Diet and exercise have long been considered the only effective way to lose weight. But in recent years, a third way—surgery—has emerged for certain men and women who are severely obese. This option, known as bariatric surgery, can result in dramatic weight loss. It has most famously physically transformed once-obese celebrities Al Roker and Carnie Wilson.

Bariatric surgery is reserved for people who have been unable to lose weight on professionally managed weight-loss programs and those who have obesity-related conditions such as diabetes, or are at risk for them. But the surgery, which restricts food intake and digestion, comes with its own serious risks and side effects.

Below, J. Christopher Eagon, MD, assistant professor of surgery at Washington University in St. Louis, discusses how this weight-loss procedure works and when it's appropriate.

When is surgery an appropriate option for weight loss?
The best candidates for the surgical treatment of obesity are patients who have a body mass index of 40 or greater, or 35 or greater and associated obesity-related conditions, such as diabetes, heart disease and sleep apnea. In terms of pounds, qualifying for surgery equates to being about 100 pounds above ideal body weight. If you take a person who is five foot, six inches tall, they might have to weigh something in the neighborhood of 260 pounds or more in order to qualify for surgery.

One of the nationally accepted criteria is that patients have to have been through some form of organized weight loss program in the past and failed to maintain that weight loss. What that exactly consists of might vary from one individual to another, and what a surgeon might accept as a failure of medical management might vary from one surgeon to another.

In general, children should not be candidates for the surgery, except when offered an operation in the setting of a trial, perhaps in an academic center where those patients will be studied quite intensely over the course of years.

How are counseling and other treatments woven into a treatment plan that might call for surgery?
We feel that it's very important to include a multidisciplinary approach to the education and screening of patients before surgery. Screening should be done in order to identify individuals who have psychopathology that might suggest they shouldn't have the surgery. And we need to educate the patients about the changes in their eating behavior that will occur with the operation. Otherwise the patients may be completely surprised to learn that in order to lose weight after surgery, they have to eat very small meals and sometimes eat very frequently in order to get adequate nutrients and liquid intake.

In addition, it's important to have the patient seen by a dietitian preoperatively to go over the current dietary choices and how those dietary choices might be affected by the operation.

How does obesity surgery work?
There are two general principles by which surgery for obesity works. One is by decreasing the capacity of the stomach. So the portion of the stomach that food gets into is much smaller, the patient feels full quicker, and so they eat fewer calories. The second general mechanism is malabsorption. Food bypasses a segment of intestine to the point that you don't absorb all of the calories that you're eating. That sometimes results in diarrhea or looser bowel movements because more unabsorbed fats are getting into the colon.

What are the different types of bariatric surgery available?
There are basically three general types of bariatric surgery that are available in America today. The simplest to understand is the adjustable gastric band. In that case, a band is wrapped around the upper part of the stomach, creating two chambers in the stomach: a very small upper chamber and a larger lower chamber. The size of the channel between the two chambers can be adjusted after the surgery in order to slow the passage of food from the upper chamber to the lower chamber.

The Roux-Y gastric bypass involves separating the stomach into two compartments: a very small upper compartment and the lower compartment, which food does not get into after the operation. They're completely blocked off from each other. Food will pass directly from the upper, small compartment into the small intestine, bypassing the lower part of the stomach. This surgery accounts for about 80 percent of bariatric surgery procedures in the United States today.

The third one is the duodenal switch. That involves two basic procedures. One is removing about 80 percent of the stomach volume, and then also bypassing the lower part of the small intestine with a long limb of intestine, but not bypassing the stomach directly. That operation results in a little bit more malabsorption in addition to a reduction in calorie intake.

What are the benefits of bariatric surgery?
The benefits can be quite dramatic. In terms of weight loss, we anticipate that patients lose about 70 percent of their excess body weight in one to two years. Among my patients, who on average weigh 390 pounds before surgery, they lose, on average, 135 pounds in a year.

In addition to the weight loss, there are dramatic improvements in obesity-related diseases. Two thirds of patients who are diabetic come off of all their diabetes medicine and have normal blood sugars. Half the patients who are taking high blood pressure medicine come off of all their high blood pressure medications, and another quarter of patients have a reduction in the dosage or the number of medicines that they take for high blood pressure. Among patients who have sleep apnea, 80 percent of those patients are able to come off of their CPAP breathing mask and have less severe sleep apnea than they had before the surgery.

In patients who don't have diabetes or high blood pressure, the operation seems to significantly reduce the future risk of developing those problems.

If the surgery goes well what can patients expect?
There's going to be a major change in their eating behavior and their relationship to food. They cannot eat large quantities of food without having pain or nausea and sometimes vomiting if they eat too rapidly or too much at one sitting.

Some people can experience dumping syndrome, which is a phenomenon caused by consuming too much sugar, such as high-calorie liquids containing a lot of sugar, and sweets. They can develop a sense of flushing, lightheadedness, dizziness and a very uncomfortable feeling. They have to lie down before they start to feel better. And they can lose their taste for sweets as consequence of that sensation.

What are the risks associated with this type of surgery?
The most serious risk is a risk of dying in the period right around the time of surgery. Overall, the risk of dying is probably in the neighborhood of 1 percent within the first month or two after surgery. Usually that occurs from one of three causes: a heart attack; a blood clot in the legs which travels up to the lungs; and a leak at the connection between the stomach and small intestine.

There are other complications, which are more common but less worrisome, such as wound infection, pneumonia, hernias and longer-term complications such as ulcers. About 35 percent of people who have their gall bladder still in place can form gallstones unless they're either treated with medication during the period of time that they're losing weight, or they have their gall bladder removed at the time of surgery.

There are certain nutritional consequences of the operation. Certain vitamins and minerals that are not absorbed after the Roux-Y gastric bypass have to be supplemented, usually just as an oral pill. For example, a multivitamin, vitamin B12, calcium supplements and iron supplements have to be taken on a daily basis after the surgery.

And finally, there's a complication known as a gastric fistula, in which the separation between the upper and lower part of the stomach reconnects over time and food can get down into the lower part of the stomach again; that can lead to regain of weight.