In the United States, ultra-processed foods (UPFs) now account for more than half of the calories people eat each day. These products include fast food and packaged snacks that are typically high in salt, added sugars and unhealthy fats. Previous research has firmly connected heavy consumption of these foods to type 2 diabetes and other chronic illnesses in adults, but their impact on younger people has received far less attention.

To help fill that gap, scientists at the Keck School of Medicine of USC conducted one of the first long-term studies focused on how UPFs affect glucose processing, a key indicator of diabetes risk. By following participants over several years, the researchers were able to observe how changes in diet were linked to changes inside the body.

Four-Year Study Tracks Early Metabolic Changes

The research team followed 85 young adults for a four-year period. Their analysis showed that participants who increased their intake of ultra-processed foods were more likely to develop prediabetes, a condition marked by elevated blood sugar levels that often precedes type 2 diabetes. Higher UPF consumption was also associated with insulin resistance, meaning the body struggled to use insulin effectively to regulate blood sugar.

The study, which received partial funding from the National Institutes of Health, was recently published in the journal Nutrition and Metabolism.

“Our findings show that even modest increases in ultra-processed food intake can disrupt glucose regulation in young adults at risk for obesity. These results point to diet as a modifiable driver of early metabolic disease, and an urgent target for prevention strategies among young people,” said Vaia Lida Chatzi, MD, PhD, a professor of population and public health sciences and pediatrics and director of the Southern California Superfund Research and Training Program for PFAS Assessment, Remediation and Prevention (ShARP) Center at the Keck School of Medicine, who is the study’s senior author.

Why Early Adulthood Is a Critical Period

Early adulthood is a stage when physical development is complete and long-lasting lifestyle habits often take shape. Choices made during this time can influence health outcomes for decades. Shifting away from packaged or restaurant foods and toward whole options such as fruits, vegetables and whole grains may lower the risk of developing type 2 diabetes later in life.

“Young adulthood is a critical window for shaping long-term health,” Chatzi said. “By focusing on young adults, we have an opportunity to intervene early, before prediabetes and other risk factors become lifelong conditions.”

How Researchers Measured Prediabetes Risk

The participants were drawn from the Metabolic and Asthma Incidence Research (Meta-AIR) study, which is part of the larger Southern California Children’s Health Study. All 85 participants were between the ages of 17-22 and completed an initial assessment between 2014 and 2018, followed by a second visit about four years later.

During each visit, participants recorded everything they ate on one weekday and one weekend day. Researchers then grouped foods into two categories: UPFs (such as candy, soda, cereal, packaged spreads, flavored yogurts, and many restaurant foods) and foods that were not ultra-processed. Using this information, they calculated the proportion of daily calories each person consumed from UPFs.

To assess how the body handled sugar, blood samples were taken before and after participants drank a sugary beverage. The team analyzed how well insulin responded to rising blood sugar levels and used statistical methods to link dietary changes with early signs of prediabetes, while accounting for age, sex, ethnicity and physical activity.

Between the first and second visits, every 10% increase in calories from ultra-processed foods was associated with a 64% higher risk of prediabetes and a 56% higher risk of impaired glucose regulation. Those who reported higher UPF intake at the beginning of the study were also more likely to show elevated insulin levels at follow-up — an early indicator of insulin resistance, when the body must produce more insulin to maintain healthy blood sugar levels.

Reducing Ultra-Processed Foods to Lower Risk

The findings make clear that the health risks linked to ultra-processed foods extend to young adults, a group that has often been overlooked in past research.

“These findings indicate that ultra-processed food consumption increases the risk for pre-diabetes and type 2 diabetes among young adults — and that limiting consumption of those foods can help prevent disease,” said the study’s first author, Yiping Li, a doctoral student in quantitative biomedical sciences at Dartmouth College who previously worked as a researcher at the Keck School of Medicine.

The researchers noted that future studies involving larger populations and more detailed dietary tracking could help identify which specific ultra-processed foods pose the greatest threat. They also plan to explore the biological pathways involved, including how certain nutrients in UPFs may affect insulin function and blood sugar control.