A dairy ingredient in junk food may be responsible for rising rates of inflammatory bowel disease and other immune disorders, research suggests.
Scientists believe milk fat upsets the stomach ecosystem, causing an influx of potentially harmful bacteria.
For some people, this can trigger an extreme immune reaction, leading to irritable bowel syndrome conditions such as Crohn's disease and ulcerative colitis.
The discovery may explain why once-rare immune disorders have become so common in the West over the past 50 years, US researchers said.
Study leader Professor Eugene Chang, from the University of Chicago, said: "This is the first plausible mechanism showing step-by-step how Western-style diets contribute to the rapid and ongoing increase in the incidence of inflammatory bowel disease.
Concentrated milk fat is a powdery substance abundant in processed foods and confectionery.
It contains a complex mix of fatty acids, most of which are of the saturated variety.
Tests showed milk fat alters the composition of bacteria in the gut.
In people with a particular genetic make-up, this can disrupt the delicate "truce" between the immune system and the trillions of bugs that live in the intestines.
Milk fats are unusually difficult to digest, requiring the liver to produce bile rich in sulphur, which allows certain harmful bacteria to thrive.