The Straight Dope | Eat Your Molybdenum 

I heard it’s possible to live for an extended period of time, not just short-term, on nothing but potatoes and milk. Is there any truth to this? If I could really get all the nutrients I need from just potatoes and milk, it sure would make my grocery shopping easier. —Joshua B., Olney, Maryland

Excellent question, Josh. With hard times coming, we’re all going to have to economize. The good news: A spuds-and-milk diet definitely has possibilities—the Irish, to cite the best-known example, got by mainly on potatoes until the infamous blight of 1845 wiped out their main course. The bad news: 1. Considering the quantities you’re going to have to eat, you’d better really like potatoes. 2. If you’re literally going to eat nothing but potatoes and milk, you risk—brace yourself—serious molybdenum deficiency.

Years ago I tackled the question of whether you could live by bread alone. Answer: Yeah, for about six months, but then you’d die of scurvy. Things won’t be anywhere near that bad on milk and potatoes. Before the Great Famine, the traditional Irish peasant meal consisted mainly of potatoes, milk, oats, beans, barley, and bread. Potatoes were the mainstay. As the years grew leaner, dairy products largely disappeared from the Irish diet, since poverty forced many farmers to sell their milk to pay rent. By the time the famine hit, the peasants were eating pretty much just potatoes, supplemented with some salt fish and oatmeal. I’ve seen it said that a third of the population lived on potatoes and nothing else, although that seems doubtful, as we’ll see. Edward Wakefield, an English land agent and amateur social scientist who traveled Ireland from 1809 to 1811, calculated each Irish peasant family member consumed 5.5 pounds of potatoes per day. An 1846 source claims a working man needed at least 8 pounds of potatoes a day to survive if nothing else were available; a typical family of six would need 26 pounds.

How did the Irish do on this diet? We can’t be certain—nobody was conducting nutrition studies in those days. But there’s reason to believe they were healthier than you might guess. In the century before the famine, Ireland had the highest birthrate in Western Europe. Some credit potatoes, saying the availability of easy-to-grow, easy-to-cook spuds made it practical to raise large families. Telling evidence on this score, one historian writes, “is that the Irish in general and Irish women in particular were widely described as healthy and good-looking.” I don’t know about you, Josh, but any diet that gets results like that is good enough for me.

OK, history lesson’s over. Could someone subsist solely on potatoes and milk? I had my assistant Una run a spreadsheet on key nutrients. Looking only at vitamins and minerals for which a recommended daily allowance has been established, we find a diet of vitamin-D-fortified whole milk and potatoes stacks up pretty well, providing at least some of all known dietary needs except molybdenum. For example, if you’re an active male between 19 and 30, of average height and weight, then one gallon of milk and eight pounds of potatoes will supply the RDA of most nutrients, falling a little short on the iron, folate, and niacin fronts, missing a lot of vitamin E, and striking out completely on molybdenum. Chug two gallons of milk with your spuds and all you’re missing is about two-thirds of your vitamin E and, of course, your molybdenum. Not so nuts about milk? Fine, cut it down to a quart and choke down 14 pounds of potatoes instead. Now you’re short on zinc, folate, niacin, vitamin E, and way low on vitamin A. And alas, still no molybdenum.

So what happens if you starve yourself of molybdenum? According to one nutritional reference book, “signs of molybdenum deficiency … are headache, rapid breathing and heart rate, nausea and vomiting, acute asthma attacks, visual problems, disorientation, and, finally, coma.” However, you don’t need much of the stuff—the RDA is only 45 micrograms—and good sources include lentils, split peas, green beans, cauliflower, and, significantly, oatmeal. Since we have no indication that a third of the Irish population was in a coma prior to 1845, my guess is that reports of potato-only diets in the pre-famine era were exaggerated and that inhabitants of the Emerald Isle were getting sufficient oatmeal and other foods to meet their micronutrient needs.

What lessons does all this hold for you, Josh? 1. While no sane nutritionist would recommend a diet consisting of just two foods, if in the interest of economy you need to heavy up on a couple, you could do a lot worse than potatoes and milk. Potato historians sing the praises of this miracle tuber for elevating humankind from the constant threat of starvation. 2. Notwithstanding the potato’s virtues, make sure you eat your oatmeal too.

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