Harvard study finds heart health depends on food quality, not just diet type

A major Harvard study has revealed that the quality of food consumed is just as important as whether a person follows a low-carbohydrate or low-fat diet when it comes to protecting heart health.

The research from Harvard T.H. Chan School of Public Health, which tracked nearly 200,000 adults over several decades, found that people who adhered to either diet but focused on high-quality, plant-based foods had a significantly lower risk of developing coronary heart disease.

The researchers are presenting their findings at NUTRITION 2025, the American Society for Nutrition’s flagship annual meeting, currently taking place (31 May–3 June) in Orlando.

“We found that what you eat on low-carb or low-fat diets matters just as much as the diet itself,” said Zhiyuan Wu, PhD, lead author and postdoctoral fellow at Harvard.

“Healthy versions of these diets – those rich in plant-based foods and whole grains – were linked to better heart health outcomes and improved metabolic function.”

Evaluating diet quality

The Harvard study analysed data from three long-running US cohort studies: the Health Professionals Follow-Up Study (43,430 men, 1986–2016), the Nurses’ Health Study (64,164 women, 1986–2018), and Nurses’ Health Study II (92,189 women, 1991–2019).  All participants completed detailed food frequency questionnaires, which enabled researchers to evaluate the quality of their diets.

Researchers assessed diet quality by classifying foods into two categories:

• High-quality foods included whole grains, fruits, vegetables, nuts and legumes
• Low-quality foods included refined grains, potatoes, processed meats, added sugars and saturated fats from animal sources

Their analysis showed that individuals who consumed high-quality foods, regardless of whether they followed a low-carb or low-fat diet, reduced their risk of coronary heart disease by around 15 percent. In contrast, those who followed diets high in processed or animal-based foods faced an increased risk.

For more than 10,000 of the study participants, the researchers also measured hundreds of blood metabolites to assess how diet quality influenced their metabolic regulation. “This approach allowed us to better understand the biological effects of these diets and strengthened our findings,” said Wu.

Importantly, the findings underline the need for food professionals and public health experts to shift focus away from macronutrient ratios alone and instead promote better food choices.

Wu concluded:

Our findings suggest that improving food quality is crucial for improving heart health. Regardless of whether someone follows a low-carbohydrate or low-fat diet, emphasising whole, minimally processed and plant-based foods and limiting refined grain, sugar and animal foods can significantly reduce the risk of coronary heart disease.

The Harvard team is now expanding its research to examine how genetics, lifestyle factors and other biomarkers may interact with diet to influence heart health, as well as exploring broader health implications such as risks of type 2 diabetes and cancer.

Researchers’ diet recommendations

For those aiming to improve their diet, the researchers recommend increasing intake of whole grains, fruits, vegetables, nuts and legumes, while reducing consumption of processed meats, refined carbs and sugary foods.

They note that it is also important to check food labels and be mindful of added low-quality ingredients, such as added sugars in juices and processed snacks.