For example, researchers found out that when they fed mice antibiotics this caused a change in their behavior. These antibiotic-fed mice exhibited an increase in their exploratory activities and certain changes in their brain chemistry. When the researcher fed the antibiotics to the same strain of mice bred in sterile conditions (i.e. no bacteria in their guts), the behavior of the mice did not change and they experienced no alteration in brain chemistry. This indicated that the changes in gut bacteria brought about by the antibiotics were responsible for the alteration of the behavior of the mice.
Another group of researchers performed an experiment where they fed groups of mice high-fat diets. The scientist treated some of the high-fat diet fed mice with a probiotic. This is a preparation of selected strains of bacteria thought to be beneficial for the intestine. Prolonged treatment with probiotics leads to a change in the bacterial makeup of the gut. The probiotic treatment prevented the mice from becoming obese, and the researchers found that this was associated with a decrease in food intake and an increase in certain blood hormones associated with the induction of satiety. When the scientists analyzed the brain of the probiotic-treated mice they found changes in the levels of expression of genes associated with the reduction of hunger and the increase in satiety.
Of course these are animal experiments, but there are several conditions in humans involving changes in gut bacteria such as irritable bowel syndrome that are accompanied by feelings of anxiety and depression, and certain psychiatric conditions are also believed to be affected by the makeup of the bacteria of the gut.
So next time you experience some mood swings during a bout of intestinal distress, take a good look at the remnants of your last meal as they disappear down the toilet. They may be related to what you are thinking or feeling more than you care to know!
Image of gut bacteria by Janice Carr, Centers for Disease Control and Prevention.
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