Their health usually improves anyway. Many with diabetes no longer need insulin. Cholesterol and blood pressure levels tend to fall. Sleep apnea disappears. Backs, hips and knees stop aching.

There are not nearly enough surgeons or facilities to operate on all the obese people who might be helped by bariatric surgery, noted Randy Seeley, director of the nutrition research center at the University of Michigan.

And many patients and doctors persist in thinking — all evidence to the contrary — that if overweight people really set their minds to it, they could get thin and stay thin.

Scientists got an unsparing look at what they were up against 50 years ago, when a clinical researcher at Rockefeller University, Dr. Jules Hirsch, did some old-fashioned experiments. He recruited obese people to stay at the hospital and subsist on a 600-calorie a day liquid diet until they reached a normal weight.

The subjects lost 100 pounds on average, and they were thrilled. But as soon as they left the hospital, the pounds piled back on.

Dr. Hirsch and Dr. Rudy Leibel, now at Columbia University, repeated the study again and again, with the same result. Eventually, they found that when a very fat person diets down to a normal weight, he or she physiologically comes to resemble a starving person, craving food with an avidity that is hard to imagine.

The lesson never really penetrated the popular consciousness. Just a couple of years ago, Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, made headlines with a study of contestants from the Biggest Loser television show. They lost enormous amounts of weight, he found, but rarely could keep it off.