The study found a small increase in the risk of death among older women who took dietary supplements.
Most people receive little benefit from taking multivitamins and many other dietary supplements and they may even be harmful, according to researchers behind a large new study.
The researchers tracked nearly 39,000 women for an average of 19 years. They found a small increase in the risk of death among older women who took dietary supplements compared with those who didn’t, according to the study, released Monday in the Archives of Internal Medicine.
An exception was calcium supplements, which are widely recommended to protect against bone fractures. They were associated with a slightly decreased risk of premature death.
“Based on existing evidence, we see little justification for the general and widespread use of dietary supplements,” unless there is a medical reason or deficiency of a particular nutrient, wrote the study authors, most of whom are affiliated with the University of Minnesota.
At least half of U.S. adults take vitamins, minerals, or other supplements, according to government estimates. Consumers spent about $11.8 billion on vitamins and minerals last year, according to the Nutrition Business Journal, which tracks supplement sales.
Among the several vitamins and minerals studied, researchers said the strongest association for an increased risk of death came with iron, which is often prescribed for people who have anemia. Women in the study who took iron supplements were 3.9% more likely to die than those who didn’t.
The risk of dying early was 2.4% higher among women who took a multivitamin compared with women who didn’t.
Miriam Pappo, director of clinical nutrition at Montefiore Medical Center in the Bronx, N.Y., said the findings likely won’t drive doctors to stop recommending multivitamins to help patients get all the nutrients they need.
“No one disputes that oral intake of food to get your vitamins and minerals is the way to go,” said Ms. Pappo. However, “in reality, most of us don’t make it up to the nine [servings] a day of fruits and vegetables.”
The study was based on self reports from women participating in the Iowa Women’s Health Study who were age 62 when it started in 1986.
The researchers, led by Jaakko Mursu, of the University of Eastern Finland in Kuopio and the University of Minnesota, Minneapolis, assessed supplement use in three questionnaires taken in 1986, 1987 and 2004, and then tracked the death rate in the women.
Supplement use rose from 63% of women in 1986 to 85% of women in 2004. During the study more than 15,000 women died.
Certain groups of people may still need supplements, such as women who are pregnant or trying to become pregnant. The nutrient folic acid, a B vitamin, has been shown to reduce the risk of birth defects.
And doctors recommend that many older women take calcium and a vitamin D supplement to protect against brittle bones.
Researchers controlled for factors such as age, diet, weight, smoking status and underlying health conditions to isolate the impact of vitamins and minerals.
So the results weren’t skewed by the fact that women who took supplements were more likely to exercise, weigh less and have a lower prevalence of diabetes or high blood pressure than women who didn’t take supplements. All of the women studied were white, so the findings may not apply to other groups or to men, the study said.
Duffy MacKay, vice president of scientific and regulatory affairs at the Council for Responsible Nutrition, a Washington, D.C., trade group that represents supplement makers, said it is difficult for most people to get all their nutrients from food, and that vitamin supplements can fill in important gaps in nutrition.
Susan Fisher, chairwoman of the University of Rochester School of Medicine and Dentistry’s Department of Community and Preventive Medicine, noted that the study looks at deaths and not debilitating health conditions that vitamins can help prevent. “If your doctor suggests you should take supplements, it is still wise to follow that direction,” Dr. Fisher said.
Still, “this study, in combination with previous studies, raises safety questions about supplements,” said Lisa Harnack, a professor of epidemiology at the University of Minnesota and a study author.
“Most Americans get plenty of micronutrients in their diets,” she said, adding many types of cereal, bread and beverages are fortified with added vitamins.
The study was partly funded by the National Cancer Institute and the Academy of Finland, which is backed by the Finnish government. It is the most recent of several studies released in the past few years to question the long-term health effects of widely used vitamin supplements.
A 2009 study involving about 161,000 postmenopausal women participating in the federally funded Women’s Health Initiative found that multivitamin use didn’t cut the risk of cancer, cardiovascular problems or the overall death rate.
Many doctors still recommend that patients take a daily multivitamin in addition to adhering to a healthy diet with lots of fruits, vegetables, whole grains and protein from fish, poultry and low-fat dairy products.
Montefiore’s Ms. Pappo agreed with the researchers in saying doctors should be cautious before prescribing iron, noting relatively few cases of anemia are caused by an iron deficiency.
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