Scientists believe milk fat upsets the gut ecosystem, causing an influx of potentially harmful bacteria.

In certain individuals, this can trigger an extreme immune reaction, leading to Irritable Bowel Syndrome (IBS) 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, U.S. researchers said.

Study leader, professor Eugene Chang, from the University of Chicago, whose findings are reported in the journal Nature, 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.

“We know how certain genetic differences can increase the risk for these diseases, but moving from elevated risk to the development of disease seems to require a second event which may be encountered because of our changing lifestyle.”

Concentrated milk fat is a powdery substance that is abundant in processed foods and confectionery.

It contains a complex mix of fatty acids, most of which are the unhealthy saturated variety.

Tests carried out by the scientists showed that 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 -- many beneficial -- that live in the intestines.

Genetically susceptible mice fed a diet high in milk fat tripled the rate at which they developed colitis.

Within six months, 60% of the animals had the condition. They also suffered far more severe symptoms than affected mice on low-fat diets.

The levels of saturated fat given to the mice closely resembled those typically consumed in Western cultures.

Chang's team found that milk fat, but not polyunsaturated fats, caused an explosion in the numbers of a normally rare gut microbe called Bilophila wadsworthia.

The bugs were almost undetectable in mice on low-fat or unsaturated fat diets, but made up about 6% of all bacteria in the guts of mice fed milk fat.

Bilophila wadsworthia has an affinity for bile, which is released into the intestines to help break down fats.

Milk fats are unusually difficult to digest, requiring the liver to produce bile rich in sulphur.

Since B. wadsworthia thrives on sulphur, this fuels its population growth.

“Unfortunately, these can be harmful bacteria,” Chang said.

“Presented with a rich source of sulphur, they bloom, and when they do, they are capable of activating the immune system of genetically prone individuals.”

By-products of the bugs' interaction with bile enhance the effect by making the bowel more permeable, the scientists said.

Immune cells then find it easier to infiltrate the bowel wall and damage tissue.

Chang added:, “Right now we can't do much about correcting genes that predispose individuals to increased risk for these diseases, and while we could encourage people to change their diets, this is seldom effective and always difficult.

“However, the balance between host and microbes can be altered back to a healthy state to prevent or treat these diseases.

“In essence, the gut microbiome can be 're-shaped' in sustainable and predictable ways that restore a healthy relationship between host and microbes, without significantly affecting the lifestyles of individuals who are genetically prone to these diseases.

“We are testing that right now.”

From the June 14, 2012, Prepared Foods’ Daily News