Bariatric surgery promotes weight loss by changing the digestive system's anatomy, limiting the amount of food that can be eaten and digested. Obesity normally is defined through the use of body mass index (BMI) measurement. Physician offices, obesity associations, nutritionists, and others offer methods for calculating BMI, which is a comparison of height to weight. Those with a BMI of 30 or higher are considered obese. However, at 40 or higher, they are considered severely obese-approximately about 100 pounds overweight for men and 80 pounds overweight for women. Many people who are obese struggle to lose weight through diet and exercise but fail. Only after they have tried other methods of losing weight will they be candidates for bariatric surgery, which today is considered a "last resort" for weight loss. In general, guidelines agree that those with a BMI of 40 or more, or a BMI of 35 to 39.9 and a serious obesity-related health problem, qualify for bariatric surgery. More than 23 million Americans are candidates for bariatric surgery. More than 100,000 of the procedures were performed in 2003 and the number of surgeries performed will probably continue to rise for many years. Bariatric surgery is not for everyone and the surgeon and other physicians will evaluate all medical conditions before allowing a patient to proceed. As a major surgery, there are associated risks and side effects. Women of childbearing age should be aware that rapid weight loss and nutritional deficiency associated with bariatric surgery may be harmful to a developing fetus. It is important that a patient reveal all current medications and conditions during any pre-operative discussions or examinations. Also, the physician will carefully evaluate the patient to ensure that he or she is prepared to make a lifelong commitment to the changes in eating and lifestyle required to make the surgery successful. Though many studies have shown general safety associated with the major surgeries, they are relatively new and research on long-term effects are not as widespread as they are for many other surgeries and procedures. When choosing a surgeon to perform the operation, patients should check with organizations such as the American Society for Bariatric Surgery for certification. A patient also should ask about the surgeon's experience in performing the particular operation.