An estimated 72 million Americans are obese, and every year some 200,000 have bariatric surgery to help them lose weight. Unfortunately, some of these patients may be trading one health problem for another. A new study released online Monday in the Journal of the American Medical Association found that the gastric bypass, which is the most common and effective weight-loss surgery, increases a person's risk of alcohol abuse. Here, a brief guide:

What is gastric bypass surgery?
It's an operation that involves shrinking the size of the patient's stomach, attaching it to a lower portion of the intestine. The changes limit the amount of food the person can eat, as well as the calories the body can absorb. Researchers say the surgery also changes how the body metabolizes alcohol. Many doctors warn patients that they'll be more sensitive to alcohol after the surgery, a difference which this new study indicates could lead to drinking problems in some cases.

How did the researchers discover the link?
They asked nearly 2,000 men and women who had any form of weight-loss surgery about their drinking habits before their operations, and again one and two years after. Overall, most said they didn't drink excessively before or in the year immediately following surgery. Even two years later, only about five percent of the patients who had stomach-banding surgery (in which an inflatable silicone device is banded around part of the stomach to reduce food consumption) drank too much — about the same as before the operations. But the figures were dramatically different for patients who had undergone gastric bypass surgery.

Did those patients drink a lot more?
In some cases, yes. Gastric bypass surgery was by far the most common — about two-thirds of the patients in the study had that procedure. Two years after the surgery, 11 percent (103 of 996 people) reported having drinking problems, a 50 percent increase over the pre-surgery numbers. The warning signs included having at least six drinks on one occasion, "needing" to drink in the morning, and drinking so much on occasion that they couldn't remember things that happened. Previous studies have shown that gastric bypass patients are more sensitive to alcohol, says the study's lead author, Wendy King of the University of Pittsburgh Medical Center. Now we know it "significantly" increases the risk of alcohol abuse.

Why the big difference?
Stomach-banding surgery merely reduces the size of the stomach, making people feel full on less food. Gastric bypass surgery does that, too, but it also sends alcohol "straight into the intestine," where it is absorbed rapidly, says Dr. Mitchell Roslin, a bariatric surgeon at in New York City. The higher absorption rate gives the drinker "a high peak and rapid fall," and makes alcohol more addictive, he said. Another theory is that the risk of alcoholism might be a byproduct of the success of this type of surgery. During the first year following surgery, patients are shedding pounds; in the second year they're getting out more, socializing... and drinking, says Leslie Heinberg of the Bariatric and Metabolic Institute of the Cleveland Clinic. "It may be an unintended consequence of doing a heck of a lot better," Heinberg says.

Sources: ABC News, Boston Globe, MSNBC, Pittsburgh Post-Gazette