People who avoid carbohydrates and eat more fat, even saturated fat, lose more body fat and have fewer cardiovascular risks than people who follow the low-fat diet that health authorities have favored for decades, a major new study shows.
The findings are unlikely to be the final salvo in what has been a long and often contentious debate about what foods are best to eat for weight loss and overall health. The notion that dietary fat is harmful, particularly saturated fat, arose decades ago from comparisons of disease rates among large national populations.
But more recent clinical studies in which individuals and their diets were assessed over time have produced a more complex picture. Some have provided strong evidence that people can sharply reduce their heart disease risk by eating fewer carbohydrates and more dietary fat, with the exception of trans fats. The new findings suggest that this strategy more effectively reduces body fat and also lowers overall weight.
The new study was financed by the National Institutes of Health and published in the Annals of Internal Medicine. It included a racially diverse group of 150 men and women who were assigned to follow diets for one year that limited either the amount of carbs or fat that they could eat, but not overall calories.
"To my knowledge, this is one of the first long-term trials that's given these diets without calorie restrictions," said Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University, who was not part of the study. "It shows that in a free-living setting, cutting your carbs helps you lose weight without focusing on calories. And that's really important because someone can change what they eat more easily than trying to cut down on their calories."
Diets low in carbohydrates and higher in fat and protein have been commonly used for weight loss since Dr. Robert Atkins popularized the approach in the 1970s. Among the long-standing criticisms is that these diets cause people to lose weight in the form of water instead of body fat, and that cholesterol and other heart disease risk factors climb because dieters invariably raise their intake of saturated fat by eating more meat and dairy.
Many nutritionists and health authorities have "actively advised against" low-carbohydrate diets, said the lead author of the study, Dr. Lydia Bazzano of the Tulane University School of Public Health and Tropical Medicine. "It's been thought that your saturated fat is, of course, going to increase, and then your cholesterol is going to go up," she said. "And then bad things will happen in general."
The new study showed that was not the case.
By the end of the yearlong trial, people in the low-carbohydrate group had lost about 8 pounds more on average than those in the low-fat group. They had significantly greater reductions in body fat than the low-fat group, and improvements in lean muscle mass — even though neither group changed their levels of physical activity.
While the low-fat group did lose weight, they appeared to lose more muscle than fat.
"They actually lost lean muscle mass, which is a bad thing," Mozaffarian said. "Your balance of lean mass versus fat mass is much more important than weight. And that's a very important finding that shows why the low-carb, high-fat group did so metabolically well."