"beriberi" is malnutrition: it means literally "i cannot".

You become .. not very able to do what you need to do, to survive, on this kind of diet, fairly quickly.

Before the 1840s, Irish farmers lived on.potatoes and milk, with the odd herring in winter when milk was harder to come by. Not a pleasant diet, and you're buggered if the potato harvest fails, but definitely doable.