Low-carbohydrate and low-fat diets were both associated with lower risk of coronary heart disease when they emphasized high-quality, plant-based foods and minimized refined carbohydrates and animal products, according to a large new study.
The research, led by investigators at Harvard T.H. Chan School of Public Health and published in the Journal of the American College of Cardiology, analyzed diet and health data from nearly 200,000 U.S. adults participating in the Nurses’ Health Study, Nurses’ Health Study II and Health Professionals Follow-Up Study.
Researchers scored participants’ diets to distinguish between healthier and less healthy versions of low-carbohydrate and low-fat eating patterns. Healthier patterns emphasized whole grains, fruits, vegetables, nuts, legumes and unsaturated fats. Less healthy patterns were higher in refined carbohydrates, animal fats and animal proteins.
Over decades of follow-up, participants who adhered to healthier versions of either low-carbohydrate or low-fat diets had about a 15% lower risk of coronary heart disease compared with those who did not. In contrast, low-carbohydrate and low-fat diets built around refined grains, animal fats and highly processed foods were associated with higher risk.
“Low-carbohydrate and low-fat diets have been widely promoted in the U.S. over the past two decades for weight control and metabolic health, but their effects on heart disease risk have remained unclear,” said first author Zhiyuan Wu, a postdoctoral research fellow in the Department of Nutrition. “Our findings help debunk the myth that simply modulating carbohydrate or fat intake is inherently beneficial, and clearly demonstrate that the quality of foods constructing low-carbohydrate and low-fat diets is what’s most important to protect heart health.”
The researchers also analyzed blood samples from subsets of participants and found that healthier versions of both diet patterns were associated with more favorable cardiovascular biomarkers, including higher HDL cholesterol and lower triglycerides.
The study was observational and relied on self-reported dietary data, meaning it cannot prove cause and effect. However, its size, long follow-up period and inclusion of biomarker data strengthen confidence in the findings.
“For clinicians, dietitians and patients, our study suggests that promoting an overall healthy eating pattern, rather than strict macronutrient restriction, should be a central strategy for the primary prevention of heart disease,” said senior author Qi Sun, associate professor in the Departments of Nutrition and Epidemiology.
The results add to growing evidence that focusing narrowly on carbohydrate or fat intake may miss the bigger picture. Diets that reduce carbohydrates by replacing them with processed meats and refined fats may carry different risks than those that substitute nuts, legumes and unsaturated plant oils. Likewise, low-fat diets high in refined grains may not offer the same benefits as those centered on whole plant foods.
In other words, the composition of a diet appears to matter more than the percentage of calories from fat or carbohydrate alone.
The Nurses’ Health Studies and Health Professionals Follow-Up Study are supported by multiple grants from the National Institutes of Health. Additional support was provided by NIH research grants and the Dana-Farber/Harvard Cancer Center.