You hit the local juice bar before work and think you’re making a healthy choice by drinking your breakfast. But Keren Gilbert, RD and founder of Decision Nutrition, says it’s not that simple. Even though you’re consuming fruits and veggies, a lot of the good stuff is left behind in the juicing process. “When you juice something, you’re taking all the vitamins and sugars and leaving out all of the fiber,” she says. Fiber slows down sugar absorption in the body, so without it, sugar is absorbed too quickly. This means that your tummy could be grumbling in an hour—which totally defeats the purpose of a diet, says Gilbert.