DNA tests have shown that herbal supplements - substances containing medicinal properties of plants and marketed to improve health - are little more than powdered rice and weeds, the New York Times reported.

Canadian researchers found that herbal products available to consumers in the marketplace may be contaminated or substituted with alternative plant species and fillers that are not listed on the labels.

Using a test called DNA barcoding, a method used to identify plants, researchers found that many popular herb supplements were often diluted -or replaced entirely - by cheap fillers like soybean, wheat and rice.

"Most of the herbal products tested were of poor quality, including considerable product substation, contamination and use of fillers," researchers concluded in the study. "These activities dilute the effectiveness of otherwise useful remedies, lowering the perceived value of all related products because of a lack of consumer confidence in them."

The researchers selected popular medicinal herbs, and then randomly bought different brands of those products from stores and outlets in Canada and the United States, the Times reported. They analyzed 44 bottles of popular herb supplements from 12 companies.   

Of 44 herbal supplements tested, one-third of the products were entirely replaced with an alternative, there was no trace of the plant advertised on the bottle - only another plant in its place.

Based on the study, many of the supplements were contaminated with ingredients not listed on the label like rice, soybean and wheat, which are used as fillers, according to the Times.  In some cases, fillers were the only plant detected in the bottle.

According to the Times, two bottles labeled St. John's wort, which is used to treat depression, contained none of the medicinal herb. The pills in one bottle were made of nothing but rice and the other bottle contained Alexandrian senna, an Egyptian shrub that is a powerful laxative.

They also found that bottles of Echinacea supplements, which are used by millions of Americans to prevent and treat colds, contained Parthenium hysterophorus, an invasive plant found in India and Australia that has been linked to rashes, nausea and flatulence.

Steven G. Newmaster, biology professor and botanical director of the Biodiversity Institute of Ontario at the University of Guelph and the study's lead author, told the Times their findings is a health concern for people with allergies or those seeking gluten-free products.

Consumer advocates and scientists said this study highlights the flaws in the herbal supplement industry. This study provides more evidence that the industry is riddled with questionable practices.

"This suggests that the problems are widespread and that quality control for many companies, whether through ignorance, incompetence or dishonesty, is unacceptable," David Schardt, a senior nutritionist at the Center for Science in the Public Interest, an advocacy group, told the Times. "Given these results, it's hard to recommend any herbal supplements to consumers."

Industry representatives argue that any problems are not widespread.

Representatives of the supplement industry told the Times while mislabeling of supplements was a legitimate concern, they did not believe that the problems were widespread.

"Overall, I would agree that quality control is an issue in the herbal industry," Stefan Gafner, the chief science officer at the American Botanical Council, a nonprofit group that promotes the use of herbal supplements, told the Times. "But I think that what's represented here is overblown. I don't think it's as bad as it looks according to this study."