Chronically restricting calorie intake can improve health by reducing weight and lowering cholesterol, but it remains to be seen whether those effects can be maintained over the long-term, according to a "groundbreaking" study published this month in the Lancet Diabetes & Endocrinology,
Cheat sheet series: Evidence-based medicine 101
Previous studies have proven that calorie restriction can extend life spans and improve health for lab animals, but until now, there's been no conclusive evidence that those same benefits can be achieved in healthy humans.
For the study, which was fully funded by the NIH, researchers observed 218 healthy men and women between ages 21 and 50 to determine the potential health benefits of a calorie restriction diet. The participants were divided into two groups: 143 were assigned to follow a calorie restriction diet, while 75 followed a control diet. For two years, the calorie-restricted participants could eat whatever type of food they wanted but were asked to cut their daily caloric intake by 25%.
To assist the participants in their new lifestyle, researchers provided an intensive training course that taught them how to cook low-calorie meals, as well as group sessions and check-ins with nutrition experts.
Ultimately, the researchers found that, despite the 25% calorie reduction target, participants in the calorie-restricted group on average managed to cut their caloric take by only about 12%. As O'Connor notes, that translated to about 300 calories per day—"the amount in a large bagel, a few chocolate chip cookies, or a small Starbucks Mocha Frappuccino."
Still, even though the participants were allowed to eat whatever they wanted, the researchers observed that they ate less fat and fewer carbohydrates, while their micronutrient intake increased.
"They were eating more healthy foods … like nuts, whole grains, green vegetables, and legumes," according to Susan Roberts, co-author of the study and senior scientist at the U.S.D.A. Human Nutrition Research Center on Aging at Tufts University. Participants' protein intake remained about the same.
And even though participants didn't hit the 25% goal, they still saw positive health effects from the more modest calorie restriction.
On average, participants lost about 16 pounds over the two-year study period. What the researchers found even more surprising were the improvements in participants' metabolic health. Participants who restricted their calories had better blood sugar control, decreased inflammation, and improved cholesterol levels and blood pressure, O'Connor reports.
William Kraus, the lead study author and a professor of medicine and cardiology at Duke University, said, "We weren't surprised that there were changes. 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."
Kraus noted that weight loss may have been responsible for some of the health benefits, but it did not explain all of the gains, suggesting that calorie restriction might have a separate effect on the body's disease pathways.
Participants who restricted their calories also reported an improvement in their sleep, energy, and mood.
The control group, which did not implement any calorie restriction, saw no improvements in any of the observed markers.
Frank Hu, chair of the nutrition department at the Harvard T.H. Chan School of Public Health, who was not involved in the research, noted the study didn't determine whether calorie restrictions could lead to improved longevity or decrease the instance of chronic disease. To demonstrate any such long-term benefits, the improvements would have to be sustained over time and observed after many decades, he said.
Hu also questioned how feasible chronic calorie restriction would be for most adults. Even with group sessions, check-ins with nutrition experts, and the cooking program, the participants still didn't meet the study's 25% calorie restriction goal. "[W]e are living in an obesogenic environment with an abundance of energy-dense, nutrient-poor foods that are cheap, accessible and heavily marketed," Hu said.
Even so, O'Connor writes, the study does offer conclusive evidence that limiting calorie intake by just 300 calories per day could result in significant health improvements.
"That's essentially an after-dinner snack," Kraus said (O'Connor, New York Times, 7/16).