A four-year study of young adults found that relatively small increases in ultra-processed food consumption were associated with substantially higher risk of prediabetes and insulin resistance — changes that can begin long before someone feels sick.
Prediabetes is easy to imagine as a problem of middle age: the consequence of decades of weight gain, inactivity or gradually worsening metabolic health.
But the biology that eventually leads to type 2 diabetes can begin much earlier. A study following young adults in Southern California for approximately four years offers a glimpse of how early those changes may emerge — and how closely they may be connected to the increasingly processed American diet.
Researchers from the Keck School of Medicine of USC studied 85 young adults who were between 17 and 22 years old when they entered the study and had a history of overweight or obesity. They found that participants who increased the proportion of ultra-processed foods in their diets were more likely to develop signs of impaired glucose regulation by the time they were evaluated again several years later.
The size of the association was striking. For every 10-percentage-point increase in the proportion of daily calories coming from ultra-processed foods, the researchers found a 64% higher risk of prediabetes and a 56% higher risk of impaired glucose regulation at follow-up.
Those numbers do not mean that eating 10% more ultra-processed food makes an individual 64% more likely to develop diabetes. The study was observational, relatively small and limited to young people who already had a history of overweight or obesity. It can identify a relationship, but it cannot prove that ultra-processed foods themselves caused the metabolic changes. Still, the findings matter because of what was already happening beneath the surface.
The researchers weren't simply looking at body weight. They were looking at the machinery that controls blood sugar — including insulin levels and the body's ability to handle a glucose challenge — and found evidence that higher ultra-processed food consumption was associated with that machinery beginning to struggle.
More than half of the American diet
Ultra-processed foods now account for more than half of the calories consumed in the United States. They include many familiar products: soft drinks, candy, packaged snacks and spreads, sweetened cereals, flavored yogurts, fast food and numerous commercially prepared meals.
What makes a food “ultra-processed,” however, is not simply that it comes in a package. The researchers classified foods using the NOVA system, which separates foods according to the degree and purpose of industrial processing. At one end are unprocessed or minimally processed foods such as fresh vegetables, fruit, eggs and meat. At the other are industrial formulations typically assembled from refined ingredients, additives, flavorings, emulsifiers, sweeteners and other substances that would rarely appear together in a home kitchen. That distinction has become increasingly important in nutrition research. A growing body of evidence has associated diets high in ultra-processed foods with obesity, cardiovascular disease, type 2 diabetes and other adverse health outcomes.
Much of that research, however, has focused on adults. Far less is known about what happens when a diet dominated by these foods becomes established during adolescence and early adulthood. That was the question the USC researchers wanted to explore.
Four years between two snapshots
The participants came from the Metabolic and Asthma Incidence Research study, itself part of the long-running Southern California Children's Health Study. They completed their first assessments between 2014 and 2018 and returned approximately four years later, between 2020 and 2022.
At both visits, researchers collected detailed dietary information. Participants reported everything they had eaten during one recent weekday and one weekend day. Each food was then classified as either ultra-processed or non-ultra-processed, allowing the researchers to estimate the percentage of daily calories coming from UPFs. The investigators also did something considerably more revealing than asking participants whether they had diabetes.
They performed an oral glucose tolerance test.
After fasting, participants consumed a glucose drink and had blood collected so researchers could observe how their bodies handled the sudden influx of sugar. The test offers a window into glucose regulation and insulin function that a simple dietary questionnaire cannot provide. The researchers could therefore compare what had changed in the participants' diets with what had changed in their metabolism.
Those who reported higher ultra-processed food consumption at the beginning of the study were more likely to show elevated insulin levels years later. And those who increased their UPF consumption over time were more likely to show signs of prediabetes and impaired glucose regulation. Elevated insulin is particularly interesting because it can appear before glucose reaches diabetic levels.
Type 2 diabetes usually does not appear overnight.
One of the earliest problems is often insulin resistance. Cells become less responsive to insulin, the hormone that helps regulate glucose in the bloodstream.
At first, the pancreas can compensate. It simply produces more insulin. Blood glucose may remain within a seemingly acceptable range, while the body is working increasingly hard to keep it there. This state of elevated insulin can persist for years before conventional glucose measurements clearly signal a problem. Eventually, compensation may become inadequate. Glucose begins to rise, prediabetes appears and, in some people, type 2 diabetes follows.
That progression is one reason the age of the participants makes the USC findings noteworthy. These weren't people being evaluated after decades of metabolic disease. Some were teenagers when researchers first examined them. The metabolic warning signs were already emerging.
But what exactly is the problem with ultra-processed food?
Ultra-processed foods tend to share several characteristics that could plausibly contribute to metabolic problems. Many are high in refined carbohydrates, added sugars, sodium and energy while containing relatively little fiber. They are often engineered to be highly palatable and easy to consume quickly. But “ultra-processed” describes an enormous category. A sugar-sweetened soda, packaged bread, breakfast cereal, protein bar, flavored yogurt and frozen dinner may all qualify under NOVA, despite having very different nutritional profiles. That makes it difficult to determine whether processing itself is responsible for the associations observed in population studies or whether the problem comes from particular nutrients, combinations of ingredients, eating patterns or the tendency of some UPFs to encourage greater calorie consumption.
The authors acknowledge that more research is needed to identify which foods — and which characteristics of those foods — are most strongly associated with metabolic risk. Their study cannot answer that question. Nor can it tell us that every food classified as ultra-processed carries the same risk.
A 64% increase sounds enormous. Here's what it means.
Relative-risk statistics can easily create misleading headlines, so the study's central finding deserves some context. The researchers did not report that 64% of people eating ultra-processed foods developed prediabetes. They found that each 10-percentage-point increase in the share of calories coming from ultra-processed foods was associated with a 64% increase in relative risk of prediabetes within this particular group.
Imagine, for example, that ultra-processed foods supplied 40% of someone's calories at the beginning of the study and 50% several years later. That would represent the kind of 10-percentage-point increase being examined. The finding suggests that the direction in which the diet was moving mattered.
But because the study included only 85 participants, its estimates also need to be confirmed in much larger populations. Dietary assessment presents another limitation. Researchers relied on two 24-hour dietary recalls at each visit — one weekday and one weekend day. Those snapshots are useful in nutritional epidemiology, but two days cannot perfectly represent what someone ate throughout four years.
The researchers adjusted their analyses for factors including age, sex, ethnicity and physical activity, but observational studies can never eliminate every possible difference between people who eat more or less ultra-processed food. In other words, this study provides a warning signal, not a verdict.
The important part may be how early the warning appears
People in their late teens and twenties are generally considered too young to worry much about chronic metabolic disease. Yet it is also the period when many people begin making their own food decisions for the first time. They leave home. They start college or work. Convenience becomes important. Meals become irregular. Restaurant food, delivery apps, packaged snacks and inexpensive ready-to-eat products can easily become the default diet. Those habits can persist for decades.
The USC researchers argue that this makes early adulthood an important window for prevention. Waiting until someone develops type 2 diabetes at 45 or 55 may mean overlooking metabolic changes that began much earlier. And the practical message from the study is not necessarily that young adults need to eliminate every packaged food from their lives. It is that the proportion of the diet matters.
Replacing some packaged and restaurant foods with less processed alternatives — vegetables, fruit, whole grains and other minimally processed foods — shifts that proportion in the opposite direction.
For people following lower-carbohydrate eating patterns, the specific foods may look different, but the principle remains relevant: meat, fish, eggs, vegetables, nuts and other minimally processed foods are fundamentally different from constructing most of a diet around industrial formulations simply because the label says “low carb,” “keto,” “high protein” or “healthy.”
Processing does not disappear because the front of the package carries a health claim.
Prediabetes doesn't have to wait until middle age
Perhaps the most unsettling aspect of the study is also the most useful: the participants were young enough that many would never have thought of themselves as candidates for prediabetes. Yet researchers could already detect differences in insulin and glucose regulation associated with the direction their diets had taken over just a few years.
That does not mean a teenager who eats fast food is destined to develop diabetes. Genetics, body composition, physical activity, sleep, socioeconomic conditions and many other factors contribute to metabolic health. Nor does this study prove that ultra-processed foods alone caused the changes researchers observed.
What it suggests is that metabolic disease has a long runway. By the time type 2 diabetes receives a diagnosis, the processes leading toward it may have been developing quietly for years. That makes young adulthood more than a period in which lifelong eating habits are formed. It may also be one of the periods when changing those habits still offers the greatest opportunity to alter what comes next.
The most important meal for preventing diabetes, in other words, may not be the one someone starts eating after receiving a frightening blood test at 50. It may be the ordinary meals they were eating decades earlier.
Study: Li Y, et al. Ultra-processed food intake is associated with altered glucose homeostasis in young adults with a history of overweight or obesity: a longitudinal study. Nutrition & Metabolism. 2025. DOI: 10.1186/s12986-025-01036-6.
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