That finding held even when the people taking the supplements were consuming lower-than-recommended levels of nutrients through their diet.
The study also found that getting excess amounts of certain nutrients was linked to an increased risk of death, particularly from cancer — but only when those excesses came from supplements.
The study adds to a long line of studies, including two major ones published last year, that have shown most people receive no health benefits — but may experience harm — from taking vitamin and mineral supplements.
“As potential benefits and harms of supplement use continue to be studied, some studies have found associations between excess nutrient intake and adverse outcomes, including increased risk of certain cancers,” said Dr. Fang Fang Zhang, M.D., the study’s senior author and a cancer epidemiologist at Tufts University, in a released statement. “It is important to understand the role that the nutrient and its source might play in health outcomes, particularly if the effect might not be beneficial.”
More than half of American adults report taking some kind of dietary supplement, and about a third say they take a multivitamin and mineral supplement, usually because they’ve been led to believe — despite the lack of any good evidence — that those pills will serve as a kind of “insurance” against cancer, heart disease and other chronic illnesses.
The monetary cost of that practice is high. This year, American consumers will spend an estimated $32 billion on dietary supplements.
How the study was done
The current study is based on data collected from a nationally representative sample of 30,899 adults who participated in six cycles (between 1999 and 2010) of the National Health and Nutritional Examination Survey (NHANES), a database that is updated every two years.
Each participant answered detailed questions about their dietary habits and supplement use at least twice during that period.
More than half of the participants (51 percent) reported using dietary supplements, and more than a third (39 percent) said they had used multivitamin and mineral supplements.
Twenty-five vitamins and minerals were assessed in the study. The most common vitamins taken by the participants were vitamin C (used by 40 percent of the participants), vitamin E (39 percent) and vitamin D (38 percent).
The most common minerals were calcium (39 percent), zinc (35 percent) and magnesium (33 percent).
The people who took supplements tended to be older, whiter, more highly educated and wealthier. They also were more likely to be physically active, to not smoke or drink heavily, and to have a healthy diet.
In fact, the supplement users took in higher levels of most (23) of the nutrients than the nonusers even when their supplement use was discounted.
Half of all the participants, however, had an inadequate intake of vitamin D, vitamin E, choline, vitamin K and potassium.
Key findings
Zhang and her colleagues followed the people in the study through the end of 2011. By that time, 3,613 of the participants had died, including 945 from cardiovascular disease and 805 from cancer.
The researchers compared the participants’ nutrient intake with their risk of early death. After adjusting the data for a long list of other factors known to affect that risk, such as age, smoking, alcohol use, physical activity and socioeconomic status, they found the following:
- People with adequate intakes of vitamin K and magnesium had a lower risk of death from any cause.
- People with adequate intakes of vitamin A, vitamin K and zinc had a lower risk of death from cardiovascular disease.
The researchers then analyzed the data based on the source of those nutrients (food versus supplements). They found that the lower risk of death occurred only when the nutrients came from foods.
The supplements had no effect on lowering the risk death from any cause — even among people who weren’t getting enough nutrients from their diet.
The study did find, however, that excessive amounts of calcium (1,000 milligrams or more per day) were linked to a higher risk of death from cancer — but only when those high amounts came from supplements. There was no link between cancer and high levels of calcium consumed through food.
Taking excessive amounts of vitamin D (more than 10 micrograms or 400 IUs daily) through supplements was also associated with an increased risk of death, including from cancer. This effect was seen, however, only in people who already got adequate amounts of the vitamin through their diet. For people with inadequate dietary levels of vitamin D, taking supplements had neither a beneficial nor a harmful effect.
Focus on healthful foods
The study comes with plenty of caveats. Most notably, it relies on self-reported data about diet and supplement use, and such reports can be unreliable.
Still, the findings are similar to those of other large studies, including ones published last year in the Journal of the American College of Cardiology and in Circulation: Cardiovascular Quality and Outcomes (a publication of the American Heart Association).
The findings are also in line with the recommendations from the United States Preventive Services Task Force (USPSTF), which says there is no good evidence that vitamin and mineral supplements protect against either heart disease or cancer.
“Although adequate nutrient intake from foods could contribute to reduced risk for death, excess intake from supplements might increase mortality,” Zhang and her colleagues conclude in their paper.
“The potential risks and benefits of dietary supplement use for health need to be further evaluated in future studies,” they add.
In the meantime, most of us should be focusing on getting our nutrients from healthy foods rather than from supplements. (Two groups who do benefit from supplements are pregnant women and individuals who have been diagnosed with specific nutrient-deficiency conditions, such as pellagra.)
Good food sources of vitamin K include leafy greens, such as kale, spinach, turnip greens and Swiss chard. Zinc is found in nuts, beans and legumes (such as lentils and chickpeas), as well as in beef, pork and lamb. And vitamin A can be found in eggs and orange and yellow fruits and vegetables, as well as in fortified milk and cereals.
FMI: You’ll find an abstract of the study on the Annals of Internal Medicine website, but the full study is, unfortunately, behind a paywall — even though it was funded primarily with public money through the National Institutes of Health.
