Everything you eat, and I do mean everything, is broken down by the body into simple sugars, that is what we run on. We use insulin produced by our pancreas to burn that sugar. If we eat a great deal of foods that break down quickly into simple sugars, like juice, like carbohydrates, our pancreas produces massive amounts of insulin to burn all that sugar efficiently. If we only drink juice and eat these easily metabolized foods, and do not eat enough protien etc, then when the insulin is used to burn the sugar, there is nothing left for the body to run on and you go into a state of hypoglycemia, low blood sugar. This leaves you feeling tired all the time. If the sugar goes low enough, it can be life threatening. You need to see a doctor or a registered dietician and learn to eat a balanced diet.