Avoiding refined and added sugars is a proven way to lose weight and prevent diabetes and heart disease, but what to do about all those natural sugars in smoothies, juices, and fruit is a bit more complicated. New research finds that when it comes to whole fruit, you can pretty much eat as much as you'd like, but you need to watch for fruit in a smoothie or juice, which can spike blood sugar in unhealthy ways. "A whole, intact apple is quite different from applesauce, even though both have all the same nutrients and fiber," says Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital. The key reason: The fibrous structure in whole fruit helps slow the absorption of fructose, the sugar mainly found in fruit, says Ludwig. Pureeing a fruit diminishes your body's ability to break down the sugar – and squeezing the fruit and filtering out its fiber (in juice) is even worse. Ludwig says that three to five servings of any kind of whole fruit is healthy, but if you're eating much more than that, go for temperate fruits such as apples, pears, peaches, and berries, which have a gentler effect on blood sugar levels than tropical fruits like bananas, mangos, and pineapples.