About the study

For the study, published in JAMA Oncology, researchers from Massachusetts General Brigham analyzed data from 29,000 women enrolled in the long-running Nurses' Health Study II.

Participants were aged 24 to 42 when they joined the study in 1989 and were followed for 24 years, from 1991 to 2015. Every four years, the participants completed detailed dietary questionnaires, and all had undergone at least one colonoscopy before age 50.

The research team evaluated whether participants developed precancerous colorectal growths — either adenomas, which are more likely to progress to cancer, or serrated lesions, which are considered lower-risk. More than 1,200 women in the study were diagnosed with adenomas.

On average, ultra-processed foods accounted for more than one-third of daily caloric intake. These foods included breads and breakfast items, sauces and condiments, and sugar-sweetened or artificially sweetened beverages. The researchers assessed participants' intake patterns against colonoscopy results to identify associations between dietary habits and early colorectal changes.

The researchers consistently found that women who consumed the most ultra-processed foods were about 1.5 times likelier to develop adenomas compared to those who ate the least. Researchers also found that diets higher in sugar and artificial sweeteners were most strongly linked to adenoma risk, followed by diets high in sauces, spreads, and condiments. No link was identified between ultra-processed foods and serrated lesions.

The researchers found that women who consumed nine to 10 servings of ultra-processed foods daily had a 45% greater likelihood of developing colon polyps before age 50 than women who consumed around three servings daily. That association remained even after adjusting for body mass index, type 2 diabetes, and dietary features like fiber intake. 

Commentary

According to Andrew T. Chan, the study's senior author and a gastroenterologist at Massachusetts General Brigham, the results highlight association rather than causation. "It gives us some clues into the potential role of diet in the development of early-onset colorectal cancer precursor lesions," he said. "And I think it is probably the best available data we have now."

Experts not involved in the study said the results strengthen current evidence on how diet may contribute to early colorectal risk. According to Leah Ferrucci, an assistant professor of epidemiology at the Yale School of Public Health, "there are very few existing epidemiologic studies in which we can investigate early-onset cancers, as they are still relatively rare."

"[T]his is an important new piece of evidence" that could help inform future research, she added.

Additional hypotheses involve gut microbiome disruption. Chan said that ultra-processed foods are known to alter microbial communities in ways that "in theory, could make cells in the gut more likely to turn cancerous."

Still, scientists caution that ultra-processed foods are a broad category, "some of which are unhealthy, and some of which may even be healthy," said Sarah Berry, a professor of nutritional sciences at King's College London and a study coauthor. "Every food is processed in a different way," she added, underscoring that risk cannot be assumed uniformly across product types.

For consumers looking to reduce intake, small, incremental changes may help. Dalia Perelman, a research dietitian at Stanford University not involved in the study, said that the findings strengthen existing evidence that ultra-processed foods may play a role in the disease process.

Her advice: shift toward simpler ingredients. "It doesn't have to be complicated," Perelman said. "And it certainly doesn't have to be perfect."

As researchers continue to investigate rising cancer rates in younger adults, Chan noted that shifts in dietary habits may be one part of the broader picture. "One thing that has been clear is that the U.S. intake of ultra-processed food has really risen in the past few decades in a way that mirrors the staggering increase in colorectal cancer cases," he said.