He added, however, that some people might find calorie restriction feasible if they combined it with other popular dietary strategies like the Mediterranean diet, intermittent fasting or reduced carbohydrate intake.

The new study provided evidence of just how difficult calorie restriction can be. The study participants went through an intensive training program where they learned to cook low-calorie meals, attended group sessions and had regular check-ins with nutrition experts. Still, they were not able to meet even half the goal of a 25 percent cut in calories. And as anyone who has dieted knows, keeping the weight off long-term can be the hardest part.

Still, the new study was groundbreaking in several ways. Funded entirely by the N.I.H. at a cost of $55 million and called Calerie — for Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy — it was the first major clinical trial to examine the effects of caloric restriction in a group of middle-aged or younger adults who were either normal weight or just slightly overweight but not obese. The goal of the trial was to look at whether caloric restriction could influence healthy aging and disease.

While the calorie target they set was steep, they did give the subjects some flexibility, allowing them to eat the foods that they wanted. What they found was that the subjects did not change their protein intake, but they did eat significantly less fat and slightly fewer carbohydrates. They also consumed more micronutrients like vitamins A, K and magnesium, indicating a big increase in fruits and vegetables, said Susan B. Roberts, a co-author of the study and a senior scientist at the U.S.D.A. Human Nutrition Research Center on Aging at Tufts University.

“They were eating more healthy foods,” she said. “Things like nuts, whole grains, green vegetables and legumes.”

For many people, dieting can be an unpleasant experience. But the researchers found that for many of the subjects, caloric restriction was surprisingly not unbearable. While more people dropped out of the diet group than the control group, the study overall had a high retention rate. The researchers looked at measures of quality of life and discovered that the calorie-restricted group reported better sleep, increased energy and improved mood. Compared to the control group, they did not have significant increases in hunger or food cravings either, Dr. Roberts said.

“There was nothing we measured that indicated that they weren’t doing well,” she said.

One question the study could not answer was whether caloric restriction could extend life span in humans the way that it can in other animals. The researchers would have to keep people on the diet and follow them for many decades to test that.