538, the new platform for stats-oriented analysis from Nate Silver at ESPN, has this good story by Emily Oster about the junky science of vitamins: “Don’t Take Your Vitamins.”
Many medical studies show positive health effects from higher vitamin levels. The only problem? These studies often can’t tease out the effect of the vitamins from the effect of other factors, such as generally healthy living. Studies that attempt to do this typically show no impact from vitamin use — or only a very tiny one on a small subset of people. The truth is that for most people, vitamin supplementation is simply a waste of time.
To get a little more concrete — and to understand how we got to that endless row of vitamins at CVS — it’s useful to look at a couple of examples: vitamin D and vitamin E. These are among the most popular vitamin supplements: In the 2009-2010 NHANES, 34 percent of adults reported taking vitamin D supplements and 30 percent reported taking vitamin E.
One can find plenty of support for this supplementation behavior in the medical literature. A recent review identified 290 observational studies on vitamin D. For the most part, these studies measure the amount of 25-hydroxy vitamin D — the marker of vitamin D concentration — in participants’ blood and analyze the relationship between that concentration and various measures of health.
Using this approach, researchers have found that higher concentrations of vitamin D are linked to less cardiovascular disease, lower overall mortality, less weight gain, less diabetes, less likelihood of getting infectious diseases, less multiple sclerosis, fewer mood disorders, better cognitive function — basically, every outcome under the sun. Based on these studies, vitamin D is pretty much the philosopher’s stone.
A bit less magical, vitamin E has also been credited (again, in observational studies) with everything from better pregnancy outcomes to lower mortality. In the most striking result, a large study published in the early 1990s found a 40 percent reduction in mortality risk from taking vitamin E supplements for two years. This effect is enormous.
But as striking as these results on both vitamin D and vitamin E are, they fall short of the standard for causality. These studies were not randomized controlled trials, which means other factors could have influenced their outcomes. The authors did try to adjust for some variables — age and whether the subjects smoke, for example — but these may not be sufficient. Yet people believe the results: 25 percent of adults reported taking vitamin E in 1989, and the share rose to almost 40 percent by 2003.
As is often the case, striking observational results like these were followed by large randomized controlled trials — many of them. A study run through the National Institutes of Health called the Women’s Health Initiative analyzed the impact of vitamin D and calcium supplementation in 36,000 post-menopausal women. Another large trial out of Harvard — the Physician’s Health Study — looked into vitamin E supplementation among 14,000 male physicians.
In these trials, participants were randomly assigned to take supplements. Because the assignment was random — and the trials were big — the demographic and health characteristics of the supplement group and the non-supplement group were similar before the study started. When researchers looked at participants’ health over the long term, they could therefore be confident that any differences they saw across groups were due to the supplements, and not some other factor.
When the results of these studies came out, they largely refuted the idea that these supplements offered benefits. Vitamin E appears to have no impact on cancer or heart disease. Results from the Women’s Health Study, released in 2005, showed no relationship between vitamin E supplementation and overall mortality. Later results from the men in the Physicians’ Health Study showed the same: no relationship.
For vitamin D, the randomized trials (nicely summarized here) refuted virtually all of the purported benefits to diabetes, weight loss and cancer. For elderly women, there is some evidence of a small reduction in mortality with supplementation, but well below what was seen in observational data and only marginally statistically significant.
Randomized controlled trials are not actually required to draw some conclusions in some cases; the problem is that it is easy and cheap to study correlations, as in those studies that show correlations between blood levels of vitamins and some health benefit. Taking vitamins is part of a constellation of habits of organized, health-conscious people, so naturally people who take vitamins tend to have many other healthy habits and so their vitamin levels often correlate with good outcomes. Researchers do the easy studies first, then get funding for the much more expensive studies to look for causation; in this case, very little causation is turned up. So don’t feel bad about taking vitamins — you can make a case for the multivitamin as insurance against deficiencies you may not be aware of. There is little downside to moderate doses of vitamins. But a good diet with diverse foods generally provides all of the vitamins most people need.
Vitamin D levels in blood correlate with low rates of dementia, for example, but that may well be because people who eat oily coldwater fish regularly are being protected by the fish oils and not the vitamin D they contain.
The “junk science” here is not the correlation studies, but the conclusion that they prove anything that should be acted on.
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