A high-fat diet changes the bacteria in the gut and makes you more likely to suffer from depression and anxiety as a result.
Researchers started investigating a possible link after noticing that obese people with type 2 diabetes were more likely to suffer from acute depression. While anyone can be depressed, the diabetic seems to feel it more keenly.
When laboratory mice are fed a high-fat diet, they become more erratic, and show signs of depression, anxiety and obsessive behaviour, say researchers from Harvard Medical School.
"Your diet isn't just making your blood sugar higher or lower, it's also changing a lot of signals coming from gut microbes, and these signals make it all the way to the brain," said lead researcher C. Ronald Kahn.
His research team noticed that mice fed a high-fat diet developed signs of anxiety and depression, and their behaviour returned to normal when they were given antibiotics, which changed the bacteria in the gut, known as the microbiome.
A similar pattern was seen when healthy mice were given bacteria from the guts of anxious and depressed mice. But their mood didn't alter when they were given the bacteria of mice whose microbiome had been changed with antibiotics.
The researchers surmise that the brain can also become insulin-resistant, a classic first sign of diabetes, on a high-fat diet. The change to the microbiome also affects neuro-transmitters, the chemicals that transfer signals across the brain.
Kahn and his team are trying to narrow down the types of bacteria that affect depression and anxiety, and, from that, the supplements that could create healthier brains.
"If we could modify those bacteria, either by putting in more beneficial bacteria or reducing the number of harmful bacteria, that might be a way to see improved behaviour," he said.