Flashback Friday: Do Vitamin C Supplements Prevent Colds but Cause Kidney Stones?

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Mainstream medicine has long had a healthy skepticism of dietary supplements, extending to the present day. “Enough is enough.” But, this commentary in the Archives of Internal Medicine argued we may have gone too far, as evidenced by our “uncritical acceptance” of supposed toxicities; the surprisingly “angry, scornful tone” found in medical texts, with words like “careless, useless,…indefensible, wasteful, [and] insidious,” as well as ignoring evidence of possible benefit.

“To illustrate the uncritical acceptance of bad news” [about supplements], they discuss the well-known concept that “high-dose [vitamin C] can cause kidney stones.” But, just because something is “well known” in medicine doesn’t mean it’s necessarily true. They couldn’t find a single reported case. We’ve known that vitamin C is turned into oxalates in the body, and if the level of oxalates in the urine gets too high, stones can form. But, even at 4,000 mg of vitamin C a day (that’s like a couple gallons of orange juice’s worth), urinary oxalates may not get very high.

But, you know, there may be rare individuals that have increased capacity for this conversion into oxalate, and so, a theoretical risk of kidney stones with high-dose vitamin C supplements was raised in a letter printed in a medical journal back in 1973. Okay, but when it’s talked about in the medical literature, they make it sound like it’s an established phenomenon. Here’s a reference to seven citations, supposedly suggesting an association between excessive vitamin C intake and “the formation of oxalate [kidney] stones.” Let’s look at the cited sources. Okay, there’s the letter about the theoretical risk—that’s legit, but this other citation has nothing to do with either vitamin C or kidney stones, and the other five citations are just references to books, which can sometimes be okay, if the books cite primary research themselves. But, instead, there’s like this circular logic, where the books just cite other books, that like cite that theoretical risk letter again. So, it looks like there’s a lot of evidence, but they’re all just expressing this opinion with no new data.

Now, by that time, there were actually studies that followed populations of people taking vitamin C supplements, and found no increased kidney stone risk among men. Then later, the same thing for women. So, you can understand this author’s frustration that vitamin C supplements appeared to be unfairly villainized.

The irony is that now we know that vitamin C supplements do indeed appear to increase kidney stone risk. This population of men was followed further out, and men taking vitamin C supplements did indeed end up with higher risk—confirmed now in a second study, though also of men. We don’t know if women are similarly at risk, though there’s now also been a case reported of a child also running into problems.

What does doubling of risk mean exactly, in this context? That means those taking like 1,000 milligrams a day may have a 1 in 300 chance of getting a kidney stone every year, instead of a 1 in 600 chance, which is not an insignificant risk—1 in 300. Kidney stones can be really painful; so, they conclude that, look, since there’s no benefits, and some risk, better to stay away.

But, there are benefits. Taking vitamin C just when you get a cold doesn’t seem to help, and regular supplement users don’t seem to get fewer colds. But, when they do get sick, they don’t get as sick, and get better about 10% faster. And, those under extreme physical stress may cut their cold risk in half. So, it’s really up to each individual to balance the potential common cold benefit with the potential kidney stone risk.

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Image credit: Kristina DeMuth. Image has been modified.