Body mass index is a way to estimate body fat by using math. 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). The DEXA scan is expensive. And you should have it only so many times because it uses radiation. 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 the latest statistics from the CDC, more than one-third of U.S. adults are obese. This means they have a BMI greater than 30. Although everyone needs a certain amount of body fat, extra body fat can put you at risk for a host of health problems.

Obesity occurs when a person takes in more calories than he or she burns. The cause of this imbalance of "calories in and calories out" differs from person to person. Genes, eating habits, physical activity, and mental health factors all play a part in obesity. Some nervous system conditions, endocrine problems, and certain medicines can cause some weight gain but usually not severe obesity.

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

A 5% to 10% weight loss can lower blood pressure and cholesterol levels, the NIDDK says. A 5% to 7% weight loss can prevent type 2 diabetes from developing 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. Weight control should be considered a lifelong effort.

Your weight-loss program should aim for a slow, steady loss. If you plan to lose more than 15 to 20 pounds, have any health problems, or take medicine regularly, discuss your plan with your healthcare provider before you begin, the NIDDK says. Your provider can look at your general health, your need to lose weight, and any health 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. You will need to continue this exercise level to maintain your weight loss. If you haven't been exercising regularly, check with your healthcare provider before beginning any exercise program. You don't have to do 30 (or 60) minutes all at one time. You can "sneak" activity into your day by taking the stairs instead of the elevator; walking and talking with a friend at lunch; or taking 2 to 3 short walking breaks at work. You can also walk on a treadmill at home while watching TV, for instance. It's important to choose an activity you enjoy so you'll stick with it.

After the first weight loss related to the medicine, a person's weight tends to level off or even increase for the remainder of treatment, the NIDDK says. Most weight-loss medicines are for short-term use only. These medicines 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 medicines are available only by prescription except for orlistat, a lipase inhibitor that is the first FDA-approved nonprescription weight-loss medicine. The prescription form of orlistat is one medicine that may be used for a year, or as directed by your healthcare provider. It blocks the absorption of about 30% of fats consumed. It may be prescribed for a person who is overweight and also has high blood pressure, high cholesterol, or diabetes. Discuss with your healthcare provider which medicine is best for you. Ask how long you should take the medicine. Any amphetamine-based medicines are not recommended for weight loss because of the potential for abuse or dependence. A person taking weight-loss medicine 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 2 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 be eaten at one sitting. Bariatric operations include laparoscopic adjustable gastric banding and vertical sleeve gastrectomy. These types of restrictive surgery don't change the way food is digested. The other type of surgery is malabsorptive surgery, which creates a new path for food, bypassing most of the 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 bypassing the duodenum and jejunum. The malabsorptive procedures include the Roux-en-Y gastric bypass and the duodenal switch with biliopancreatic diversion. One risk with this type of surgery is nutritional deficiencies.