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The Junk Food Hangover—Explained!

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For the most part, the 80/20 rule is a pretty sweet deal. You get all the body benefits of clean eating, and can enjoy the occasional, guilt-free indulgence as well. But sometimes, that 20 percent comes back to bite you in the butt, and you wake up feeling headache-y, groggy, bloated—really, kind of hung over. But it wasn’t one too many glasses of wine that did you in, it was one too many bites of cheesecake. What’s up with that?

“A food hangover is your body giving you feedback. Your gut is basically communicating to your brain, sending it a warning signal about what you just ate,” says Robynne Chutkan, M.D., author of Gutbliss. As crappy as it feels at the time, this reaction is a good thing, she says. “If that didn’t happen, we’d all be devouring Doritos and hamburgers every day. And that’s bad news, not just for your weight, but for the health of your entire body.”

Just as certain alcohols deliver a worse next-day headache (hello, Champagne and whiskey), certain foods are also more hangover-inducing than others, says Chutkan. Namely, anything salty, fatty, and sugar-y or carb-y. (Good news for oenophiles: Scientists Are Making Hangover-Free Wine.)

Salt dehydrates you, which can trigger a headache and cause your body to retain water, making you feel puffy. Fat takes a long time to digest, so the fries you ate last night may still be hanging out in your stomach this morning—another recipe for bloating, and acid reflux to boot. And sugar and carbs will spike your blood sugar levels, leading to jitteriness and more head pain when levels fall again.

These foods also damage the good-for-you bacteria that live in your intestinal tract, says Gerard E. Mullin, M.D., author of The Gut Balance Revolution. “Within 24 hours, you can transform your gut bug population from good to bad.” And gut bacteria imbalances can cause damaging body-wide inflammation, digestive issues, and weight gain.

On top of all this, eating more than you normally would in one sitting can result in a food hangover too, says Chutkan. To help you digest that large load, your body diverts blood away from your brain, lungs, and heart to you GI tract, which causes fatigue and brain fog. (6 Ways Your Microbiome Affects Your Health.)

Take heart: You can enjoy the 20 part of the 80/20 rule without suffering from a food hangover every time. Just be mindful of portion sizes when you’re indulging, drink plenty of water along with your treat, and consider taking a daily probiotic to keep your intestinal flora in check. And always check in with yourself the morning after indulging. Everyone’s different; you may find that certain junk foods just don’t agree with you, while others are totally fine. If the ones you can’t tolerate are the ones you love most, check out these Smart, Healthy Alternatives