Both low-carbohydrate and low-fat diets were linked to about a 15 percent lower risk of coronary heart disease, but only when they leaned on plant-based whole foods rather than refined carbs and animal products.
That is the message from a Harvard T.H. Chan School of Public Health analysis of nearly 200,000 US adults, published February 11, 2026 in the Journal of the American College of Cardiology.
When either diet was built on refined carbohydrates, processed foods and animal fats, the picture flipped. Those versions were tied to a higher risk of heart disease and worse metabolic profiles.
For readers who have spent years choosing between low-carb and low-fat camps, the practical upshot is short. The macro label matters less than what is actually on the plate.
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What the study looked at
The researchers pooled data from three long-running cohorts, the Nurses' Health Study, Nurses' Health Study II and the Health Professionals Follow-up Study.
Together they covered 198,473 US adults and more than 5.2 million person-years of follow-up, with 20,033 cases of coronary heart disease documented over roughly three decades, according to a press release from the American College of Cardiology.
Diets were scored on quality, not just macronutrient ratios. Blood samples were used to check cardiovascular biomarkers such as HDL cholesterol, triglycerides and inflammation. Healthy versions of both diet patterns showed higher HDL, lower triglycerides and reduced inflammation.
This is an observational study, so it can show a link but not prove cause and effect. Its strength lies in the size of the population and the length of the follow-up.
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What high quality actually means
The study drew a clear line between the healthy and unhealthy versions of each diet, as Harvard T.H. Chan School of Public Health explained in its summary of the findings.
High-quality choices meant whole grains, fruits, vegetables, legumes, nuts and olive oil, along with plant-based proteins and fats. Fish and low-fat dairy also fit the healthy pattern, according to nutrition experts quoted in coverage of the study by Stat News.
Low-quality choices meant refined carbohydrates, added sugars, processed foods and animal proteins and fats. A low-carb plate heavy on red and processed meat was not protective. Neither was a low-fat plate built on white bread, sugary cereal and snack foods.
Lead author Zhiyuan Wu, a postdoctoral research fellow at Harvard T.H. Chan, said in the university's release that "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."
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What it means for your grocery list
Corresponding author Qi Sun, an associate professor at Harvard T.H. Chan, argued the shift should reach clinics too. "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," Sun said in the school's release.
Andrea Deierlein, director of public health nutrition at NYU's School of Global Public Health, told Stat News that "there are many ways of combining those foods to create a healthy dietary pattern when focusing on beans, peas, legumes, fruits and vegetables, lean animal proteins."
Five simple swaps line up with what the study calls high quality. White bread for whole grain bread. Sugary breakfast cereal for oatmeal with nuts. Chips for a handful of almonds or walnuts. Processed deli meat for beans, lentils or fish. Butter for olive oil.
Anyone with heart disease, diabetes or other chronic conditions should talk with a doctor or dietitian before overhauling their diet, especially if they take medication that reacts to changes in eating patterns.
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