This transcript has been edited for clarity.
Welcome to Impact factor, your weekly dose of comments on a new doctor’s office. I’m Dr. F. Perry Wilson of Yale School of Medicine.
Vitamins. If you’re like most American adults, you’ve recently taken a vitamin or supplement. Over-the-counter sales of these products in this country total over $ 30 billion annually. This is more than the statin market, and vitamins are not covered by insurance.
For there to be a $ 30 billion market, there must be some pretty compelling evidence that vitamin supplements work to improve health, right?
Well, in the most in-depth meta-analysis to date, Kaiser-Permanente researchers crunched the numbers of virtually every randomized trial of vitamin supplements in adults to conclude that, in practice, they do nothing.
Or, as they say in nephrology: Vitamins give you an expensive pee.
As many of you know, the US Preventive Services Task Force (USPSTF) provides evidence-based recommendations to the American people on a wide variety of health behaviors, from aspirin for primary prevention to lung cancer screening.
The USPSTF tasked researchers to update the data on vitamin supplementation with two important findings in mind: cancer and cardiovascular death. Why vitamins? Because the observational data are clear and convincing. People with vitamin deficiencies are at greater risk for these negative results.
People with lower levels of some vitamins, not in the deficiency range, are also at increased risk for cancer and cardiovascular disease. It makes sense that if lower levels are associated with negative results and supplements prevent you from having lower vitamin levels, supplements could improve those results.
The researchers identified 87 adult RCTs in which at least one vitamin or multivitamin was evaluated. Warning: These were general population studies, not studies of people with known vitamin deficiencies. Findings should not necessarily be generalized to those with known deficiencies or disease states that promote the deficiency.
There are a lot of vitamins, so there’s a lot to cover, but I’ll hit some of the highlights.
Of all the multiple potential links between vitamins and outcomes, only one – the link between multivitamin use and cancer – showed a sign of benefit.
This is a bit frustrating as “multivitamin” can mean a lot of things. There were nine randomized trials evaluating “multivitamins” which, when combined, show this effect, but the specific types of multivitamins were different, from a custom antioxidant cocktail to Centrum Silver. So no, I don’t know which multivitamin you should be taking.
To be fair, the effect isn’t even that impressive: a 7% relative reduction in cancer incidence. And the relative risks really tend to overestimate the size of the effect. In absolute terms, multivitamins reduced the incidence of cancer by approximately 0.2%. This means that you would have to treat 500 people with a multivitamin to avoid a case of cancer.
And while these studies didn’t specifically enroll patients with vitamin deficiencies, some of those enrolled may have. What we could see is a small population effect based on the benefit gained on a small number of people who were truly vitamin deficient.
And this really is the best discovery of the whole study if you are a lover of vitamins.
No analysis of individual vitamins – beta-carotene, vitamin A, vitamin E, vitamin D (with 32 randomized trials) and calcium supplements – has shown significant benefits in terms of cardiovascular disease or cancer. They just don’t seem to do much.
So what makes the act of taking a vitamin so addicting? Why do so many of us, even knowing that the data doesn’t actually support it, continue to take a pill every day? I think there are a couple of reasons.
First, we must recognize the fact that vitamins are generally quite inexpensive and have a very low rate of side effects. They don’t make you dizzy, nauseous, fast hearted or tired. They don’t hear much of anything.
Given the low risk, something akin to Pascal’s bet develops here. Sure, vitamins may not help, but they don’t seem to hurt, so why not take them, just in case.
Well, the truth is, they could actually hurt a little. The authors also analyzed adverse events in all of these vitamin studies, albeit to assess their harms also including observational studies. This may seem unfair: evaluate the benefits only with randomized studies but the harms through randomized studies and observational studies. But I think that’s actually fine, as the bias direction in observational studies tends to favor vitamins given the “healthy user effect”. This is the idea that people who choose to take vitamins tend to make other healthy lifestyle choices, so if you see harm from taking a vitamin in your viewing environment, you’ll probably want to pay attention to it.
Notable results for the injury analysis included evidence that use of vitamin A could increase the risk of hip fracture, that use of vitamin E could increase the risk of hemorrhagic stroke, and that use of vitamin A could increase the risk of haemorrhagic stroke. C or calcium could increase the risk of kidney stones.
Why are observational data showing lower vitamin levels linked to worse outcomes so powerful and data from randomized supplementation studies so weak? This is the classic confusion. Basically, healthier people have higher vitamin levels and healthier people have less cardiovascular disease and cancer. Vitamin levels are an indicator of overall health, not a determinant of overall health.
But to be honest, there’s probably not too much harm in taking that daily vitamin. We shouldn’t ignore the ineffable value of the ritual here. Taking a vitamin, although it is a small gesture, is still an act of self-care – a moment that we take only for ourselves and for ourselves – a commitment to try to be healthy. A brief moment of positivity in the morning may not reduce heart attacks or cancer rates, but it may still have some benefits.
F. Perry Wilson, MD, MSCE, is an associate professor of medicine and director of the Yale Clinical and Translational Research Accelerator. His science communication work can be found in the Huffington Post, on NPR and here on Medscape. He tweets @fperrywilson and hosts a repository of his communication work at www.methodsman.com.
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