There are so many reasons why a variety of fruits and vegetables should be a large portion of our diets. They taste delicious and have a very high nutrient to calorie ratio (lots of nutrients, few calories). They are the best and most accessible source of many essential vitamins and minerals. And they are an essential source of fiber in our diets. Fiber is the indigestible carbohydrates (including cellulose and lignin) that makes up the structure of plants. But since we can’t digest it, why bother eating it? Wouldn’t it be better just to squish all of the vitamins and minerals out of the fruits and vegetables and just drink them instead?
You could do that. It would provide a very fast, easily digestible dose of some vitamins, minerals and some sugars. But since it is a liquid, it would pass through your system quickly and barely satiate you. This recipe for “healthy green juice” calls for 1 green apple, 2 celery stalks, ½ a cucumber, 3 kale leaves, ¼ a lemon and a ½” piece of ginger for a single serving. That is a full daily serving of vegetables for an adult, and enough to make up an entire meal (which would be a pretty awesome salad). And if you ate that it would keep you full because your body needs time to break down all that tough vegetal matter (oh HEY). While it slowly passes through your system, you absorb more of the nutrients present because they are in contact with digestive system cell walls longer.
So, why would you ever buy enough produce for an entire meal, but throw out all of the parts that actually make you feel full (or spend your money buying juice, when you could buy produce)? Hunger is our body telling us that we need more nutrients, so if you consume something and are hungry soon after then that food is not doing its job. Juicing yourself or just drinking pre-made juice (and especially as part of juice cleanses, ugh) are not the answer to your dietary problems. Eating more fruits and vegetables, without throwing away the bulk of what you buy (or buying just the carbs that straight juice contains), is a much better and more economical approach. Putting fruit and vegetables into smoothies is a slightly better approach, because you are still consuming the whole plant, but whirling it up still makes it much easier to digest so you’re missing out on prolonged fullness. A juice or smoothie every once in a while is fine, but for the most part don’t drink what you could eat.
Further Reading (Updated 9/11/2014): Stop Juicing, Lay Off the Juice Cleanse Diet. You’ve Been Lied to.