Do multivitamins cause breast cancer? An observational cohort study conducted in Sweden, recently published in the American Journal of Clinical Nutrition, suggests they may.
In such trials, people answer questions about their lives, and are then observed to see what happens to whom. These studies can be powerful when large -- this one followed nearly 35,000 women for close to ten years -- but they are never as definitive as intervention trials in which people are randomly assigned to treatment A or treatment B. People who decide to do 'A' may differ in a whole variety of ways from people who decide to do 'B.'
In this case, they did. Women who took multivitamins also used oral contraceptives and hormone replacement therapy more, and exercised less, for example, than the women who did not take the supplements.
Roughly 25 percent of the women in the study routinely took a multivitamin, and were 19 percent more likely to be diagnosed with breast cancer after adjusting for other potential explanations.
There's the headline, but let's work those numbers over just a bit. That 19 percent risk increase, if real, is a "relative" risk increase. How big is the absolute risk?
A total of 681 cancers developed over roughly 10 years in 26,312 women not routinely taking multivitamins. The risk of breast cancer in these women in any given year was thus about 0.26 percent.
In the 9,017 women taking multis routinely, there were 293 breast cancers over that same decade. Among these women, then, the absolute risk of breast cancer in any given year was 0.32 percent.
The relative difference between a risk of 0.32 percent and 0.26 percent is, indeed, about 19 percent. But the absolute difference is 0.06 percent. 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 one extra case of breast cancer occurred.
So, clearly, there is no cause for panic.
But there is cause for reflection, and perhaps reorientation. After all, we take multivitamins in the hope they will do us good, not in the hope they won't do us harm. And while evidence is scant that they do us good, this study is not the first to hint of potential harm -- other researchers have found a similar association between multis and breast cancer.
There are plausible mechanisms. Tumors grow less well when certain nutrients -- folate prominent among them -- are in rate-limiting supply. A multivitamin might "feed" cells in a tumor.
If folate is the relevant nutrient in Sweden, it may not be relevant in the U.S., since we fortify our food supply with folate (doing so dramatically reduces the occurrence of a congenital anomaly called 'neaural tube defect') and the Swedes do not. Even Americans NOT taking multivitamins are getting supplemental folate. Folate, however, is just one potential explanation for the findings.
Of course, if what prevents a tumor from growing is having too little of a nutrient to feed the tumor cells, it raises a question: is there enough of the nutrient to feed healthy cells optimally? Not getting cancer is important, but so is being vital and energetic. This study could not address that issue.
If we want optimal nutrients for healthy cells but don't want to feed tumors, the source of nutrients may be crucial. The best source -- the source strongly and consistently associated with lower risk of just about every disease -- is wholesome foods. No supplement is a substitute for them.
But something called a "whole food based" supplement may come close. Products such as Juice Plus, currently under study in my lab, take all of the nutrients from plant foods and concentrate them into capsules for those who simply can't or won't eat the recommended servings of fruits and vegetables daily (that's most Americans!). Unlike multivitamins which take nutrients out of context and repackage them, whole food supplements maintain the natural array and concentration of nutrients -- thousands of them -- found in the foods themselves. It may be that nutrients only work as they should in concert, like the various instruments in a symphony orchestra. There is both science and theory to support this notion, although no decisive evidence yet that whole food supplements promote health over the long-term while avoiding potential harms of standard multivitamins. But it seems plausible to me that this might be true, and further study is well justified.
Finally, not all nutrients are equal when it comes to breast cancer risk. Supplementation with calcium, and possibly vitamin D, in the Swedish study were actually associated with reduced risk. So along with "don't panic," let's add: 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 with omega-3 oils. I have not yet abandoned use of multivitamins, but am growing steadily less enthusiastic.
If multivitamins increase breast cancer risk, the increase is very, very small. While quite meaningful at the population level, it is very unlikely to make a difference in your life. Still, the association could be real -- and there are other ways to optimize your nutrient intake. The best of these is to eat those fruits and vegetables Mom recommended all along.
Dr. David L. Katz; www.davidkatzmd.com