It is generally recommended not to combine fruits with vegetables for the simple reason that vegetables contain starch. The starch in vegetables is not something you would usually want to combine with the sugar from fruits. Sugar + starch = fermentation and alcohol! Fermentation and alcohol can create bloating, gas, and sometimes constipation.
There’s an exception, however, for one class of vegetable—greens. These do not contain the same type of starch as other vegetables, and so will not cause these unpleasant effects in the body, when combined with fruits.
In her revolutionary book Green for Life, Victoria Boutenko notes that greens should actually be a food group of their own, as they are technically not vegetables at all. Says Victoria, “Placing greens in the same category as vegetables has caused people to mistakenly apply the combining rules of starchy vegetables to greens.” What’s more, greens are the only foods that help to digest whatever other food they are eaten with: Greens help stimulate the secretion of digestive enzymes, and they are indispensable for helping alkalize the body, which is so very important.
Additionally, when you make a smoothie, the blender essentially "chews" the fruits and vegetables much more efficiently than your teeth can, thereby creating the maximum amount of surface area for your stomach to digest them. Well chewed food to the point of liquification is ideally what we are supposed to do with all of our food before swallowing, because it makes everything much easier to digest.
Lemon juice can be added to green drinks.