Balancing the Risks and Benefits of Vitamin C Supplements

Mainstream medicine has long had a healthy skepticism of dietary supplements, extending to the present day with commentaries like “Enough is enough.” In an essay entitled “Battling quackery,” however, published in the Archives of Internal Medicine, it’s argued that we may have gone too far in our supplement bashing, as evidenced by our “uncritical acceptance” of supposed toxicities; the surprisingly “angry, scornful tone” found in medical texts using words like “careless,” “useless,” “indefensible,” “wasteful,” and “insidious”; and ignoring evidence of possible benefit.

“To illustrate the uncritical acceptance of bad news” about supplements, the authors discussed the “well-known” concept that high-dose vitamin C can cause kidney stones, as I highlight in my video Do Vitamin C Supplements Prevent Colds but Cause Kidney Stones? Just because something is well-known in medicine, however, doesn’t mean it’s necessarily true. In fact, the authors 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, which is like a couple gallons’ worth of orange juice, urinary oxalates may not get very high, as you can see at 1:10 in my video. Of course, there may be the rare individuals who have an increased capacity for this conversion into oxalates, 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.

When the theoretical risk was discussed in the medical literature, however, the researchers made it sound as if it were an established phenomenon: “Excessive intake of vitamin C may also be associated with the formation of oxalate stones.” Sounds less like a theoretical risk and more like an established phenomenon, right? That statement had 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, which you can see from 1:47 in my video. One reference is the letter about the theoretical risk, which is legitimate, but another listed citation, titled “Jaundice following the administration of niacin,” has nothing to do with either vitamin C or kidney stones. What’s more, the other five citations are just references to books. That may be acceptable if the books cited primary research themselves, but, instead, there was a kind of circular logic, where the books just cite other books citing that theoretical risk letter again. So, while it looks as if there’s a lot of evidence, they’re all just expressing this opinion with no new data.

By that time, there actually were studies that followed populations of people taking vitamin C supplements and found no increased kidney stone risk among men, then later, the same was shown in women. So, you can understand the frustration of the authors of “Battling quackery” commentary that vitamin-C supplements appeared to be unfairly villainized.

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

What does doubling of risk mean exactly in this context? Those taking a thousand milligrams or so of vitamin C a day may have a 1-in-300 chance of getting a kidney stone every year, instead of a 1-in-600 chance. One in 300 “is not an insignificant risk,” as kidney stones can be really painful, so researchers concluded that since there are no benefits and some risk, it’s better to stay away.

But there are benefits. Taking vitamin C just when you get a cold doesn’t seem to help, and although regular supplement users don’t seem to get fewer colds, when they do get sick, they don’t get as sick and get better about 10 percent 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.


What about intravenous vitamin C? I’ve got a whole video series on that, including:

If you’re not taking vitamin C supplements for pharmacological effects and just want to know how many vitamin C-rich fruits and vegetables to eat every day, check out my video What Is the Optimal Vitamin C Intake?.

Is there anything we can put into our mouth that really might help prevent colds? These videos will point you in the right direction:

And, if you’re interested in learning about the most important steps you can take to prevent and treat kidney stones, look no further than my videos How to Prevent Kidney Stones with Diet and How to Treat Kidney Stones with Diet.

What about high-oxalate vegetables such as rhubarb, spinach, beet greens and swiss chard? I’d encourage a moderation in intake. If you’re going to take my advice to ideally eat cups of dark green leafy vegetables a day I’d recommend sticking with other greens such as kale or collards.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations:

How Much Vitamin C Should You Get Every Day?

“For many years, the RDA [recommended daily allowance] for all vitamins were based on preventing deficiency, with a margin of safety,” but the miniscule amount of vitamin C needed to avoid scurvy, for example, is not necessarily the ideal intake for optimal health. What might the optimal intake of vitamin C be? To find out, let’s ask the body. But how? By seeing how much the body absorbs and excretes, which I go through in my video What Is the Optimal Vitamin C Intake?.

When we swallow 15 mg of vitamin C, the amount we’d get eating about a quarter of an orange, our body absorbs nearly 90 percent of it. If we instead take a supplement containing 1,250 mg of vitamin C, our body seems to realize that’s too much and clamps down on absorption at the intestinal lining level, and we end up absorbing less than half. By doing experiments where the level of intake is ratcheted up slowly, we can see when the body starts to say, “Okay. That’s enough.”

That magic level of intake appears to be about 200 mg a day. When we take up to 200 mg daily, our body absorbs it all. Above that level, however, the body tries to block further absorption, suggesting that our “intestinal vitamin C transport mechanisms… have evolved to fully absorb up to about 200 mg of vitamin C” a day.

In addition, vitamin C is reabsorbed in our kidneys back into our bloodstream to maintain our vitamin C blood levels around 70 or 80 micromoles per liter, which is what we reach at a vitamin C intake of about 200 mg a day. Even if we take ten times as much in vitamin C supplements, 2,000 mg a day, our body will just pee and poop out the excess to keep our blood levels in that narrow range of 70 to 80 micromoles per liter. Based on these kinds of data, one might “propose that 200 mg is the optimal daily intake of vitamin C…”

We can confirm that hypothesis using disease data. For example, at what daily intake of vitamin C is there the lowest stroke risk? Apparently, at about 200 mg a day. While dietary vitamin C intake was associated with lower stroke risk, vitamin C supplements were not, which is consistent with the overall body of evidence showing that antioxidant supplements in general don’t seem to protect against heart attacks or strokes.

Is it possible to get up to an intake of 200 mg of vitamin C a day without taking supplements? No problem. A single serving of fruits and vegetables may have about 50 mg each, so just five servings of fruits and veggies a day could get us to ideal blood levels.


Doctors can circumvent our body’s natural barriers to vitamin-C overload by dripping high levels of the vitamin directly into the bloodstream of cancer patients. Is this a good idea? See:

What other benefits or drawbacks might vitamin C have? See:

What about other common supplements? See, for example:

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations:

Do DHA Supplements Improve Brain Function?

The concept of vitamins was first described by none other than Dr. Funk. In his landmark paper in 1912, he discussed the notion that there were complex compounds our body couldn’t make from scratch, so we had to get them from our diet. By the mid-20th century, all the vitamins had been discovered and isolated, but it wasn’t until the 1960s that we realized that certain fats were essential, too.

In 1929, the necessity for fat was definitively settled… “in the diet (of the rat),” but when one of the researchers tried a 99 percent fat-free diet on himself for six months, ironically, he felt better. His high blood pressure went away, he felt more energetic, and his migraines disappeared. This one-man experiment “fortif[ied] the medical profession’s doubt that essential fatty acids had any relevance to humans,” until TPN—Total Parenteral Nutrition, meaning feeding someone exclusively through an IV—was developed in the 1960s. TPN was initially developed for babies born without working intestines. Because we didn’t think humans needed fat, “the first preparations were fat free, and they rapidly induced severe EFA [essential fatty acid] deficiencies, ultimately convincing the medical community” that some fats are indeed essential. They started out using safflower oil, but, as they discovered in a young girl given the oil after an abdominal gunshot wound, we don’t just need fat—we need specific fats like omega-3s. So, when they switched from safflower oil to soybean oil, she was restored to normal.

The fact it took so long and under such extreme circumstances to demonstrate the essential nature of omega-3s illustrates how hard it is to develop overt omega-3 deficiency. Of course, the amount required to avoid deficiency is not necessarily the optimal amount for health. The vitamin C in a spoonful of orange juice would be enough to avoid scurvy (the overt vitamin C deficiency disease), but no one considers that enough vitamin C for optimal health.

As I discuss in my video Should We Take DHA Supplements to Boost Brain Function?, what would optimal omega-3 status look like? Well, doubt has been cast on its role in heart health (see Is Fish Oil Just Snake Oil?), which appears to have been based on a faulty premise in the first place (see Omega-3s and the Eskimo Fish Tale), so taking extra omega-3s for our heart might not make any sense (see Should We Take EPA and DHA Omega-3 for Our Heart?). But what about for our baby’s brain (see Should Pregnant and Breastfeeding Women Take DHA?)? Extra DHA may not help pregnant or breast-feeding fish-eaters, but those who want to avoid the contaminants in fishes can take supplements of pollutant-free algae oil to get the best of both worlds for their babies (see Should Vegan Women Supplement with DHA During Pregnancy?). What about adults? There doesn’t appear to be any apparent psychological (see Fish Consumption and Suicide) or neurological (see Is Fish “Brain Food” for Older Adults?) benefit of DHA supplementation for the general public, but what about in those who don’t eat fish?

The famous Alpha Omega Trial randomized thousands of people over three years to get either long-chain omega-3s from fish, short-chain omega-3s from plants, or placebo. The result? The study found no significant benefits for any kind of omega-3 supplementation on global cognitive decline. However, most of the subjects were eating fish, thereby already getting pre-formed DHA in their diets. General population studies like this, that find no benefit, can’t fully inform us about the role of DHA in brain health. It would be akin to giving half these people oranges, finding no difference in scurvy rates (zero in both groups), and concluding vitamin C plays no role in scurvy.

In 2013, for the first time, DHA supplementation was found to improve memory and reaction time among young adults who rarely ate fish. Previous randomized, controlled trials failed to find such a benefit among18- to 45-year-olds, but they only lasted a few months at most, whereas the 2013 study lasted for six months. If all the studies showed either no effect or a positive effect, one might give it a try. But in one of those shorter trials, DHA supplementation didn’t just fail to show benefit—it appeared to make things worse. After 50 days, those who consumed the DHA had worse memory than those taking the placebo. So, out of the six randomized controlled trials for DHA supplementation, four showed nothing, one showed a benefit, and one showed a harm. If it were just about boosting brain function in the short term, I’d err on the side of caution and spend my money elsewhere.


What about the long term though? See Should Vegans Take DHA to Preserve Brain Function?.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: