Ultra-Processed Foods Push Young Adults Toward Prediabetes

The foods that come in crinkly wrappers and neon bottles are not just empty calories. In a first-of-its-kind longitudinal study, Keck School of Medicine of USC researchers followed 85 young adults for roughly four years and found that rising ultra-processed food intake was linked to disrupted glucose control, elevated insulin, and sharply higher odds of prediabetes. The work, published in Nutrition & Metabolism on November 10, 2025, adds urgent weight to concerns about what an ultra-processed diet does to bodies that are still solidifying lifelong habits.

More than half of U.S. calories now come from ultra-processed foods, or UPFs, a catchall for items like fast food, packaged snacks, sweetened drinks, flavored yogurts, and many restaurant meals. Prior research pinned UPFs to type 2 diabetes in older adults, but evidence in youth has been thin and often cross-sectional. This study tracked the same individuals over time, offering a clearer window into cause-adjacent changes in glucose homeostasis.

Participants, ages 17 to 22 at enrollment, completed two 24-hour dietary recalls at each of two clinic visits, then underwent blood testing before and after a sugary drink to gauge how their bodies handled glucose. Researchers categorized every reported item by NOVA processing level and calculated the share of each diet that was ultra-processed by weight. Between the first and second visits, average UPF intake rose from about 20 percent to almost 24 percent.

The metabolic signals moved in the wrong direction. A 10 percentage-point increase in UPF consumption was associated with 51 percent higher odds of prediabetes and 158 percent higher odds of impaired glucose tolerance at follow-up. Higher baseline UPF intake also predicted higher insulin levels two hours after the glucose challenge and a larger insulin area under the curve, classic footprints of mounting insulin resistance. Meanwhile, the Matsuda index, a whole-body insulin sensitivity metric, fell as UPFs rose.

“These results point to diet as a modifiable driver of early metabolic disease, and an urgent target for prevention strategies among young people.”

Those numbers matter because they point to risk before a diagnosis is etched into the chart. Prediabetes is the biological rumble strip before type 2 diabetes, and in young adults it can set decades of vascular, renal, and neurological complications in motion. The authors note that nutrients common to UPFs, including saturated fats and added sugars, are known to stress pancreatic beta cells and blunt insulin sensitivity, potentially exhausting the very systems that hold blood sugar in check.

Inside The Study: Who, What, And How

The longitudinal design is a key strength. Participants were drawn from the Metabolic and Asthma Incidence Research cohort, part of the Southern California Children’s Health Study, and all had a history of overweight or obesity in early adolescence. That focus sharpened the lens on youth at elevated risk, while repeated, standardized testing at both time points reduced the fuzziness that can plague diet research.

At each visit, investigators used well-established dietary recall tools and then mapped foods to NOVA categories, classifying items as unprocessed, processed culinary ingredients, processed, or ultra-processed. Oral glucose tolerance tests captured fasting and post-load glucose and insulin at five time points, allowing calculation of insulin sensitivity and secretion indices. Analyses adjusted for age, sex, ethnicity, energy intake, and physical activity; sensitivity checks that added smoking and alcohol did not budge the results.

Not everything shifted significantly. Body composition measures trended upward over time, as one might expect in late adolescence and early adulthood, but associations between UPF change and adiposity were positive and not statistically significant in this small sample. Even so, the convergence of higher insulin, lower sensitivity, and worse tolerance makes the glucose story hard to ignore.

Why It Matters Now

Young adulthood is when independence meets convenience. Late classes, long shifts, and limited budgets make heat-lamped fries and shelf-stable snacks a normal dinner. The study’s most practical takeaway is also the simplest: swap a few of those items for whole or minimally processed foods and you may nudge insulin and glucose back toward safer ground. Fruits, vegetables, legumes, whole grains, and unsweetened dairy or water are unglamorous, but metabolically protective.

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

There are caveats. Eighty-five participants is not a population, and dietary recalls can miss what a camera on the fridge would catch. Odds ratios can also overestimate risk when outcomes are common. Yet the findings echo a larger body of adult research and, crucially, bring detailed physiology into view for youth. Bigger cohorts with finer-grained diet tracking could identify which specific UPFs do the most damage and why.

Until then, the guidance is clear enough to act on. If UPFs inch up, metabolic risk follows. For families, clinicians, and campus health teams, that is a lever worth pulling early and often.

Nutrition & Metabolism: 10.1186/s12986-025-01036-6