For decades, dietary advice in the United States has centered on modulating carbohydrate and fat intake. But a new study suggests that simply focusing on the amount of these macronutrients may be misguided. Instead, the quality of carbohydrates and fats—where they come from—appears to be the key determinant of heart health, according to research published in the Journal of the American College of Cardiology.
Researchers at Harvard T.H. Chan School of Public Health analyzed dietary data and health outcomes from nearly 200,000 health professionals over a 30-year period. The findings indicate that both low-carbohydrate and low-fat diets can be associated with a lower risk of coronary heart disease (CHD)—but only when they emphasize high-quality, plant-based foods and limit animal products and refined carbohydrates.
Quality Over Quantity
The study challenges the long-held belief that merely reducing carbs or fats is inherently beneficial for the heart. “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,” explained Zhiyuan Wu, PhD, a postdoctoral research fellow in the Department of Nutrition at Harvard and first author of the study. “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.”
Specifically, the research revealed that low-carb and low-fat diets rich in refined carbohydrates and high in animal fats and proteins were actually associated with a higher risk of CHD. In contrast, those emphasizing healthy carbohydrates – like whole grains – were linked to a 15% reduction in risk. A similar benefit was observed with low-fat diets prioritizing plant-based fats.
What Does “Quality” Mean?
The distinction lies in the source of the macronutrients. “It’s the quality of your diet that matters,” stated Dr. Qi Sun, a specialist in nutrition and epidemiology at Harvard T.H. Chan School of Public Health. This means prioritizing whole, unprocessed foods over refined and processed options.
For carbohydrates, this translates to choosing whole grains, fruits, and vegetables instead of white bread, sugary drinks, and pastries. For fats, it means opting for sources like olive oil, avocados, nuts, and seeds, while limiting saturated and trans fats found in red meat and processed foods. Protein sources should lean towards fish, beans, lentils, and poultry, rather than processed meats.
Metabolic Pathways and Heart Health
The study also explored the metabolic effects of these different dietary patterns. Researchers found that healthy low-carbohydrate and low-fat diets may exert their protective effects through common pathways that contribute to a favorable cardiovascular risk profile and lower CHD risk. The details of these pathways are outlined in a related publication in in JACC, detailing the impact on metabolomic indices.
Observational Study Considerations
It’s important to note that this study was observational, meaning it identified associations between dietary patterns and health outcomes, but it could not definitively prove cause, and effect. Researchers followed a large cohort for a long period, which strengthens the findings, but other factors could also contribute to the observed relationships. As noted by Camilla Dalby Hansen, a researcher at the University of Southern Denmark, Odense, who was not involved in the study, the long-term follow-up and large sample size are “impressive.”
Implications for Public Health
The findings have significant implications for public health recommendations. Rather than focusing solely on restricting carbohydrates or fats, health professionals may need to emphasize the importance of dietary quality. This means encouraging individuals to adopt eating patterns rich in plant-based foods, whole grains, and unsaturated fats, while limiting refined carbohydrates, processed foods, and animal products.
The research reinforces the benefits of dietary approaches like the Mediterranean diet, which naturally emphasizes these healthy food choices. “Because this was an observational study, researchers can’t directly prove certain diets actually caused study participants to get heart disease,” but the data strongly suggests a link between food quality and cardiac health.
While the study focused specifically on coronary heart disease, the principles of prioritizing food quality likely extend to overall health and well-being. The message is clear: when it comes to diet, what you eat is just as important as how much you eat.
