Can your multivitamin give you breast cancer?

One recent study in the American Journal of Clinical Nutrition says yes. The study followed nearly 35,000 women for close to ten years; 25% of the women in the study routinely took a multivitamin and the other 75% did not. Authors found that multivitamin use was associated with a 19% increase in the likelihood of being diagnosed with breast cancer.

But I'm not convinced that multis trigger breast cancer, and you have no cause to panic. First, let's talk about the study.

1) The numbers sound scarier than they are. In women not taking multis, the risk of breast cancer in given person in any given year was about 0.26%. Among multivitamin users, the absolute risk of breast cancer in any given year was 0.32%. The relative difference between a risk of 0.32% and 0.26% is, indeed, about 19%. But the absolute difference is 0.06%. In other words, if multivitamins are truly the cause of the apparent risk difference, they would increase your breast cancer risk by considerably less than one tenth of one percent; 1,667 women would need to take multivitamins for a year before the exposure resulted in one extra case of breast cancer.

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2) The women who took multis differed in a variety of ways from women who did not. This was an observational cohort study, which means that people answer questions about their lives (Ex: Do they take vitamins? Do they exercise?) and are then observed to see what happens to whom. Such studies can be powerful when they are large, but they are never as definitive as intervention trials in which people are randomly assigned to treatment A, or treatment B. Why? Because people who decide to live one way as opposed to another may differ in a whole variety of ways from people who choose to live another way. In this case, multi takers used both oral contraceptives and hormone replacement therapy more than non-users, and did less exercise. The association between multi use and breast cancer persisted when all of this was factored in, but it did weaken.

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3) The study was in Sweden, so results to American women may not translate. Some important things about living in Sweden differ markedly from the U.S. In this case, namely the fact that we fortify our food with folate and the Swedes do not. Tumors grow less well when certain nutrients, including folate, are in rate-limiting supply. Because our food is fortified with this vitamin, even American women who don't take multis are apt to have a good intake of it, so there may be fewer differences in cancer risk between multi takers and non-takers.

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So while there is no cause for panic, there may be cause for reflection. As the authors note, other studies have found a similar association between multi use and breast cancer, and there are plausible mechanisms by which this could occur.

Remember, the best source of nutrients-the source strongly and consistently associated with lower risk of just about every disease-is wholesome foods. There is no substitute for healthful eating. Supplement, not substitute. And keep in mind that not all nutrients are equal when it comes to breast cancer risk. Supplementation with calcium, and possibly vitamin D, in the Swedish study was actually associated with reduced risk. So don't toss out the baby with the bathwater. I favor vitamin D supplementation, vary my calcium recommendations depending on diet, and routinely encourage supplementation of omega-3 oil. I have not abandoned use of multivitamins, but am growing increasingly less enthusiastic. In contrast, I am more and more interested in "whole food"-based supplements, which preserve the natural balance of nutrients found in fruits and vegetables.

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In conclusion: If multivitamin use does increase breast cancer risk, it increases risk by a very, very small amount. This is still quite meaningful at the population level, but is very unlikely to make a difference in your life. If you're concerned, there are other ways to optimize your nutrient intake. Among these, eating a diet close to nature, rich in fruits and vegetables, is my most enthusiastic recommendation.

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