But a new study, one of the largest and most rigorous trials of the subject to date, suggests that eating a diet low in carbohydrates and higher in fats may be beneficial for your cardiovascular health if you are overweight.
The new study, which was published in the American Journal of Clinical Nutrition, found that overweight and obese people who increased their fat intake and lowered the amount of refined carbohydrates in their diet — while still eating fiber-rich foods like fresh fruits, vegetables, nuts, beans and lentils — had greater improvements in their cardiovascular disease risk factors than those who followed a similar diet that was lower in fat and higher in carbs. Even people who replaced “healthy” whole grain carbs like brown rice and whole wheat bread with foods higher in fat showed striking improvements in a variety of metabolic disease risk factors.
The study suggests that eating fewer processed carbs while eating more fat can be good for your heart health, said Dr. Dariush Mozaffarian, a cardiologist and dean of the Friedman School of Nutrition Science and Policy at Tufts University, who was not involved with the research. “I think this is an important study,” he said. “Most Americans still believe that low-fat foods are healthier for them, and this trial shows that at least for these outcomes, the high-fat, low-carb group did better.”
“It’s a well-controlled trial that shows that eating lower carb and more saturated fat is actually good for you, as long as you have plenty of unsaturated fats and you’re mostly eating a Mediterranean-type diet,” Mozaffarian added. Many doctors recommend a traditional Mediterranean style diet, rich in fruits and vegetables, fish and heart-healthy fats like nuts and olive oil, for cardiovascular health. Other rigorous studies have found that following a Mediterranean diet can help to ward off heart attacks and strokes.
The new study included 164 overweight and obese adults, mostly women, and took part in two phases. First, the participants were put on strict, low-calorie diets that lowered their body weights by about 12%. Then they were each assigned to follow one of three diets in which 20%, 40% or 60% of their calories came from carbohydrates.
Protein was kept steady at 20% of calories in each diet, with the remaining calories coming from fat. The participants were fed just enough calories to keep their weights stable. The participants followed the eating plans for five months, with all of their meals provided to ensure that they stuck to their diets.
The average American gets about 50% of his or her daily calories from carbs, most of them in the form of highly processed starchy foods like pastries, bread and doughnuts and sugary foods and beverages. In the new study, the low-carb group ate significantly fewer carbs than the average American. But they were not on a super-low-carb ketogenic diet, which severely restricts carbs to less than 10% of daily calories and forces the body to burn fat rather than carbohydrates. Nor did they eat unlimited amounts of foods high in saturated fats like bacon, butter and steak.
Instead, the researchers designed what they considered practical and relatively healthy diets for each group. All of the participants ate meals like vegetable omelets, chicken burritos with black beans, seasoned London broil, vegetarian chili, cauliflower soup, toasted lentil salads and grilled salmon. But the high-carb group also ate foods like whole wheat bread, brown rice, multigrain English muffins, strawberry jam, pasta, skim milk and vanilla yogurt. The low-carb group skipped the bread, rice and fruit spreads and sugary yogurts. Instead, their meals contained more high-fat ingredients such as whole milk, cream, butter, guacamole, olive oil, almonds, peanuts, pecans and macadamia nuts, and soft cheeses.
After five months, people on the low-carb diet did not experience any detrimental changes in their cholesterol levels, despite getting 21% of their daily calories from saturated fat. That amount is more than double what the federal government’s dietary guidelines recommend. Their LDL cholesterol, the so-called bad kind, for example, stayed about the same as those who followed the high-carb diet, who got just 7% of their daily calories from saturated fat. Tests also showed that the low-carb group had a roughly 15% reduction in their levels of lipoprotein(a), a fatty particle in the blood that is strongly linked to the development of heart disease and strokes.
The low-carb group also had improvements in metabolic measures linked to the development of Type 2 diabetes. The researchers assessed their lipoprotein insulin resistance (LPIR) scores, a measure of insulin resistance that looks at the size and concentration of cholesterol-carrying molecules in the blood. Large studies have found that people with high LPIR scores are more likely to develop diabetes. In the new study, people on the low-carb diet had their LPIR scores drop by 15% — reducing their diabetes risk — while those on the high-carb diet had their scores rise by 10%. People on the moderate carb diet had no change in their LPIR scores.
The low-carb group had other improvements as well. They had a drop in their triglycerides, a type of fat in the blood that is linked to heart attacks and strokes. And they had increases in their levels of adiponectin, a hormone that helps to lower inflammation and make cells more sensitive to insulin, which is a good thing. High levels of body-wide inflammation are linked to a range of age-related illnesses, including heart disease and diabetes.
The low-carb diet that was used in the study largely eliminated highly processed and sugary foods while still leaving room for “high quality” carbs from whole fruits and vegetables, beans, legumes and other plants, said Dr. David Ludwig, an author of the study and an endocrinologist at Harvard Medical School. “It’s mainly focused on eliminating the processed carbs, which many people are now recognizing are among the least healthful aspects of our food supply,” said Ludwig, who is co-director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital.
Ludwig stressed that the findings do not apply to the very-low carb levels typical of ketogenic diets, which have been shown to cause sharp elevations in LDL cholesterol in some people. But he said the study does show that people can gain metabolic and cardiovascular benefits by replacing the processed carbs in their diets with fat, including saturated fat, without worsening their cholesterol levels.
The new study cost $12 million and was largely funded by the Nutrition Science Initiative, a nonprofit research group. It was also supported by grants from the National Institutes of Health, the New Balance Foundation and others.
Linda Van Horn, a nutrition expert who served on the federal government’s dietary guidelines advisory committee and who was not involved with the new study, noted that the low-carb group consumed large amounts of unsaturated fat and fiber-rich vegetables — both of which are known to have beneficial effects on cholesterol and cardiovascular risk markers. The low-carb group, for example, consumed an average of 22 grams of fiber per day, which is more than the average American consumes, she said.
“While the study is valuable and carefully designed, as always in nutrition research, there are many dietary factors that influence cardiometabolic risk factors that can help to explain the results,” said Van Horn, who is also chief of nutrition in the department of preventive medicine at the Northwestern University Feinberg School of Medicine.
Mozaffarian said his take-home message for people is to adopt what he calls a high-fat Mediterranean style diet. It entails eating fewer highly processed carbs and sugary foods and focusing on fruits, vegetables, nuts, seeds, fish, cheese, olive oil and fermented dairy products like yogurt and kefir. “That’s the diet that America should be focusing on,” he said. “It’s where all the science is converging.”