Body mass index is a mathematical calculation that estimates the amount of body fat a person has. A BMI of 30 or greater is considered obese. A BMI of 25 to 29.9 is considered overweight.

Women usually have more body fat than men. Measuring body fat is difficult. The most accurate methods are to weigh a person underwater or with an X-ray test called a dual energy X-ray absorptiometry (DEXA). Because of their expense and difficulty, these methods are only used in a research setting. Simpler methods are much less accurate; they involve measuring the thickness of the layer of fat just under the skin or sending a harmless amount of electricity through a person's body. These methods are popular in health clubs.

According to 2004 statistics, the latest available, about one-third of U.S. adults are obese, meaning they have a BMI greater than 30, the CDC says. Although everyone needs a certain amount of body fat, excess body fat can put you at risk for a host of health problems.

Obesity occurs when a person consumes more calories than he or she burns. What causes this imbalance of calories in and calories out differs from person to person. Genetics, eating habits, physical activity, and psychological factors may all play a part in obesity. Some neurological conditions, endocrine problems, and certain medications can cause weight gain.

Obesity is also linked to several kinds of cancer, including colorectal, breast (in postmenopausal women), and uterine lining (endometrium). Other health problems associated with obesity include osteoarthritis, gallstones, liver disease, gout, sleep apnea, and infertility, the NIDDK says.

A 5 to 10 percent weight loss can lower blood pressure and cholesterol levels, the NIDDK says. A 5 to 7 percent weight loss can prevent type 2 diabetes in people who are at high risk for the disease. But any weight-loss program should be for the long term. Obesity is a chronic problem, and weight control should be considered a lifelong effort.

Your weight-loss program should be aimed at a slow, steady loss. If you plan to lose more than 15 to 20 pounds, have any health problems, or take medication regularly, discuss your plan with your doctor before you begin, the NIDDK says. Your doctor can assess your general health, your need to lose weight, and any medical conditions that may be affected.

Some experts say you may need to work up to 60 minutes of moderate level of activity on most days of the weeks to lose weight, and you will need to continue this exercise level to maintain your weight loss. If you haven't been exercising regularly, check with your doctor before beginning any exercise program. You don't have to get the 30 (or 60) minutes all at one time. You can "sneak" activity into your day by taking the stairs instead of the elevator; walk and talk with a friend at lunch; or take two or three short walking breaks at work. It's important to choose an activity you enjoy so you'll stick with it.

After that, a person's weight tends to level off or even increase for the remainder of treatment, the NIDDK says. Most weight-loss medications are for short-term use only—a year or less. And these medications are usually meant for people who are obese, not for those who are overweight or who want to lose a few pounds. Most of the medications are available only by prescription. Xenical, a lipase inhibitor, is one medication that may be used for more than a year; it blocks about 30 percent of fats consumed. It may be prescribed for a person who is overweight and also has high blood pressure, high cholesterol, or diabetes. One other medication available for longer-term is sibutramine. Any amphetamine-based medications are not recommended for weight loss because of the potential for abuse or dependence. A person taking weight-loss medication should also increase physical activity and improve his or her diet to maintain the weight loss for the long term.

This surgery may also be recommended for men who are more than 100 pounds overweight, or for women who are more than 80 pounds overweight. One of two types of procedures may be used. One type is called restrictive surgery, in which parts of the stomach are closed off to make it smaller and "restrict" the amount of food that can enter. This type of surgery does not alter the way food is digested. The other type of surgery is malabsorptive surgery, which creates a new path for food, bypassing the lower stomach, the duodenum (the first segment of the small intestine), and the jejunum (the second segment of the small intestine). This surgery reduces the amount of nutrients the body can absorb from food. Malabsorptive surgery may also entail removing portions of the stomach and, as above, bypassing the duodenum and jejunum. One risk with this type of surgery is nutritional deficiencies.