Credit...Casey Zhang for The New York Times

In the mid-1990s, Carlos Monteiro, a nutritional epidemiologist in Brazil, noticed something alarming: Obesity rates among children in his country were rising rapidly.

To understand why, he and his colleagues at the University of São Paulo scrutinized data on the food buying patterns of Brazilian households to see if they had changed in recent years. The researchers found that people were purchasing less sugar, salt, cooking oils and staples like rice and beans, and more processed foods like sodas, sausages, instant noodles, packaged breads and cookies.

To describe that second category of food, Dr. Monteiro said, the team introduced a new term into the scientific literature — ultraprocessed foods, or UPFs — and defined it. They would later link UPFs to weight gain in children and adults in Brazil.

Since then, scientists have found associations between UPFs and a range of health conditions, including heart disease, Type 2 diabetes, obesity, gastrointestinal diseases and depression, as well as earlier death.

That’s concerning, experts say, since ultraprocessed foods have become a major part of people’s diets worldwide. They account for 67 percent of the calories consumed by children and teenagers in the United States, for example.

But many questions remain. What are ultraprocessed foods, exactly? And how strong is the evidence that they’re harmful? We asked experts to answer these and other questions.