Diet quality health study finds processing level may matter less

A new diet quality health study suggests the nutritional makeup of what people eat may be more closely tied to long-term health than whether foods are heavily processed. The American Society for Nutrition said the findings could affect how doctors, dietitians and public health officials talk about ultra-processed foods.

The research, led by Xiaowen Wang, a postdoctoral research fellow in the Department of Nutrition at Harvard T.H. Chan School of Public Health, examined data from more than 200,000 U.S. adults. Wang is scheduled to present the findings at NUTRITION 2026, the American Society for Nutrition’s annual meeting, held July 25-28 in National Harbor, Maryland.

According to the American Society for Nutrition, overall diet quality showed a strong association with risk of heart disease, type 2 diabetes and death. Those links did not substantially change when researchers compared diets by food processing level.

Does processing matter more than diet quality?

In this study, researchers found that diet quality mattered more. Healthy diets were linked with lower risk, while unhealthy diets were linked with higher risk, and those patterns were nearly the same whether the foods were categorized as ultra-processed or minimally processed, according to the American Society for Nutrition.

Wang said the results do not mean all ultra-processed foods are good choices. Many are high in added sugar, sodium and refined carbohydrates, she said, and should generally be limited or avoided. Her point, as described by the American Society for Nutrition, is that processing by itself does not make a food harmful.

The researchers used data from three long-running studies that followed U.S. adults from the 1980s into the 2020s. They assessed diets with a plant-based diet index, which measures how much a person emphasizes plant foods versus animal foods, along with the quality of those plant-based choices.

The analysis was not limited to vegetarians or vegans. To make comparisons consistent, the researchers focused on plant-derived parts of participants’ diets rather than animal-derived foods, according to the American Society for Nutrition.

How did researchers classify the diets?

The team used the Nova system to classify foods by processing level. Nova groups foods according to how much industrial processing they undergo, but it does not judge whether a food is nutrient-rich or nutrient-poor.

Researchers then sorted participants into four broad diet patterns: healthy and ultra-processed, healthy and minimally processed, unhealthy and ultra-processed, and unhealthy and minimally processed. Examples of healthy ultra-processed foods included whole-grain breakfast cereal with fortified plant-based milk, plant-based yogurt, whole-grain bread and canned fruit.

Healthy minimally processed examples included oatmeal with fresh fruit and nuts, quinoa and vegetable salad with olive oil, and homemade lentil soup. The unhealthy ultra-processed group included fast food, fries, soda, packaged desserts and refined snacks, while the unhealthy minimally processed group included white rice, fruit juice, potatoes and homemade desserts made with refined flour and sugar.

About half of the participants eventually died or developed heart disease or type 2 diabetes, according to the American Society for Nutrition. Across those outcomes, researchers reported that people eating healthier diets had lower risk than those eating less healthy diets.

The findings challenge the idea that processing level alone is a reliable shorthand for healthy eating. The American Society for Nutrition said Wang suggested dietary guidance should keep emphasizing established high-quality eating patterns, including Mediterranean and healthy plant-based diets, while food makers could help by improving the nutrition of packaged foods.

This story draws on original reporting from Medical Xpress.