Are Multivitamins Useless?

Linus Pauling Institute, Oregon State University, USA

“A study published recently (1), which followed 161,808 women from the Women's Health Initiative over eight years, claimed to provide "convincing evidence that multivitamin use has little or no influence on the risk of common cancers, cardiovascular disease, or total mortality in postmenopausal women." This message was immediately sent around the world by the news media, leading people everywhere to believe that taking a daily multivitamin does no good and is a waste of money. Is it really? Actually, nothing could be further from the truth.

The study was an observational study, not a randomized controlled trial. Both types of studies are called "epidemiological" or population-based studies, but there is a fundamental difference between the two of them. As its name implies, an observational study "observes" what people do, what they eat, what dietary supplements they take, how they live, and what kind of diseases they develop. Randomized controlled trials take a group of subjects and randomly assign half of them to get a specific treatment, for example, a certain pharmaceutical drug or vitamin, and the other half gets a dummy pill, or placebo. After several years, researchers assess whether those who got the actual treatment develop less disease than those who got the placebo.

Every epidemiologist will tell you that observational studies cannot establish cause-and-effect relationships; they only can observe associations. For example, a study may find that intake of a certain vitamin was associated with a lower incidence of a specific disease. Whether that vitamin was the cause for the decreased disease risk cannot be answered by an observational study. In order to answer that question, a randomized controlled trial is necessary. In other words, every epidemiologist knows that observational studies are only good enough to generate a new hypothesis, like "multivitamins might not lower risk of heart disease", but this hypothesis needs to be tested in randomized controlled trials to either prove it, establishing a cause-and-effect relationship, or refute it. Unless and until such trials have been conducted, one cannot draw any conclusions regarding causality, let alone make recommendations for the public.

Observational studies are only hypothesis-generating because they are notoriously difficult to evaluate and interpret. For example, the data are based on information collected from the participants, which is often selective and inaccurate (called "recall bias"). Behavior can change appreciably over eight years of observation. Multivitamin formulations vary considerably, and participants may have changed brands during the study. Most importantly, people who volunteer to be part of these studies are generally healthier than the average person – they are more health conscious, have a healthier diet, exercise more, etc., which can significantly affect the outcome of the study (…).

Given these considerations, it appears inappropriate for the authors of the study to conclude that it provides "convincing evidence" for multivitamins having little or no effect on cancer or cardiovascular disease risk. The evidence is far from convincing; it is suggestive at best. In addition, while endpoints like cancer, heart disease, and death are important, it is possible, for example, that a daily multivitamin helps protect against other diseases, improves immune or brain function, or promotes general health. Also, eight years of multivitamin supplementation in women over 50 years of age, as assessed in the study, may be too little too late to have a significant effect. Obviously, the data do not apply to men, because they have a different risk profile for cardiovascular diseases and hormone-dependent cancers, among many other reasons.

The reality is that most people in the U.S. have a poor diet and don't come close to consuming the recommended nine servings of fruit and vegetables every day. As a consequence, high percentages of the U.S. population do not meet the recommended dietary allowances — set by the U.S. Institute of Medicine — for many vitamins and essential minerals, including vitamin A, vitamin C, vitamin E, and vitamin K, vitamin B9 (folic acid), zinc, magnesium, and calcium. For example, data from the National Health and Nutrition Examination Survey indicate that over 90% of the population doesn't meet the recommended dietary intake for vitamin E, over 40% for vitamin A, 30% for vitamin C, and 50% for magnesium. And evidence is accumulating that most people in the U.S. are vitamin D deficient.

Given the reality that people will not improve their diet and often cannot afford to buy more fruits and vegetables, the next best thing and most cost-effective solution is to take a multivitamin. Despite the cynics' assertion that "popping vitamins is a waste of money," taking a daily multivitamin costs less than 10 cents a day. Even Dr. JoAnn Manson, a principal investigator of the Women's Health Initiative and co-author of the study, acknowledges that "the research doesn't mean multivitamins are useless. Multivitamins may still be useful as a form of [health] insurance for people with poor eating habits." And let's not fool ourselves, that's the large majority of the people in this country!”