What makes a diet healthy-or not? According to a new study, nutritional quality matters more than processing level when it comes to long-term health.

The study of over 200,000 people found that the overall quality of a person's diet was strongly associated with their risk of heart disease, type 2 diabetes, and death. However, these associations did not appear to substantially differ based on food processing levels.

While many ultra-processed foods tend to be of poor nutritional quality and should generally be limited or avoided, processing itself is not inherently harmful."

Xiaowen Wang, PhD, postdoctoral research fellow, Department of Nutrition, Harvard T. H. Chan School of Public Health

Based on the study results, she said individuals should "focus on what you're eating overall, not just whether it's processed. A healthy plant-based diet rich in whole grains, fruits, vegetables, nuts, legumes and healthy plant oils is beneficial, even if it includes some processed foods."

Wang will present the findings at NUTRITION 2026, the flagship annual meeting of the American Society for Nutrition, held July 25–28 in National Harbor, Maryland, just outside Washington, D.C.

The nutritional quality of a person's diet and the amount of ultra-processed foods they consume are often intertwined. Many ultra-processed foods are high in added sugars, sodium and refined carbohydrates, so eating a lot of these foods is associated with poor dietary quality. On the other hand, ultra-processed foods like fortified breakfast cereals, whole grain products, canned beans, or plant-based dairy can still provide important nutrients, fiber, antioxidants and other beneficial compounds, contributing to a healthy diet.

For the study, researchers sought to separate overall diet quality from the level of food processing. Using data from three studies that collectively followed over 200,000 U.S. adults from the 1980s through the 2020s, they used a framework known as the plant-based diet index to assess the degree to which each participant emphasized plant versus animal food products in their diet, the overall quality or healthfulness of their diet, and their consumption of plant-based foods across a wide spectrum of processing levels. The study was not limited to vegans or vegetarians; however, to enable consistent comparisons, the analysis focused on the plant-derived components of participants' diets rather than animal-derived foods.

After assessing each participant's overall diet and using the Nova classification system to categorize the processing level of the foods they consumed, researchers grouped participants according to their plant-based eating habits:

• Healthy/ultra-processed (for example, whole grain breakfast cereal with fortified plant-based milk, plant-based yogurt, whole-grain bread, and canned fruit);
• Healthy/minimally-processed (for example, oatmeal with fresh fruit and nuts, quinoa and vegetable salad with olive oil, and homemade lentil soup);
• Unhealthy/ultra-processed (for example, fast food meals, French fries, soda, packaged desserts, and refined snacks); and
• Unhealthy/minimally-processed (for example, white rice, fruit juice, potatoes, and homemade desserts made with refined flour and sugar).

About half of the study participants eventually died or developed heart disease or type 2 diabetes. According to the results, the risk of these outcomes varied significantly depending on overall diet quality: healthy diets were associated with lower risk, while unhealthy diets were associated with higher risk. Within each category, however, the risks were almost identical regardless of whether the diet was ultra-processed or minimally-processed.

The results call into question the use of processing level as a key indicator of healthy eating. Since the Nova classification does not take into account the nutritional value of foods, healthy foods can be classified as ultra-processed while minimally-processed foods can still be unhealthy. 

"Advising people to universally 'avoid ultra-processed foods' can lead to lower diet quality since healthy ultra-processed foods are beneficial and unhealthy minimally-processed foods are detrimental," said Wang. "People should prioritize the quality of foods rather than focusing solely on the level of processing. Aim for more nutrient-dense, high-quality foods to help lower your risk of chronic diseases."

Based on the findings, she suggested that dietary guidelines should continue to promote established high-quality diets, like the Mediterranean or healthy plant-based diets, rather than focusing on processing level. She noted that encouraging food manufacturers to improve the nutritional content of packaged foods could also help to make following a healthy plant-based diet more convenient and accessible.