Bariatric surgery is an option that obesity medicine specialists say is too often ignored or dismissed. Yet it is the only option that almost always works to help very heavy people lose a lot of weight and that also can mysteriously make some chronic conditions vanish. Here are some answers about bariatric surgery and what it does.

HOW MANY AMERICANS ARE ELIGIBLE FOR BARIATRIC SURGERY?

Twenty-four million, according to the American Society for Metabolic and Bariatric Surgery. The criteria are a body mass index above 40, or a B.M.I. of at least 35 along with other medical conditions like diabetes, hypertension, sleep apnea or acid reflux.

HOW MANY HAVE THE SURGERY EACH YEAR?

Fewer than 200,000.

WHAT ARE THE OPERATIONS?

There are four in use today. The two most popular procedures are the Roux-en-Y gastric bypass and the gastric sleeve. Both make the stomach smaller. The bypass also reroutes the small intestine. A simpler procedure, the gastric band, is less effective and has fallen out of favor. And a much more drastic operation, the biliopancreatic diversion with duodenal switch, which bypasses a large part of the small intestine, is rarely used because it has higher mortality and complication rates.

HOW MUCH DO THE OPERATIONS COST?

The average cost of a sleeve gastrectomy is $16,000 to $19,000, and the average cost of a gastric bypass is $20,000 to $25,000. Most insurance plans cover the cost for patients who qualify, though some plans require that patients try dieting for a certain amount of time first.