I highly doubt in our (America) society that the vast majority of people are undereating. In fact, its quite the opposite actually. People eating too much and the wrong kind of food. We use to be the fattest country in the world. I think Mexico has surpassed us now.
So the undereating is not likely to be a cause of AF. And like I said before, undereating alone is probably not going to send someone into AF. Rather a combination of stress and a lot of other damaging things combined for a long period of time will send someone into AF. And if you happen to not be consuming proper and adequate nutrition, you'll likely see AF sooner rather than later.