Many studies have found that being slightly overweight doesn't increase the risk of premature death. That's good news for the 35 percent of adult Americans who carry 30 or 40 extra pounds.

But for the 10 million or more who are at least 100 pounds overweight, prospects for a long life are distinctly poorer. While medical treatment may control such obesity-linked conditions as high blood pressure and diabetes, science has yet to come up with a diet, exercise or drug guaranteed to make these dangerously fat people thin. Still, there is one reliable solution: bariatric surgery, in the form of gastric bypass or gastric banding.

Gastric bypass accounts for about 80 percent of weight-loss surgeries in the United States. It involves a permanent rerouting of the digestive system that leaves patients with a 1- to 2-tablespoon stomach pouch, created with staples, and detours food around the upper 20 to 60 inches of small intestine. Bypass patients feel full after eating tiny portions of food, and -- for at least the first year or so after surgery -- are digestively rebuked if they eat high-carb foods such as cookies and candy.

Approved in the United States since 2001, gastric banding is the most-often performed weight-loss surgery in the rest of the world. In it, a surgeon straps an inflatable silicon cuff around the stomach, creating a small pouch at the top with a restricted opening to the rest. The pouch fills quickly and empties slowly, creating a feeling of fullness. Overfilling causes undigested food to back up into the esophagus -- an unpleasant sensation that patients learn to avoid by eating small portions and chewing thoroughly. Banding can be reversed with removal of the band.

Results of bariatic surgery have been remarkable: Most studies indicate patients lose an average of 60 to 70 percent of their excess weight through gastric bypass; 40 to 55 percent through banding. As well, surgery dramatically improves, or even cures, diabetes, high cholesterol and high blood pressure.

Yet the procedures are not without risk, and -- once done -- require drastic, lifelong changes in eating habits.

Gastric bypass, for example, can lead to bowel obstruction or wound infection in the short term, and -- because the truncated intestinal tract compromises absorption of vitamins and minerals -- can in the long term cause osteoporosis and even beriberi if deficiencies are not corrected.

Gastric banding, which includes creation of a subcutaneous syringe port through which the inflatable cuff is adjusted, can lead to infection in the short term and serious problems associated with band slippage or tube rupture over time.

For weight-loss-surgery patients on the whole, the risk of dying during or shortly after the operation is low -- 1 percent or less -- and lower when done by an experienced bariatric surgeon.

While the long-term effectiveness of bariatric surgery is largely unknown, some studies lasting three or more years found that bypass patients began to regain a bit of weight, whereas banding patients were still losing weight. But averages mask significant variations in weight loss, researchers say, and it's rare that a bariatric-surgery patient actually reaches a normal body weight.

If you're considering weight-loss surgery, first make sure you're eligible: In most cases, that means having 100 or more excess pounds, and not having a drinking or drug problem. If your health plan covers the surgery -- procedures paid for out-of-pocket can cost $20,000 and up -- it may require you to undergo at least six months of a medically supervised weight-loss program beforehand.

Research shows that surgical centers handling 125 or more weight-loss surgeries a year have much better outcomes than those handling far fewer. For a list of centers that meet strict standards, check the Web site of the Surgical Review Corp. at www.surgicalreview.org, or call toll-free 1-866-790-4772.