Older women who took a daily vitamin supplement — even just a multivitamin — had an increased risk of dying of cardiovascular disease and cancer, according to a study published Monday in the journal Archives of Internal Medicine.

The study highlights concerns about the long-term use of supplements and vitamins in people who do not have severe nutritional deficiencies, the authors say. An accompanying editorial notes that findings "add to the growing evidence demonstrating that certain supplements can be harmful."

Previous studies have raised questions about the value of supplements and vitamins, but researchers and nutrition experts call these new findings "puzzling" and say more research is needed.

About half of adults in the USA take multivitamins. Annual vitamin and supplement sales total more than $20 billion.

"I think the main message is researchers are finding very little benefit from these substances," says lead author Jaakko Murso, a nutritional epidemiologist at the University of Minnesota in Minneapolis. "Other studies have not shown the mortality risk our study shows, but those studies have not seen any positive effect either."

Researchers used data from the Iowa Women's Health Study to examine the link between vitamin and mineral supplements and death rates among 38,772 women, average age 61.6. Women filled out questionnaires about supplement use in 1986, 1997 and 2004. "Out of 15 studied supplements, seven are associated with increased total mortality risk," Murso says.

Among the findings:

Use of multivitamins, vitamin B6, folic acid, iron, magnesium, zinc and copper were associated with increased risk of death.

The link between supplement intake and death risk was strongest with iron.

Calcium supplements were associated with reduced risk.

The study's authors advise that vitamins "be used with a strong medically based cause," not for prevention alone. Murso says the findings focus on the higher risk of death from cardiovascular disease, cancer and "other causes," but the study didn't examine how supplements and vitamins might affect health. "There is much more research needed to begin to understand that," he says.

"This study is very puzzling and calls for more research," says Miriam Pappo, director of clinical nutrition at Montefiore Medical Center in New York. "I wouldn't conclude from this that you stop taking a standard multivitamin. Very few people eat the required amount of fruits and vegetables a day. It's best to get your daily needs from food, but few people do that."

A spokesman for the vitamin industry was skeptical. "The study may make for interesting scientific water cooler discussion, but certainly does not warrant sweeping, overstated concerns for elderly women," said a statement from Duffy MacKay of the Council for Responsible Nutrition.

Among the 38,772 women who started the study in 1986, 15,594 died within 19 years. Self-reported supplement use increased substantially from 1986 to 2004: 62.7 percent of women reported using at least one supplement daily in 1986, 75.1 percent in 1997 and 85.1 percent in 2004.