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Your body is programmed for it.

It's true: You are what you eat. Eat highly processed foods, and you'll train your body to ask for them. For one, "the bacteria in your gut gets used to those foods and will crave them," says Harari. That means you'll be more likely to get stuck in a cycle of junk food, which, normally low in nutrition and high in sugar, salt and fat, are all too easy to overeat. What's more, research from Kansas State University shows that a high-fat, high-sugar diet drives brain changes that increase impulsivity, making it difficult to hold back in the face of temptation.