Dietary Supplements: Healthy or Harmful

Do you take a dietary supplement? If so, you're in good company: More than half of all U.S. adults take at least one daily or occasionally. But a group of researchers are now saying that vitamin and mineral supplements may be doing some people more harm than good. Two recent studies are questioning the long-term safety of multivitamins for older women and vitamin E supplements for men.

Supplements are needed by people with vitamin or mineral deficiencies. But many healthy people take supplements to prevent chronic disease—a benefit that hasn't been scientifically established or has proved inconsistent at best.

The Iowa Women’s Health Study

The ongoing Iowa Women's Health Study is monitoring women's postmenopausal health and cancer incidence. The results from one portion of the study, published in the Archives of Internal Medicine, suggest a link between multivitamins and an increased risk of death for older women.

Beginning in 1986, experts collected data from 39,000 women, ranging in age from 55 to 69 years old, about their dietary supplement habits. About 19 years later, investigators discovered that women who took multivitamins, iron, copper, magnesium, zinc, vitamin B6 and folic acid had a greater risk of dying earlier than those who didn't take supplements.

Most supplements accounted for a 2.4 percent (multivitamins) to 5.9 percent (folic acid) greater risk of dying among older women over almost two decades. An exception was copper, accounting for an 18 percent increase. Iron, in particular, concerned researchers: The higher the dose taken, the higher the death risk. Only calcium was associated with lower death rates.

But don't throw away your multivitamins just yet. Researchers found no direct relationship between supplements and premature death. They're suggesting a link but have no evidence as to what it is. Also, the study was large and well designed but had limitations that may have influenced the outcome:

  • Results of observational studies like this one are difficult to interpret because they can't prove that multivitamins caused an increased risk of dying—other factors may have played a role.
  • Once a healthy woman hits menopause, her iron needs decrease and she no longer needs an iron supplement. The postmenopausal women in the study might have overloaded on iron, which increases risk of heart, liver and other diseases.
  • Some women might have started taking more supplements later in life after being diagnosed with chronic conditions such as cardiovascular disease and cancer. This could have swayed the results toward a higher death rate. (At the start, 66 percent of the women used at least one dietary supplement; by the study's end, 85 percent used one or more.)
  • The women in the study were Caucasian, so outcomes may not apply to other ethnic populations.

What does all this mean? For starters, try to get most of your nutrients naturally instead of through supplements. Prepare varied healthful meals with nutrient-rich foods such as fruits, vegetables, whole grains, nuts, legumes and beans. If you're postmenopausal, you may still need calcium and vitamin D supplements.

Men’s cancer prevention trial

A stronger case has been made against taking vitamin E supplements to prevent prostate cancer through the Selenium and Vitamin E Cancer Prevention Trial (SELECT). Researchers found that, instead of protecting men against prostate cancer, vitamin E supplements may actually increase the risk of developing prostate cancer by 17 percent. It's estimated that about half of all men over age 60 currently take a supplement that contains vitamin E.

SELECT's original purpose was to determine whether selenium and vitamin E supplements could help prevent prostate cancer. More than 35,000 men ages 50 and older were recruited for the study. SELECT was stopped ahead of schedule in 2008 when it was apparent that neither supplement was beneficial. Although participants stopped taking the supplements, researchers continued to monitor their health.

When experts reviewed the participants' latest data in 2011, they found that men in the group who took only vitamin E supplements (400 international units a day) were developing prostate cancer at a significantly higher rate than men who took either selenium only, a combination of selenium and vitamin E or a pill containing no medicine (a placebo).

The men took the supplements for an average of five and a half years. The increased rates didn't appear until after the men stopped taking vitamin E, which points to the long-lasting effects of the supplement. Unfortunately, scientists haven't yet been able to pinpoint how vitamin E increases prostate cancer risk. They also need to explore why men who took both selenium and vitamin E saw no increased risk, which could suggest a beneficial interaction between the two.

A word of caution

Be cautious about the supplements you take and why you're taking them. Don't take supplements before consulting with your doctor or a dietitian for guidance. He or she can identify any nutritional gaps you may have and make recommendations.

Source: Prepared by the Editors of The Johns Hopkins Medical Letter: Health After 50

Publication Review By: the Editorial Staff at

Published: 17 Jul 2013

Last Modified: 17 Mar 2015