make people eat an amount of food that provides the right amount of energy for their needs. The feeding centers make people want to eat. The satiety centers act as a brake on the feeding centers. They make people feel satisfied and stop eating.
The feeding and satiety centers are extremely complicated mechanisms. Their function may be affected by emotional pressures or physical processes. For example, strong emotions, such as great disappointment, cause some people to stop all physical activity. At such times, these people eat more than they usually do and gain weight. Other people may respond to disappointment by moving around more, eating less, and losing weight.
Children and pregnant women need a surplus of calories for growth. But even these groups can gain body fat if their food intake is extreme. Some scientists believe that any period of intense overeating in childhood or later life can lead to development of excessive numbers of fat cells. These experts think that these cells store fat so readily that people who develop excessive numbers may be obese for the rest of their lives.
Physical inactivity is a leading cause of obesity among all age groups, but especially among children and teen-agers. Most obese young people do not eat more than young people of average weight. But they are so inactive that, even with a moderate appetite, they eat more than they need and accumulate excess fat.
Exercise burns up calories. The more vigorous the activity, the more calories it uses. A 150-pound (68-kilogram) person walking at 31/2 miles (5.6 kilometers) per hour will use up 502 calories--the number of calories in a milk shake--in 97 minutes. That person will use up the same number of calories in 61 minutes by riding a bicycle, or in 26 minutes by running.
The number of calories used is proportional to a person's weight. If you weigh 75 pounds, for example, you will burn half as many calories as a 150-pound person by doing the same exercise for the same length of time.
The appetite of active people can increase if they become very active. If they become inactive, their appetite will not necessarily decrease. Appetite tends to remain above a minimum level even if activity drops.
Heredity. Scientists have learned much about the relationship between heredity and obesity in animals, especially mice. This relationship is based on genes, the chemical units in cells that determine inherited characteristics. Genes determine whether excess weight is stored as lean or fat tissue. Scientists have discovered that mice and certain other animals have a gene that causes the satiety center not to operate. Other genes in some mice cause their bodies to overproduce certain hormones. As a result of these hormones, the mice make body fat too easily or use it up with difficulty. Still other genes cause some mice to become obese more quickly than others when physically inactive or when given a diet high in fat.
The role of genes in human obesity is an active area of study. Scientists suspected that genes might be involved in human obesity because the weight of children is often related to the weight of their parents. For example, in some studies of high school students, only approximately 8 percent of students with thin parents were obese. But if one or both parents were obese approximately 75 percent of the students were overweight. Further, the weight of adopted children does not appear to be related to the weight of their adoptive parents. In 1994, scientists announced the discovery of the first gene believed to be associated with obesity in human beings. Researchers have since found other genes that appear to play a role in human obesity.
Diseases and other causes. Obesity may result from a number of diseases. Some ailments of the endocrine glands cause these glands to release too much of a hormone into the bloodstream. The