I like to defer to Elaine on this subject. Here's a great link. Look especially at #2 and #3.
http://www.breakingtheviciouscycle.info/knowledge_base/kb/fos_fructooligosaccharides.htm
So we play it safe and get FOS from natural sources like burdock root, Jerusalem artichokes, chicory root, dandelion root, salsify root, onions, bananas, asparagus, leeks, garlic and globe artichoke.
Okay....full disclosure. I copied that list....we pretty much just consume bananas, onions, garlic and asparagus!