The decades-old saying "you are what you eat" has grown increasingly true as we learn just how important our diets are for immunity, longevity and even mental health.

"Many people don't realize that there's a way in which the food we eat impacts our mental wellbeing," according to Dr. Uma Naidoo, a Harvard-trained nutritional psychiatrist and author of "Calm Your Mind with Food."

"The food we eat, as it gets digested, interacts with the trillions of microbes in the gut microbiome, and gets broken down into different substances, which then subsequently over time impact our mental wellbeing. Some of the foods that are less healthy, if we're eating them, set the gut up for inflammation."

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Inflammation in the gut can lead to several adverse mental health symptoms, Naidoo says, including anxiety.

"By tweaking your diet back to a healthier norm, you can actually help to relieve some of these symptoms," she notes.

Here are four simple changes you can make to your diet for better brain health.