So for those medical conditions, it probably wouldn’t matter if a person lost 100 pounds or 150 pounds over two years.

BMI is a measure of weight relative to height. A BMI between 18.5 and 24.9 is considered normal weight, and above 30 is obese.

The patients in the current study were a fairly healthy group despite having a BMI of 50 or more.

After two years, those who’d had a duodenal switch showed a greater improvement in their cholesterol levels. But those levels were near-normal (in both groups) to begin with, Livingston noted.

That begs the question, he said, of whether the extra weight loss “actually accomplished anything.”

“The answer is ‘no,’” Livingston said.

In the U.S., where about 220,000 people underwent weight-loss surgery in 2009, gastric bypass is the most common form chosen.

During gastric bypass, the upper portion of the stomach is stapled off to create a smaller pouch that restricts the amount of food a person can eat at one time. The surgeon also creates a bypass around the rest of the stomach and a portion of the small intestine, which limits the body’s absorption of food.

Duodenal switch is more extensive. The surgeon removes part of the stomach, and the remaining “sleeve”-like stomach is attached to the final section of the small intestine; that puts even greater limits on the body’s absorption of calories and nutrients.

Both surgeries carry the long-term risk of nutritional deficiencies, and people need to take supplements and carefully monitor their diets for a lifetime afterward.

But the risks of nutrient deficiencies are greater with duodenal switch, and include some not seen with gastric bypass, Livingston pointed out.

Some people, for instance, may have severe calcium and vitamin D loss leading to weak and fragile bones. Other problems include severe protein deficiency and night blindness caused by vitamin A deficiency (which is also reversible with extra vitamin A treatment).

In this study, three of the 29 duodenal switch patients developed protein malnutrition, two developed night blindness, and one had a severe iron deficiency that needed to be treated with iron infusions.

Livingston said his advice to people considering weight-loss surgery is to avoid duodenal switch. “It’s just not a good operation,” he asserted.

But he also questioned the value of weight-loss surgery, in general, for severely obese people who do not have medical conditions that stand to improve or resolve after surgery.

In general, experts say that surgery could be an option for anyone with a BMI of 40 or higher; that translates, roughly, to a man who is at least 100 pounds overweight or a woman who is 80 pounds overweight.