Wild claims about vitamin supplements fly all over the internet. My Facebook feed is little more than a series of ads for vitamins, potions, and blood tests that promise to turn me into an immortal David Hasselhoff. So what does the best science tell us about vitamin supplements for people who haven’t been diagnosed with a nutritional deficiency by their doctor? Will they help us live for 115 years?
Physicians’ Health Study
The biggest and longest study of multivitamin supplements is the Physicians’ Health Study II. The study, which ran from 1997 to 2011, enrolled nearly 15,000 male doctors over the age of 50. The doctors were randomly assigned to take real or placebo versions of a common multivitamin (Centrum Silver), and their health outcomes were followed for around 11 years.
For the first few years, the doctors were also randomized to vitamin C, vitamin E, and beta-carotene. These supplements were part of shorter trials that, in the end, showed that those supplements had no effect on cardiovascular disease or cancer.
When results of the multivitamin trial were published in 2012, it was “the only large-scale trial testing the effects of long-term use of a common multivitamin.” After some searching on Google Scholar, I believe that’s still the case today. Other studies of vitamin supplements are smaller, shorter, and typically focus on only one or a few vitamins.
The study researchers looked particularly for any differences between the placebo and treatment groups in cancer incidence (excluding non-melanoma skin cancers, which are common, rarely fatal, and mediated largely by sun exposure) and major cardiovascular events such as heart attack and strokes. The study also measured total mortality.
The study found almost exactly no difference in the incidence of major cardiovascular events, an 8% decrease in cancer that barely reached statistical significance, and a 6% decrease in total mortality that didn’t quite reach significance.
Effect on life expectancy
The 6% relative reduction in mortality came with a wide uncertainty interval, from a 2% increase to a 12% decrease, which is why it didn’t reach statistical significance. But let’s assume that the 6% value is correct. How does that translate to life expectancy? It’s tempting to guess that remaining life expectancy should increase by 6%, but as a committed anti-aging geek, I know that’s not the right way to think about it.
The correct approach is to start with the baseline probability of dying in any given year, then to multiply that probability by 0.94 (i.e., 6% or 0.06 less than without a multivitamin) to calculate a new life expectancy. I’ve done the calculation for you in this Google Sheet using the Social Security Administration’s actuarial tables as a basis for the unsupplemented life expectancy. The life expectancy at age 50 rises from 29.7 to 30.3 years for a man and from 33.2 to 33.8 years for a woman. The plot below shows the survival probabilities over time.
Cost v. extra life
An extra 0.6 years might sound measly, but compare that to the cost. Centrum Silver is on Amazon for seven cents a pill. My grocery store had another brand with identical nutritional content for even less. At a cost of seven cents a day, a lifetime supply of Centrum Silver on your fiftieth birthday would set you back $758 if you’re a man or $849 if you’re a woman. So the hit to your wallet won’t be significant. Taking one pill a day is easy, and price wise it’s an absolute bargain.
The only significant side effect of the multivitamin was a 7% increase in rashes, but it’s worth noting that the reliability of this is statistically questionable. The authors tested for 13 different side effects, but the standard p-value calculation assumes you’re only testing one hypothesis. If you’re testing 13 hypotheses, you’re more likely to get a false positive and should accordingly adjust the threshold of significance. The authors don’t seem to have done that, so it’s plausible the apparent increase in rashes happened by chance.
If a multivitamin supplement truly increases the risk of rashes, it’s hardly a game-changer: You could easily stop taking it and see a doctor about the rash.
There were also side effects reported regarding minor bleeding in multivitamin-takers, but these were inconsistent. The treatment group showed a small decrease in blood in the urine, a small increase in bloody noses, and no change in bruising. The authors don’t mention this as meaningful.
I’ll be the first to admit how imperfect my above survival calculation is. I’ve already pointed out the wide uncertainty around the 6% relative reduction in mortality. Plus the Physicians’ Health Study was run on a group of highly educated, highly paid men, most of whom exercised regularly and very few of whom smoked. It’s not clear how the benefits transfer to a more typical man or a woman. Finally, as the authors themselves point out, they didn’t check whether the study subjects were deficient in any nutrients to start with. Such deficiencies could make a real difference to the effect of a multivitamin supplement.
Still, it’s reasonable to conclude that taking a daily multivitamin starting in middle age is more likely than not to extend life expectancy on the order of months. I once tested very low for vitamin D, so my own doctor has told me to take a multivitamin with at least 1,000 IU of vitamin D – which includes Centrum Silver – even though I’m well below 50. In retrospect, the cost of the multivitamin is so low compared to the test, I should have just skipped the test and gone straight to the supplement. Based on the Physicians’ Health Study II, the multivitamin might have something beneficial in it even besides the Vitamin D.