Well, it's more based on my personal experience than anything else.. and certain fruits and vegetables have different acids and enzymes so it depends on what you are mixing. But in my experience certain fruits and vegetables do not mix well together at all in the stomach. Like mixing milk and orange juice.. it just doesn't work.. at least for me. But some fruits and vegetables DO work well together so I usually just judge it on a case-by-case basis.. but as a general rule I try to avoid mixing them in the same meal to avoid unpleasant chemical reactions. Citrus fruits in particular, I have found do not tend to agree with a number of vegetables and starches and dairy products. It has to do with digestion.. and having the proper PH level in your stomach, so you don't have food rotting in there because it's not being digested properly. I don't know the science behind it, I just know what works for me and I like my digestive tract to be at zen with itself.