They lost weight and body fat. Their cholesterol levels improved, their blood pressure fell slightly, and they had better blood sugar control and less inflammation. At the same time, a control group of 75 healthy people who did not practice caloric restriction saw no improvements in any of these markers.
Some of the benefits in the calorie restricted group stemmed from the fact that they lost a large amount of weight, on average about 16 pounds over the two years of the study. But the extent to which their metabolic health got better was greater than would have been expected from weight loss alone, suggesting that caloric restriction might have some unique biological effects on disease pathways in the body, said Dr. William Kraus, the lead author of the study and a professor of medicine and cardiology at Duke University.
"We weren't surprised that there were changes," he said. "But the magnitude was rather astounding. In a disease population, there aren't five drugs in combination that would cause this aggregate of an improvement."
Calorie restriction may be a useful tool for better health and weight loss, but it's unclear whether the changes in the new study will ultimately translate into longevity and reductions in chronic disease, said Dr. Frank Hu, chairman of the nutrition department at the Harvard T.H. Chan School of Public Health, who was not involved in the research.
He said the modest improvements experienced by the participants would have to be sustained over time to produce long-term benefits. But he questioned whether caloric restriction would be practical for most people given that "we are living in an obesogenic environment with an abundance of energy-dense, nutrient-poor foods that are cheap, accessible and heavily marketed."
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 per cent 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 NIH at a cost of US$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 USDA 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 with the control group, they did not have significant increases in hunger or food cravings either, 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 animals. The researchers would have to keep people on the diet and follow them for many decades to test that.
But ultimately, caloric restriction did have a beneficial impact on a wide range of risk factors for diabetes and heart disease, two conditions that cause death and disability for millions of Americans, especially as they get older.
Kraus at Duke said that he hoped to study the participants again in 10 years to see if the benefits of the caloric restriction experiment persist, a phenomenon known as legacy effects, which can occur with intense exercise training and other health interventions. But for now, he said that one takeaway for the public is that people can benefit from shaving as little as 300 calories a day from their diets.
"It's not that hard to achieve that amount of caloric restriction," he said. "That's essentially an after-dinner snack."
Written by: Anahad O'Connor
© 2019 THE NEW YORK TIMES
