The researchers found that following a high-GI diet led to higher numbers of activated immune cells in the brain and more genes associated with inflammation.
The findings of the study, published in Molecular Psychiatry, may also indicate a potential cause of the complex neurodevelopmental disorders.
The glycemic index (GI) is a form of ranking given to foods containing carbohydrates according to the overall effect they have on blood sugar levels. The quicker that foods are absorbed, the higher their GI score is.
Many foods commonly found in the Western diet such as bread, cereals and sugary processed foods lead to rapid increases and decreases in blood sugar levels. These foods are quick to digest and, therefore, typically have high GI scores.
A low-GI diet is mainly comprised of foods such as vegetables and whole grains that take longer to digest. Consuming carbohydrates that are slow to digest reduces the peaks in blood sugar levels that typically follow meals. As a result, a low-GI diet can help people with diabetes to keep their blood sugar levels under control.
"One thing that's driving a lot of general physiological changes in people is changes in the diet," says corresponding author Pamela Maher, a senior staff scientist at the Salk Institute for Biological Studies in La Jolla, CA.
For the study, the researchers investigated whether lowering the level of a specific byproduct of sugar metabolism would reduce the autism-like symptoms of in specially-bred mice. To assess this, two groups of pregnant mice and their offspring were fed with either high- or low-GI diets, consuming the same amount of calories.
After the newborn mice had finished weaning, the researcher conducted a series of behavioral and biochemical tests. They found that the mice following a high-GI diet exhibited the expected symptoms of autism - impaired social interactions, repetitive actions with no apparent purpose and extensive grooming.
Following a high-GI diet may alter the gut microbiome
More dramatic than this, however, were the changes observed by the researchers in the brains of the mice eating the high-GI diet. Compared with the mice following the low-GI diet, these mice had less doublecortin - a protein indicator of newly developing neurons - in their brains, and particularly in the region of the brain associated with memory.
The mice following the high-GI diet also had higher numbers of activated immune cells in the brain and more genes associated with inflammation were expressed in their brains, compared with the mice following the low-GI diet.
In previous human studies of mothers and children with autism, the activation of the immune system has been implicated, explains Maher, though these studies have tended to focus on infections causing sudden bouts of inflammation rather than a high-GI diet that can cause chronic inflammation at low levels.
The researchers found that the high-GI diet may have also altered the gut microbiome - the population of microbes in the intestine. They found evidence in the blood of changes to the breaking down of complex starches by bacteria in the large intestine.
"We were really surprised when we found molecules in the blood that others had reported could only be generated by gut bacteria," says Maher. "There were big differences in some of these compounds between the two diets."
Following on from these findings, the team will now examine gut bacteria and how inflammation affects neuron development and more directly. They also plan to analyze how changes in the timing of exposure to specific diets affect outcomes.
Previously, Medical News Today reported on a study finding that mothers of young children diagnosed with autism spectrum disorders are more likely to report gastrointestinal symptoms of constipation, diarrhea and food allergy or intolerance.