Here’s the headline most of us actually care about: it’s not whether you cut carbs or fat that matters most for your heart—it’s what those carbs and fats are made of.
A large new study from researchers at Harvard T.H. Chan School of Public Health adds some long-needed clarity to the low-carb versus low-fat debate. The takeaway is refreshingly straightforward. Diets that are lower in carbohydrates or lower in fat can support heart health—but only when they’re built around high-quality, mostly plant-based foods. When those same diets lean heavily on refined carbs and animal-based fats and proteins, heart disease risk goes the other direction.
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What the study actually examined
The findings, published in the Journal of the American College of Cardiology, push back against the idea that simply manipulating macronutrients is a shortcut to better health. “Low-carbohydrate and low-fat diets have been widely promoted in the US over the past two decades for weight control and metabolic health, but their effects on heart disease risk have remained unclear,” lead author Zhiyuan Wu said in a statement.
“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.”
To get there, the research team analyzed decades of dietary and health data from nearly 200,000 adults enrolled in the Health Professionals Follow-Up Study, the Nurses’ Health Study, and the Nurses’ Health Study II. Participants’ eating patterns were scored to distinguish “healthy” versions of low-carb and low-fat diets—think whole grains, fruits, vegetables, legumes, nuts, and plant oils—from “unhealthy” versions heavy in refined carbohydrates and animal-derived foods. Researchers then tracked who went on to develop coronary heart disease, while accounting for lifestyle and health factors that could skew results.
The contrast was clear. People following low-carb or low-fat diets anchored in high-quality, plant-based foods had about a 15 percent lower risk of coronary heart disease. Meanwhile, low-carb or low-fat approaches dominated by refined grains, animal fats, and animal proteins were linked with a higher risk. Blood markers told a similar story: healthier versions of both diets were associated with higher levels of HDL cholesterol and lower triglycerides, both considered favorable for cardiovascular health.
What this means if you’re eating low-carb or low-fat
What makes this study especially relevant is how closely it mirrors the way people actually eat. Plenty of shoppers already buy low-carb or low-fat products, assuming the label alone signals something heart-healthy. This research suggests that assumption is shaky at best. A low-carb diet built around vegetables, beans, nuts, and olive oil looks very different—in terms of heart risk—from one stacked with processed meats and refined starches, even if the carb count technically checks out.
Hannah Sunderani
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For anyone feeling whiplash from years of diet trends, the guidance from the researchers is intentionally broad. “For clinicians, dietitians, and patients, our study suggests that promoting an overall healthy eating pattern, rather than strict macronutrient restriction, should be a central strategy for the primary prevention of heart disease,” said corresponding author Qi Sun.
In other words, you don’t need to swear allegiance to carbs or fat. You just need to care about where your food comes from—and how processed it is.
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