About 1 in 2 American adults — or 52 percent of them — takes a dietary supplement, a new study suggests.

Although this overall percentage of Americans taking supplements has not changed in recent years, there were changes during the 13-year study period in which supplements people take, the researchers found. For example, there was a decrease in the percentage of people taking a daily multivitamin, the researchers found. Adults taking a multivitamin/multimineral product fell from 37 percent in 1999 to 31 percent in 2012, according to the findings, published today (Oct. 11) in The Journal of the American Medical Association (JAMA).

The findings are based on nationally representative data, and give a comprehensive picture of supplement use among U.S. adults in all age groups, including the most recently available data from 2012, said lead author Elizabeth D. Kantor, an epidemiologist at Memorial Sloan Kettering Cancer Center in New York City.

In the study, the researchers looked at data collected from about 38,000 U.S. adults ages 20 and older. These men and women were participants in the National Health and Nutrition Examination Survey (NHANES), which is a yearly health interview conducted in people's homes by researchers from the Centers for Disease Control and Prevention. [5 Wacky Things That Are Good for Your Health]

All participants were asked whether they had taken any prescription or over-the-counter dietary supplements — vitamins, minerals, herbs or other supplements — within the last 30 days. Because interviewers met with each participant in their homes, they would also ask to see the supplement bottles, making the data more reliable than studies that rely primarily on a person's memory, the researchers said.

Supplement trends

Multivitamins were not the only product to show a downward trend in use between 1999 and 2012: The use of echinacea for colds, ginkgo biloba to preserve memory, garlic to promote heart health and antioxidants, such as vitamins C and E, also decreased in use during the study period, according to the findings.

On the other hand, the use of probiotics and coenzyme Q10 supplements were on the rise. The analysis also revealed that the number of adults taking vitamin D, which may be beneficial for heart disease, some cancers and fractures, went from 5.1 percent in 1999 to 19 percent in 2012. [Don't Be Fooled: 5 Probiotics Myths]

What's more, there was also a sevenfold increase in adults taking omega-3 fatty acids (often sold as fish-oil supplements), which may help prevent heart disease and some cancers. Its usage rate grew from 1.9 percent in 1999 to 13 percent in 2012.

The analysis also found an increase in supplement use as people got older.

The researchers did not specifically investigate the reasons for the trends. "It's hard to say what are the driving factors behind these trends in supplement use," Kantor said.

But factors such as the economic downturn in the late 2000s as well as the increased scrutiny of multivitamins, antioxidants and some other supplements following several studies showing no health benefit might have contributed to the decline in the use of those products, Kantor told Live Science.

While the new study offers a glimpse at supplement use among adults, it does not provide information on the frequency or dose taken, and it might not reflect current patterns in 2016, the researchers said.

During the study period, a steady stream of high-quality studies evaluating dietary supplements has yielded predominantly disappointing results about their potential health benefits, said Dr. Pieter A. Cohen of the Cambridge Health Alliance in Cambridge, Massachusetts, who wrote an editorial accompanying the findings appearing in the same issue of JAMA.

Consumers may still be using these supplements out of habit, or because they may be unaware of negative results, or because manufacturers can still make vague health claims, such as "preserves heart health" or "maintains mental alertness," about these products, wrote Cohen, who was not involved in the current research but has studied dietary supplements.

Because the negative results from high-quality studies have had only a modest impact on people's supplement use, "future efforts should focus on developing regulatory reforms that provide consumers with accurate information about the effectiveness and safety of supplements," Cohen wrote.

Originally published on Live Science.