The Little Crystals Can Get You

A few days ago I learned, from the Vital Signs column in Discover magazine, why it is that mega-mega-doses of vitamin C are dangerous.

I've known for a while that plants that contain oxalic acid are both edible (and tasty) and toxic if eaten in high enough amounts. The common weed yellow wood sorrel, Oxalis stricta, which has a pleasant sour taste, is a good example. The more obviously culinary French sorrel, Rumex acetosa, has the same property.

But I didn't really know why.

Well, it turns out vitamin C itself has the same property, and in the same way, the danger is in the dose. It has to do with how our kidneys work.

After filtering out waste, the part of the kidneys called tubules return blood back into circulation. Those tubules are pretty narrow. 

When vitamin C (ascorbic acid) gets to the kidneys, it's transformed into oxalate — a derivative of oxalic acid — and the reason oxalate is dangerous to us is because it takes the form of tiny sharp-edged crystals. When there are enough of them, they block the kidneys' tubules.

I hate to say it, but I liked this explanation because it was so mechanical and simple: I could visualize it. 

The only mystery is how much oxalate it takes to cause a problem, and it seems as though no one knows exactly. Many cultures eat foods made with plants that have oxalic acid in them, and obviously vitamin C also. Our bodies need vitamin C.

The patient in the Vital Signs column had been mega-dosed intravenously with 100 grams (grams!) of vitamin C — 50 times the recommended dose. So that was clearly beyond the pale.

But for someone eating wood sorrel from their yard, or cooking with French sorrel, how much is too much? It sounds as though it's partly a balance with how much liquid you consume, and that rather than destroying your kidneys in the long run, it might lead to kidney stones. 

Which are clearly not a great outcome, but they're better than dialysis or worse.

But one thing to remember: if something is good for you (like vitamin C), more is not always better.

Common wood sorrel in bloom. Photo from the Wikimedia Commons by Cbaile19