The quality of a low-carbohydrate or low-fat diet may matter more than the amount of carbohydrates or fat consumed when it comes to reducing heart disease risk, according to a new study published in JACC, the flagship journal of the American College of Cardiology. Researchers found that versions of both diet patterns emphasizing macronutrients from healthy foods were associated with a lower risk of coronary heart disease (CHD), while versions high in refined carbohydrates and animal products were linked to higher risk and adverse metabolic profiles.
Low-carbohydrate and low-fat diets are widely practiced in the U.S., yet their long-term cardiovascular effects and the role of diet quality within these diet patterns have remained unclear. This study is among the most comprehensive to date to analyze diet quality, metabolomic profiles and CHD risk in nearly 200,000 U.S. adults with more than 30 years of follow-up.
"Our findings highlighted that it's not simply about cutting carbs or fat, but it's about the quality of foods people choose to construct those diets," said Zhiyuan Wu, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health and lead author of the study. "The efficacy of low-carb and low-fat diets in reducing CHD risk is a topic of ongoing debate, and past studies showed mixed findings."
Wu added that one reason for these conflicting findings may be that people can follow the same type of low-carb or low-fat diet using either healthy or unhealthy foods.
"Focusing only on nutrient compositions but not food quality may not lead to health benefits," he said.
To address these knowledge gaps, researchers analyzed data from 198,473 participants across three large-scale cohort studies: the Nurses' Health Study (NHS), NHSII and Health Professionals Follow-up Study. Participants were followed for a combined total of more than 5.2 million person-years, during which 20,033 cases of CHD were documented. Diet patterns were assessed using food frequency questionnaires, and indices were created to distinguish healthy and unhealthy low-carbohydrate and low-fat diets based on food sources and macronutrient quality.
Key findings include:
- Healthy versions of both low-carb and low-fat diets, emphasizing plant-based foods, whole grains and unsaturated fats, were associated with a significantly lower risk of CHD.
- Unhealthy versions of both diets, high in refined carbohydrates and animal-based fats and proteins, were linked to higher risk of CHD.
- Healthy diet patterns were linked to lower triglycerides, higher HDL cholesterol and lower levels of inflammation.
- Metabolomic analyses supported these findings, showing favorable biomarker profiles associated with healthy dietary patterns.
"These results suggest that healthy low-carbohydrate and low-fat diets may share common biological pathways that improve cardiovascular health," Wu said. "Focusing on overall diet quality may offer flexibility for individuals to choose eating patterns that align with their preferences while still supporting heart health."
Study limitations include that findings may not be generalizable to more extreme dietary patterns with very low carbohydrate or fat intake, such as ketogenic diets, and should be interpreted within the observed ranges of macronutrient consumption. Also, dietary intake was self‑reported and therefore subject to measurement error. Lastly, study findings may have limited generalizability since participants consisted of health professionals with higher health awareness and better access to care, however, underlying biological mechanisms are unlikely to differ drastically across populations.
"This study helps move the conversation beyond the long-standing debate over low-carbohydrate versus low-fat diets," said Harlan M. Krumholz, MD, Editor-in-Chief of JACC. "The findings show that what matters most for heart health is the quality of the foods people eat. Whether a diet is lower in carbohydrates or fat, emphasizing plant-based foods, whole grains, and healthy fats is associated with better cardiovascular outcomes."
