There’s a reason our stomachs are sometimes called our second brains. Our gut produces about 90 percent of our serotonin, the chemical responsible for making us feel good. So in an effort to stay our calm, upbeat selves, we chatted with Rachel Kelly, mental health advocate and author of The Happiness Diet (in stores and online September 26), to find out which foods might be putting us in a bad mood. (Et tu, fries?)

RELATED: These 11 Foods Are Scientifically Proven to Boost Your Mood

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Diet Drinks

Aspartame, the artificial sweetener in your beloved Diet Coke, contains an amino acid called phenylalanine. In some scientific studies, like this one from the National Institutes of Health, phenylalanine has been shown to hinder our production of serotonin (the happy neurotransmitter that regulates mood). You (and your mood) are better off getting your caffeine fix from black iced tea or coffee.

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Cereals and Snack Bars

There’s no need to avoid these guys altogether, but make sure you compare labels and go with one that has 8 grams of sugar or less. Many brands contain mountains of sugar, which provides a temporary energy boost swiftly followed by a crash. When our blood sugar levels dip below normal, stress hormones are released, leading to mood swings.

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Packaged Sweets

It’s no secret that sugar is a culprit (and yes, the occasional doughnut isn’t going to kill you). But store-bought sweets are also often made with artificial colors, which have been linked to reduced attention spans and hyperactivity—particularly in children. So if you’re going to splurge, you’re better off going to your local bakery than raiding the convenience store aisles.

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Fast food

That order of XL fries is rich in trans fats. The partially hydrogenated oils extend the shelf life of most fast foods but also disturb your balance of omega-3s. Research from the University of Pittsburgh Medical Center suggests that low levels of the fatty acid are associated with pessimism, depression and aggression.

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Margarine and Shortening

Fun fact: Even the humble cockroach won’t eat margarine. It’s basically pure trans fat, which, as we’ve mentioned above, prevents you from being a pleasant human. Same goes for shortening. We hope you weren’t cooking with it, but if you were, reach for the real butter instead.

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Salty Snacks

Check the ingredient list the next time you open a bag of chips or canister of peanuts. Many contain MSG (or monosodium glutamate), a flavor enhancer commonly found in processed foods, which can give you a terrible headache and cause you to feel weak and fatigued.

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Canned Food

Canned beans and soups can also hide MSG and excess sodium, but if you’re feeling a slump after your bowl of chicken noodle, check the label again. Look for BPA-free cans. Some studies, like this one from the Yale School of Medicine, have shown it interferes with areas of the brain that regulate mood and formation of memory (scary!).

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