The effect occurs so rapidly because liquids digest faster than just about anything else. When you drink juice on an empty stomach, it races through the digestive system like water disappears down the drain. Just as water empties from a sink into the drain, fluids move through the stomach and into the intestine, where the sugars are quickly transferred to the bloodstream.
Juice goes through your system like water goes down the drain.
But wait — you may say, "I thought fruit juice was healthy!" Although it's true that fruit is filled with vitamins, minerals, antioxidants, and many nutrients, fruit juice also contains a lot of natural sugar in liquid form. Eating fruit is part of a healthy, balanced diet, but it's better to eat the fruit, not drink the juice.
Eating a serving of fruit two or three times each day is recommended, but don't eat all three portions at one time because large portions of fruit consumed in one sitting can cause a sharp rise in blood-glucose levels. When you drink a glass of fruit juice, you are having multiple servings of fruit at one time. The juice of one orange may fill your glass about one inch. A glass of juice is more like several fruits at one time.
Vegetables have many of the same vitamins and minerals that fruits have but with lower sugar content. You may find that you can enjoy a glass of vegetable juice without the same blood-sugar spike that fruit juice often causes. Portion size matters, so aim for about 4–8 ounces of vegetable juice at a time. An 8-ounce portion may contain close to 10–15 grams of carbohydrate, so you may not want to drink large volumes.
Before you nix juice completely, think about this: Fruit juice can actually come in handy in a couple of key situations. There's no need to keep large bottles of juice in the fridge, though. Single-serving (4–6 ounce) containers can be used for treating and preventing low blood-glucose levels.