Some dietary supplements are associated with an increased risk of death in older women, according to a study released Monday in the Archives of Internal Medicine.
In an analysis of about 39,000 women tracked over 19 years, researchers led by a team at the University of Minnesota found that those who took multivitamins, vitamin B6, folic acid, magnesium, zinc, copper and especially iron died at higher rates during the course of the study than those who did not take supplements.
Of 15 supplements analyzed by the researchers, only calcium was associated with lower risk.
"This paper contributes to the growing amount of studies showing no benefits for supplement use in the prevention of chronic diseases," said study first author Jaakko Mursu, a postdoctoral researcher in nutrition at the University of Minnesota's School of Public Health who also is affiliated with the University of Eastern Finland.
Mursu and colleagues analyzed data from the Iowa Women's Health Study, a large research effort designed to study factors related to cancer. The average age of the women was 62 at the start of the study, in 1986. Participants filled out health questionnaires, including information about diet and supplement use, several times over a period of 19 years.
The team found that supplement use was widespread and increased over the years. In 1986, 65% of the women reported taking one supplement daily. By 1997, that number had gone up to 75%; by 2004, to 85%.
As in the broader population, women in the study who took supplements tended to be healthier — with lower rates of diabetes and high blood pressure, and lower body mass index — than women who didn't. But with the exception of the women taking calcium, they died at slightly higher rates.
The research did not explore whether supplements contributed to the causes of death among the women.
In the case of iron, Mursu said the result could be an indication that iron is toxic in high amounts, or it could reflect the possibility that the women who took iron supplements were more likely to be sick from other causes and died from their underlying disease.
"We don't have the detailed information why the women were using it," he said.
Such limitations led some to question the significance of the findings.
"I wouldn't recommend anyone change what they're doing based on this study," said Dr. David Heber, director of the UCLA Center for Human Nutrition. "It's very hard to conclude cause and effect."
But Bonnie Jortberg, a registered dietitian and senior instructor at the University of Colorado School of Medicine in Denver, said the research bolstered arguments against using supplements other than in cases of known nutritional deficiency.
"Millions of Americans take these, but there just don't appear to be a lot of benefits," Jortberg said. "People take handfuls of supplements and they think that's a substitution for a good diet. It's just not."
Americans spend about $27 billion on dietary supplements each year, according to the Council for Responsible Nutrition, an industry group.