Women taking multivitamins may not necessarily live longer than those who get their nutrients from food alone, according to a U.S. study that found the group appears to have slightly higher death rates.

About half of adult Americans take dietary supplements, and the industry now boasts of annual sales as high as $20 billion, according to the study.

There is very little evidence showing that common dietary supplements would be beneficial in prevention of major chronic diseases, said Jaakko Mursu of the University of Minnesota in Minneapolis, who worked on the study.

Some unregulated substances, such as vitamins A and E, may be harmful in high doses, suggested the study, published Tuesday with the study in the Archives of Internal Medicine.

The survey asked about use of multivitamins, vitamins A, C, D and E as well as beta-carotene, B vitamins and minerals such as calcium, copper, magnesium, selenium and zinc.

Unless you are deficient, there is hardly any reason to take them, Mursu told Reuters Health.

Researchers used data from nearly 39,000 older women who participated in the Iowa Women's Health Study and filled out questionnaires starting in 1986.

During the study, supplements became increasingly popular, and between 1986 and 2004, the proportion of women who said they took one or more jumped from 63 percent to 85 percent.

Only calcium supplements were linked to a lower risk of death over 19 years of follow-up, with 37 percent of users dying compared to 43 percent of non users.

For instance, 41 percent of multivitamin users died versus 40 percent of non-users, and the gap became even wider when adjusting the numbers based on health problems like diabetes, high blood pressure and overweight in the two groups.

One possible exception is vitamin D, which one recent study suggests may help women live a little longer.

Mursu also cautioned that his study doesn't prove supplements cause harm. I would rather conclude that there is no evidence for benefits, he said.

The 2010 U.S. dietary guidelines recommend getting nutrients from food, not supplements. The guidelines also urge people 50 and older to get extra vitamin B12 from fortified foods or supplements.

However, women of reproductive age are advised to get extra folic acid and those who are pregnant may want to take iron supplements if their doctor suggests it.