Rastas are generally adherents to a natural lifestyle. The Rasta way of living (livity) is generally referred to as Ital (it Ital, it vital). I suppose the nature of this could have derived from adversion to food changed by Babylon, but I think it is much more simple than that. Rastas simply want to exist on and with what Jah provides. There is a lot of sense in that, I think.
To some, living Ital means that one will only eat food that is organic and natural. Ital is generally a strict diet that is fairly difficult to accomplish, depending on one's views. Some refrain from dairy products, flesh, table salt (chemically created), etc. Others choose to eat white meat and fish, limiting themselves by the Old Testament dietary restrictions -- but much like Rasta itself, it depends on the person. One common theme throughout the Ital views, however, is nature. This includes nothing polluted with chemicals, or even packaged like foods in supermarkets. In the "civilized" West, this lifestyle is difficult to follow as there are unfortunately few places available to shop in this manner. Recent studies show that in the U.S.A., only around 1% of agricultural acreage is organic. That means that 99% of produce is riddled with potentially cancer-causing pesticides, herbicides, etc.
Basically, Ital is living from what Jah gives and only altering it minimally (e.g. cooking). Therefore, marijuana does not conflict with this philosophy. Substances such as alcohol (man-made) and tobacco filled with chemicals are generally frowned upon. For additional info, including a Biblical basis for Ital and even some Ital recipes, visit Bongo Kosmo's Ital site. Remember, Ital is vital! Rasta food is a food of energy, love, and simplicity.