The Low Carb vs Low Fat Debate Just Took a Major Turn

Low-carb and low-fat diets can both protect your heart — but only if they’re built on healthy, high-quality foods.

A new study published today (February 11) in JACC, the flagship journal of the American College of Cardiology, suggests that when it comes to lowering heart disease risk, the quality of a low-carbohydrate or low-fat diet may be more important than simply reducing carbs or fat. Researchers found that both eating patterns were linked to a lower risk of coronary heart disease (CHD) when they emphasized nutrient-rich, whole foods. In contrast, versions heavy in refined carbohydrates and animal products were associated with a higher risk of CHD and less favorable metabolic health markers.

Low-carbohydrate and low-fat diets remain popular across the United States. However, their long-term impact on heart health, and how food quality influences outcomes within these diets, has not been fully understood. This investigation is one of the most extensive so far, examining diet quality, metabolomic data, and CHD risk in nearly 200,000 U.S. adults followed for more than three decades.

Why Food Choices Within Diets Matter

“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 explained that one reason earlier research may have produced conflicting results is that people can follow a low-carb or low-fat diet in very different ways. Some choose vegetables, whole grains, and healthy fats, while others rely on refined grains, processed foods, and animal-based fats.

“Focusing only on nutrient compositions but not food quality may not lead to health benefits,” he said.

Large Cohort Study With 30 Years of Data

To explore this issue in greater depth, researchers analyzed information from 198,473 participants enrolled in three major long-term studies: the Nurses’ Health Study (NHS), NHSII and Health Professionals Follow-up Study. Over a combined total of more than 5.2 million person-years of follow-up, 20,033 cases of CHD were recorded.

Participants regularly completed food frequency questionnaires. Using this data, scientists developed scoring systems to separate healthy and unhealthy versions of low-carbohydrate and low-fat diets based on food sources and overall nutrient quality.

Healthy vs Unhealthy Low Carb and Low Fat Diets

The results revealed clear differences:

• Healthy versions of both low-carb and low-fat diets that prioritized plant-based foods, whole grains and unsaturated fats were linked to a significantly lower risk of CHD.
• Unhealthy versions that emphasized refined carbohydrates and animal-based fats and proteins were associated with a higher risk of CHD.
• Healthier dietary patterns were also tied to lower triglyceride levels, higher HDL cholesterol and reduced inflammation.
• Metabolomic analyses reinforced these findings, showing more favorable biomarker patterns among those following higher-quality diets.

“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 and Broader Context

The researchers noted several limitations. The findings may not apply to more extreme eating plans with very low carbohydrate or fat intake, such as ketogenic diets, and should be interpreted within the typical ranges of macronutrient intake observed in the study. In addition, participants self-reported their diets, which can introduce measurement error. The study group consisted of health professionals who may have higher health awareness and better access to medical care than the general population, although the biological processes involved are not expected to differ substantially across groups.

“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.”

Reference: “Effect of Low-Carbohydrate and Low-Fat Diets on Metabolomic Indices and Coronary Heart Disease in U.S. Individuals” by Zhiyuan Wu, Binkai Liu, MS, Xiaowen Wang, Hala Alessa, ScD, Oana A. Zeleznik, A. Heather Eliassen, ScD, Clary Clish, Molin Wang, Kenneth J. Mukamal, Eric B. Rimm, ScD, Yang Hu, Frank B. Hu and Qi Sun, 11 February 2026, Journal of the American College of Cardiology.
DOI: 10.1016/j.jacc.2025.12.038

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