Taking some vitamin supplements later in life can't be recommended for older women because they may do more harm than good, according to a new study.

For the study in this week's Archives of Internal Medicine, researchers followed a group of 38,772  women in their 60s in Iowa for about 20 years. Taking multivitaminsfolic acid and especially iron supplements were associated with a higher mortality risk, the researchers found. 

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The use of vitamin and mineral supplements as a preventive measure can't be recommended, a journal commentary said. (Christopher Furlong/Getty)

Use of multivitamins was associated with an absolute risk increase of 2.4 per cent, Jaakko Mursu of the University of Eastern Finland in Kuopio, Finland, and co-authors reported.

In contrast, calcium was associated with a lower mortality rate.

Participants in the study filled in questionnaires about their use of supplements three times during the study period. By 2004, 85.1 per cent of women said they took at least one supplement daily.

The most commonly used supplements were:

  • Calcium.
  • Multivitamins.
  • Vitamin C.
  • Vitamin E.

Supplement users were more likely to have a poorer quality diet overall, with a lower intake of energy, total fat, and saturated fatty acids, the researchers found.

"Although we cannot rule out benefits of supplements, such as improved quality of life, our study raises a concern regarding their long-term safety," the study's authors concluded.

"Based on existing evidence, we see little justification for the general and widespread use of dietary supplements. We recommend that they be used with strong medically based cause, such as symptomatic nutrient deficiency disease."

The study included a large sample that was followed for a long period of time.

The investigators took factors such as educational level, high blood pressure, body mass index, hormone replacement therapy, smoking status and physical activity into account in their analysis.

More not better

But the researchers noted that use of supplements is related to a healthier lifestyle. The study also included only white women, so generalizing the findings to other populations, ethnic groups, or men could be questioned.

The study was well designed and well conducted, Dr. Goran Bjelakovic of the University of Nis in Serbia and Christian Gluud of Copenhagen University Hospital said in a related journal commentary.

Since the study was observational, the researchers cannot draw cause-and-effect conclusions and other factors could be playing a role, the commentary noted.

"We think the paradigm 'more is better is wrong,'" Bjelakovic and Gluud said.

"We cannot recommend the use of vitamin and mineral supplements as a preventive measure, at least not in a well-nourished population. Those supplements do not replace or add to the benefits of eating fruits and vegetables and may cause unwanted health consequences. Consumption of a varied, healthful diet seems to be a prudent preventive strategy."

The findings add to the growing evidence showing that certain supplements, such as vitamin E, vitamin A, and beta carotene, can be harmful, the pair said.

Both the researchers and commentators noted that dietary supplements, unlike drugs, do not require rigorous testing using the gold-standard approach of randomized control trials to be approved.

The commentators called on politicians and regulators to allow only safe products on the market.

The Council for Responsible Nutrition oversees the supplement industry, and maintains supplements can be a part of healthy lifestyle.

"The study may make for interesting scientific water cooler discussion, but certainly does not warrant sweeping, overstated concerns for elderly women," Duffy MacKay, the group's vice-president of scientific and regulatory affairs, said in a statement.

The study was funded by the U.S. National Cancer Institute and the Academy of Finland, and the Fulbright program.