Of all the nutritional recommendations that are passed around, eating more fruits and vegetables seems like a tip that should be universally good for everyone. Like every guideline, even this one has caveats. Many plant foods contain compounds that are irritating to our digestive system, immune system, and even nervous system. These compounds can cause certain plant foods to do more harm than good. In some cases, proper preparation of the foods can mitigate the ill effects of the compounds, but in other cases, the foods should be avoided in their entirety.