Low-carb and low-fat diets associated with lower heart disease risk only if

rich in high-quality, plant-based foods, low in animal products

 Key points:

• Low-carbohydrate and low-fat diets rich in high-quality, plant-based foods and low in animal products and refined carbohydrates were linked with lower risk of heart disease, while the same diets that were rich in refined carbohydrates and high in animal products and other low-quality foods were associated with a higher risk of heart disease. The study suggests that it’s the quality of the macronutrients composing these diets that make a difference for heart health, rather than the quantity.
• According to the researchers, the findings help debunk the myth that simply modulating carbohydrate or fat intake is inherently beneficial. 

Boston, MA—Low-carbohydrate and low-fat diets rich in high-quality, plant-based foods and low in animal products and refined carbohydrates were linked with lower risk of heart disease, according to a new study led by researchers at Harvard T.H. Chan School of Public Health. The same diets that were rich in refined carbohydrates and high in animal fats and proteins were associated with a higher risk of heart disease. The study suggests that it’s the quality of the macronutrients composing these diets that make a difference for heart health, rather than the quantity.

The study will be published Feb. 11, 2025, in JACC.

“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, 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.”

Previous studies have shown that diets emphasizing healthy sources of carbohydrates and fats (such as whole grains, fruits, vegetables, nuts, legumes, and olive oil) are associated with lower risk of coronary heart disease (CHD). The researchers sought to examine this evidence specifically in the context of low-carbohydrate and low-fat diets. To do so, they used diet and health data from nearly 200,000 men and women participating in the Health Professionals Follow-Up Study, Nurses’ Health Study, and Nurses’ Health Study II. The researchers analyzed participants’ diets and scored them to distinguish healthy and unhealthy low-carbohydrate and low-fat diets. They assessed scores in the context of whether participants developed CHD, controlling for various health and lifestyle confounders. The researchers also analyzed participants’ blood samples and measured levels of cardiovascular risk biomarkers.

The study found that low-carbohydrate and low-fat diets emphasizing high-quality carbohydrates and plant-based sources of proteins and fats were associated with about a 15% lower risk of CHD. In contrast, the same two diets rich in refined carbohydrates and animal proteins and fats were associated with a higher CHD risk. In their analysis of the blood samples, the researchers also found that healthy versions of low-carbohydrate and low-fat diets were associated with improved cardiovascular biomarkers, such as higher HDL (good) cholesterol and lower levels of triglycerides.