In other news from around the Internet, today I caught this Straight Dope column, in which the question is asked: "Could I survive on nothing but potatoes and milk?" The answer, surprisingly, is: almost. You could limp along with potatoes and (vitamin-D fortified whole) milk with not much to worry about except "molybdenum deficiency." Dude. I'd never even heard of molybdenum before today. Which, it turns out, is because I wasn't familiar enough with the periodic table, because it's an element. The deficiency of it is associated with a whole host of nasty symptoms, but also you don't need much of it to keep going. Even more relevant to me, it is apparently plentiful in oatmeal, which these days I not only eat on a more or less daily basis, but now also make cookies out of. So, no, I'm not now suddenly worried about molybdenum deficiency, thanks. (It's also to be found in lentils and split peas, both of which I quite enjoy, but don't eat too often. Also it's covered by my multivitamin. Which is...you know...the reason I take a multivitamin.)
Anyway, that wasn't the point. The point was...to tackle your nutritional requirements, the example daily dose of potatoes and milk you'd need was first suggested at a gallon of milk (ok), and eight pounds of potatoes. Jesus CHRIST! That's...that's a lot of potatoes. So...let's see what we're really looking at here.
Two pounds of potatoes... Well, thanks to a recent cooking adventure, I was able to determine that two pounds of potatoes is about three good-sized baking potatoes. So, picture a meal...a meal consisting of three baked potatoes. Now...wash that down with a quart of milk. Okay? Now...eat that meal four times a day. Breakfast, lunch, dinner, and midnight snack...you get three baked potatoes and a quart of milk. Throw in a bowl of oatmeal every once in a while, and you could probably live on that. But...would you want to?
EDIT: I was pondering actually trying it out for a day, just to see. After a little consideration, it turns out that the potatoes-and-milk thing doesn't really make much sense in any direction. It's not that economical...it would probably cost me close to $10 a day. Not only that, though, but the whole shebang probably runs something like 5,700 calories. So, uh, no.