If you’re concerned about cardiac health and can’t decide which is better, a low-fat or a low-carbohydrate diet, take heart. Healthy versions of either diet are linked to a lower risk of hardening of the arteries, the most common form of heart disease, new research suggests.
In a long-term, observational study of nearly 200,000 adults, low-fat and low-carb diets rich in plant-based foods, whole grains and unsaturated fats were associated with reduced risk of coronary heart disease. Coronary heart disease occurs when fatty deposits called plaque build up in the heart’s arteries, preventing them from delivering oxygen-rich blood to the heart muscle. It can result in chest pain, heart attack or cardiac arrest when the heart suddenly stops pumping.
On the other hand, low-fat and low-carb diets high in refined carbs and animal-based fats and proteins were associated with higher risk. The findings were published Wednesday in the Journal of the American College of Cardiology.
“It’s the quality of your diet that matters,” not any particular macronutrient like fat or carbohydrates, said lead study author, Dr. Qi Sun, a specialist in nutrition and epidemiology at the Harvard T.H. Chan School of Public Health.
Because the study focused on coronary heart disease, Sun said, the findings wouldn’t necessarily apply to other conditions such as heart failure or arrhythmias.
In addition to his school, researchers from Kuwait University, Harvard Medical School, the Broad Institute of MIT and Harvard, and Beth Israel Deaconess Medical Center in Boston studied the self-reported eating habits of health professionals in three databases which were started in the 1970s and 1980s.
The first was the Nurses’ Health Study, which enrolled women between the ages of 30 to 55. The Nurses’ Health Study II recruited somewhat younger women, and the Health Professionals Follow-Up Study enrolled male health professionals, ages 40 to 75 years.
After excluding participants who reported having a history of diabetes, cardiovascular disease or cancer when they signed up, the researchers ended up with a study population of 198,473 people.
Participants completed diet questionnaires every two to four years. Researchers classified low-fat or low-carb diets into 5 categories, including total, healthy, unhealthy, animal or vegetable. They then ranked how well people in each category adhered to those diets by tracking the foods they ate.
Over more than 30 years of follow-up, 20,033 people developed confirmed coronary heart disease.
Within the healthy low-carb diet category, people who most diligently followed that diet had a 15% lower risk of coronary heart disease compared to the least diligent people. The risk reduction by strictly adhering to a healthy low-fat diet was 13%.
A healthy low-fat or low-carb diet emphasizes foods such as whole grains, fruits (minus juice), vegetables (minus potato for low-carb), and plant-based proteins and fats (fewer fats in the low-fat diet), according to the researchers.
“Those are relatively large risk reductions” for a lifestyle change, said Kristina Petersen, an associate professor of nutritional science at Penn State who studies diet and risk of cardiovascular disease and was not involved in the study
People whose low-carb diet was ranked as the most unhealthy had a 14% increased risk compared to people whose unhealthy low-carb diet was the least extreme. The increased risk for the most unhealthy low-fat diet was 12%.
The researchers defined unhealthy diets as relying more on refined grains and animal sources of protein and fat.
Dr. Clyde Yancy, chief of cardiology at Northwestern University’s Feinberg School of Medicine, said the study was particularly strong because of its size and duration and the way it drilled down into categories of low-fat and low-carb diets.
Another key to the study is that it didn’t rely solely on self-reports from questionnaires, which can be flawed because people may not remember accurately what they ate.
Researchers examined blood samples from more than 11,000 participants and confirmed the study’s findings by looking at metabolites in the blood — small molecules like amino acids, cholesterol and triglycerides — that are associated with positive and negative health outcomes, including coronary heart disease.
People with the healthiest diets had a healthy metabolite profile and a significantly lowered risk of heart disease. The opposite was true for people with the unhealthiest diets.
The strength of this approach is that metabolites are objective measures and “cannot lie” and so give greater confidence in the study’s findings, Sun said.
“The study confirms what we’ve learned over the last 20 years,” said Dr. Dariush Mozaffarian, a cardiologist and director of the Food is Medicine Institute at Tufts University who wasn’t part of the study.
Rather than focusing on fat, carbohydrates or protein, it’s the quality of the foods that can drive down heart disease risk, he said. Quality means more nutrient-dense foods, such as fruits, vegetables, whole grains, nuts, legumes and fewer foods that are higher in added sugar, saturated fat and salt, Petersen said.
The paper had limitations. While the researchers accounted for factors beyond diet that could have affected people’s heart health and skewed the results, such as physical activity, smoking status, family history and body mass index, that adjustment is imperfect, the researchers said. In addition, the study participants were health professionals, who are more health conscious and have better access to health care and so the findings may not apply to the general population.
The study’s results support some and contradict other recommendations of the latest dietary guidelines from the Trump administration.
While the guidelines encourage Americans to eat fresh fruits and vegetables, nuts, legumes and whole grains, they also recommend red meat and full-fat dairy, including butter, which are all high in saturated fats.
“In this study, we see that if you choose a low-carb diet that is heavily based on animal foods — so fatty meats and animal fats in the form of butter or tallow — then that would increase your risk of coronary heart disease,” Petersen said.
Specifically, people who adhered most closely to an animal-based low-carb diet had a 7% increased risk of developing coronary heart disease.
If people combine a high quality diet with exercise, no smoking, good blood pressure control and cholesterol levels, research suggests they “can reduce the likelihood of having heart disease, including a heart attack, by 75% or 80%,” Yancy said.