What the scientists found was that a 10% increase in the proportion of ultra-processed foods in the diet was associated with a significant increase of greater than 10% in risks for overall cancer and breast cancer.
"Ultra-processed fats and sauces, sugary products and drinks were associated with an increased risk of overall cancer," the study says. "Ultra-processed sugary products were associated with an increased risk of breast cancer."
People who tended to eat more ultra-processed food also tended to smoke more and exercise less than the others, but the authors controlled for these issues and still found the elevated cancer risk.
"What people eat is an expression of their lifestyle in general and may not be causatively linked to the risk of cancer. So it is necessary to rule out what are called cofounding factors," said Tom Sanders, scientific governor of the British Nutrition Foundation and an emeritus professor at King's College London.
Sanders, who was not involved in the study, said the authors made statistical adjustments to accommodate for some of that, but he cautions that "the approach of categorizing dietary patterns that depend on industrially processed food in relation to disease risk is novel but probably needs refining before it can be translated into practical dietary advice."
The nonprofit trade group Association of Food Industries did not respond to requests for comment.
"This study doesn't mean that people should think 'if I eat this cracker, I'm going to get cancer,' " McCullough said. "The overriding message of this study was really to look at an overall diet pattern rather than a specific ingredient, and it supports a lot of what we already know."
For example, she said, people eating more highly processed foods are probably eating fewer healthy foods, which may help prevent cancer. Nutritionists recommend a diet rich in whole grains, whole fruits and vegetables instead of foods that have little nutritional value.
Animal studies have shown that some additives are "quite good candidates" for being carcinogenic, Touvier said, "but that would need to be seen if they are also carcinogenic in the human population."
If you are starting to worry about what you've brought for lunch, Touvier cautions not to be "too alarmist" about this research.
A balanced and diversified diet should be considered one of the most important public health priorities, the authors advise. "Eat real food and try to limit ultra processed items," Touvier said. "At least until we know more."