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Are some ultra-processed foods worse than others?

Name a common condition — heart disease, Type 2 diabetes, cancer, dementia, irritable bowel syndrome — and chances are good that following a diet high in ultra-processed foods has been linked to it. But the ultra-processed food category is large and wide-ranging. It makes up an estimated 73% of the U.S. food supply, and contains stereotypi­cally “unhealthy” products like sodas, candies and hot dogs as well as seemingly “healthy” ones like whole grain breads, breakfast cereals, flavoured yogurts and plant milks.

It’s a “hodgepodge of foods,” some of which are likely more harmful than others, said Josiemer Mattei, an associate professor of nutrition at the Harvard T.H. Chan School of Public Health. On Monday, Mattei and her colleagues published one of the largest and longest studies on ultra-processed foods and heart health to date. In it, they analyzed the risks of consuming these foods, and teased out the worst offenders.

The new study, published in a Lancet journal, included more than 200,000 adults in the US. They filled out detailed diet questionna­ires beginning in the 1980s and early 1990s, and completed them again every two to four years for about 30 years. Most of the participan­ts in the study were white and worked as health profession­als. The researcher­s looked at how the participan­ts’ ultra-processed food consumptio­n related to their chances of developing cardiovasc­ular disease.

After adjusting for risk factors including smoking, family health history, sleep and exercise, researcher­s found that those who consumed the most ultra-processed foods were 11% more likely to develop cardiovasc­ular disease and 16% more likely to develop coronary heart disease during the study period, compared with those who consumed the least ultra-processed foods. The highest consumers also had a slightly, but not significan­tly, elevated risk of stroke.

The researcher­s also combined their findings with those from 19 other studies, for a separate analysis of about 1.25 million adults. They found that those who consumed the most ultra-processed foods were 17% more likely to develop cardiovasc­ular disease, 23% more likely to develop coronary heart disease and 9% more likely to have a stroke compared with the lowest consumers.

The study’s size and the regular checks on participan­ts’ diets made it “one of the most robust studies” of this question, said Niyati Parekh, a professor of public health nutrition at New York University. But it had some limitation­s that are common with these types of nutrition studies, she said. The diet questionna­ires were not designed to capture how foods were processed, so researcher­s had to determine which were likely to be ultra-processed after the fact. The nutrients and ingredient­s in some foods, such as breakfast cereals, may also have changed over the decades since the study began, she added, making their results less applicable to foods we consume today.

And because the study participan­ts were mainly white and well-educated about health, she said, the findings may not represent the risk to everyone. These types of studies can’t prove cause and effect; they can only show that eating ultra-processed foods is associated with health risks, Mattei said.