Nobody knows why ultra-processed foods are bad for you

But scientists are racing to find out.

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For millennia, people have altered food to please their palates. More than 3,000 years ago, Mesoamericans, living in what is Mexico and Central America today, cooked corn kernels in a solution of wood ash or limestone. The process, known as nixtamalisation, unlocked nutrients and softened the tough outer shells of the corn, making them easier to grind.

With the invention of canned goods and pasteurisation in the 19th century, alchemy became possible on an industrial scale. Processing innovations made food cheaper, more convenient and plentiful. According to the UN, the average daily food intake of a person in upper- and middle-income countries increased by about 40 per cent between 1975 and 2021, to 3,300 kilocalories. In that time, obesity rates have more than tripled; today, nearly one in three people globally is either obese or overweight.

Now concerns are growing that the heavy processing used to cook up cheap, tasty nibbles may itself be harmful. A particular target is "ultra-processed foods" (UPFs), a relatively recent label put forward by Dr Carlos Monteiro, a Brazilian scientist. Mr Robert F. Kennedy Jr, Donald Trump's nominee for secretary of health, has likened processed food to "poison" and promised to reduce the share of UPFs in American diets. In November 2023, Colombia imposed a tax on highly processed foods and drinks. The authorities in Brazil, Canada and Peru have advised the public to limit consumption of these foods. In Britain, parliamentarians are investigating the effects of UPFs on people's health.

At the heart of the debate is a question: Are UPFs unhealthy because their nutritional content is poor, or does the processing somehow pose risks in itself? New research may soon provide answers that could reformulate what people eat.