It takes several oranges to make one 6-ounce glass of OJ, but when you drink juice, you consume all the calories from those oranges without the natural fruit fibers that fill you up. It's why "even 100 percent juice is just empty calories and another blood sugar spike,".Another thing: Fructose, the natural fruit sugar that makes fruit and fruit juice taste sweet, tricks your body into gaining weight by blunting your body's ability to recognize when it's full, says Melissa Rifkin, a registered dietitian at Montefiore Medical Center in New York and a Rise nutrition coach. This makes you eat more, and increases your risk of developing insulin resistance and diabetes.