Studies show how a diet high in fat and sugar impairs immune cell function.

The impact of diet on health is really a no-brainer – even leading to calls for GPs to prescribe fruit and vegetables before writing out a drug prescription.

Now, US researchers report in the journal Cell Host & Microbe that they’ve found a mechanism to explain how obesity caused by an unhealthy junk food diet can induce inflammation in the gut.

“Our research showed that long-term consumption of a Western-style diet high in fat and sugar impairs the function of immune cells in the gut in ways that could promote inflammatory bowel disease or increase the risk of intestinal infections,” says lead author Ta-Chiang Liu, from Washington University.

This has particular relevance for Crohn’s disease – a debilitating condition that has been increasing worldwide and causes abdominal pain, diarrhoea, anaemia and fatigue.

A key feature of the disease is impaired function of Paneth cells, immune cells found in the intestines that help maintain a healthy balance of gut microbes and ward off infectious pathogens.

When exploring a database of 400 adults with and without Crohn’s disease, the researchers discovered that higher body mass index (BMI) was associated with progressively more abnormal looking Paneth cells, captured under a microscope.

Armed with their discovery, they studied two strains of mice genetically predisposed to obesity and were surprised to find that the animals’ Paneth cells looked normal.

To dig deeper, the researchers fed normal mice a diet in which 40% of the calories came from fat or sugar, typical of a Western diet.

After two months the mice became obese – and their Paneth cells became abnormal. They also had associated problems such as increased gut permeability, a key feature of chronic inflammation that allows harmful bacteria and toxins to cross the intestinal lining.

“Obesity wasn’t the problem per se,” says Lui. “Eating too much of a healthy diet didn’t affect the Paneth cells. It was the high-fat, high-sugar diet that was the problem.”

Importantly, switching from junk food back to a standard diet completely reversed the Paneth cell dysfunction.

Further experiments revealed that a bile acid molecule known as deoxycholic acid, formed as a by-product of gut bacteria metabolism, increased the activity of immune molecules that inhibit Paneth cell function.

Liu and colleagues are now comparing the individual impact of fat and sugar on Paneth cells.

Whether the damaged cells respond to a healthy diet in humans remains to be seen, but preliminary evidence suggests diet can alter the balance of gut bacteria and alleviate symptoms of Crohn’s disease.