Ultra-processed foods now make up more than two-thirds of the calories consumed in the diets of young people in the US, a startling new study reveals – and researchers are warning that convenience could be taking priority over health when it comes to food choices.

These types of processed foodstuffs accounted for 61 percent of total caloric intake in 1999, the analysis found, and jumped to 67 percent by 2018. That's based on surveys of 33,795 children and teenagers aged between 2 and 19.

The biggest jump – from 2.2 percent to 11.2 percent of calories – came from ready-to-eat or ready-to-heat dishes including frozen pizza, burgers, and takeouts. The second biggest rise in calories was from sweets and snacks: 10.6 percent to 12.9 percent.

Across the same period, calories from unprocessed or minimally processed foods, which are usually healthier, fell from 28.8 percent to 23.5 percent. The remainder was made up from moderately processed food (like cheese and canned fruits) and flavor enhancers (including sugar and butter).

Of course, not all processed foods are equally problematic when it comes to negative health impacts.

"Some whole grain breads and dairy foods are ultra-processed, and they're healthier than other ultra-processed foods," says cancer epidemiologist Fang Fang Zhang, from Tufts University. "Processing can keep food fresher longer, allows for food fortification and enrichment, and enhances consumer convenience.

"But many ultra-processed foods are less healthy, with more sugar and salt, and less fiber, than unprocessed and minimally processed foods, and the increase in their consumption by children and teenagers is concerning."

There was some good news: calories from sugar-sweetened beverages dropped from 10.8 percent to 5.3 percent of overall calories consumed. According to Zhang, that's a direct result of a concerted campaign to reduce the consumption of sugary drinks – and now more work is needed in other categories.

Ultra-processed foods are often high in simple carbohydrates, such as sucrose and fructose, and don't typically contain much in the way of the fiber, proteins, vitamins and minerals that help to fuel our bodies.

The issue here is far from simply shaming individual choices. Extensive research has established that consuming too much processed food has been linked with a range of health problems, including a rise in obesity, cardiovascular issues, and certain types of cancer.

All of these impacts can negatively affect a person's wellbeing and, in aggregate, place larger burdens on the health system. That makes it important to tackle this rise in processed food consumption in kids, the researchers say.

Health experts say marketing, pricing and availability in schools is driving the take up of ultra-processed food in schools – at a time when dietary habits can be set for life.

Parental education and family income didn't have a significant impact on the level of calories contributed by ultra-processed foods – something that the researchers say shows how pervasive these types of food have become.

The team behind the new study wants to see more detailed tracking and analysis carried out in the future.

"Food processing is an often-overlooked dimension in nutrition research," says Zhang. "We may need to consider that ultra-processing of some foods may be associated with health risks, independent of the poor nutrient profile of ultra-processed foods generally."

The research has been published in JAMA.