We all know how much the Romans loved their wine; watch any gladiator movie, and you see the drink flowing.
Over the years, historians have postulated that the Romans' practice of boiling ingredients for wine in lead pots contributed to lead poisoning in the ruling class. The Romans would boil unfermented grape juice to concentrate the sugar, then they would add the sweet syrup to lower quality wines and sell them to the rich.
Lead poisoning causes brain damage and infertility, among other things.
Certainly not every historian buys into this theory, but wine-induced brain damage in the ruling class certainly couldn't have helped the empire in times of crisis.