Low-carbohydrate and low-fat eating plans are common in the United States. However, scientists have not fully understood their long-term effects on heart health, especially when considering the quality of foods included in each pattern. This research stands out as one of the largest and most detailed investigations to date, examining diet quality, metabolomic data, and CHD risk in nearly 200,000 U.S. adults followed for more than 30 years.

Why Food Sources Matter in Low Carb and Low Fat Diets

“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 noted that earlier research may have produced conflicting results because people can follow a low-carb or low-fat diet in very different ways. Some prioritize vegetables, whole grains, and healthy fats, while others rely more heavily 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 Long Term Study of Nearly 200,000 Adults

To better understand these differences, researchers analyzed data from 198,473 participants enrolled in three major cohort studies: the Nurses’ Health Study (NHS), NHSII and Health Professionals Follow-up Study. Together, participants were tracked for more than 5.2 million person-years, during which 20,033 cases of CHD were recorded.

Dietary habits were measured using food frequency questionnaires. The research team then developed scoring systems to separate healthier and less healthy versions of low-carbohydrate and low-fat diets, based on the types of foods consumed and overall nutrient quality.

Healthy vs Unhealthy Diet Patterns

The analysis revealed several important patterns:

• Healthy versions of both low-carb and low-fat diets that emphasized plant-based foods, whole grains and unsaturated fats were associated with a significantly lower risk of CHD.
• Unhealthy versions that were high in refined carbohydrates and animal-based fats and proteins were linked to a greater risk of CHD.
• Higher-quality diets were also associated with lower triglyceride levels, higher HDL cholesterol and reduced inflammation.
• Metabolomic testing supported these findings, showing favorable biomarker patterns among people who followed healthier diet 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 and Broader Implications

The researchers cautioned that the findings may not apply to more extreme dietary approaches with very low carbohydrate or fat intake, such as ketogenic diets. The results should be interpreted within the ranges of macronutrient intake observed in the study. In addition, dietary information was self-reported, which can introduce measurement error. Because participants were health professionals with generally higher health awareness and better access to care, the findings may not fully represent the general population, although the biological mechanisms involved are unlikely to differ substantially.

“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.”