Fructose is essential in our existence, but causes metabolic dysfunction if consumed in excess. But we have an incredible ability to convert calories from fructose to fat. It's hard get too much fructose from just eating apples, for instance, but fructose from sugary drinks (soda and 'healthy' fruit smoothies) as part of an already high calorie diet, overwhelms our body and is stored as fat. Dr. Attia beaks this down really well in chapter 6.