Health conscious shoppers read labels on the foods they eat and the supplements they take, but new research suggests that some labels leave a lot out.

Researchers at the University of Guelph in Canada sampled 44 herbal products sold in the United States and Canada by 12 companies. They found that the majority contain ingredients not listed on the label, including fillers and cheaper substitutes.

Using DNA bar coding technology, researchers found that nearly 60 percent of the products they tested contained plant species not listed on the label, while 32 percent used product substitution.

More than 20 percent included fillers, such as rice, soybeans, and wheat not listed on the label, which can be dangerous for people with food allergies.

Read More: 20 Reasons Not to Trust Cosmetics Labels

“It's common practice in natural products to use fillers such as these, which are mixed with the active ingredients,” Steven Newmaster, lead study author and an integrative biology professor at Guelph, said in a statement. “But a consumer has a right to see all of the plant species used in producing a natural product on the list of ingredients.”

The study was published in the latest issue of the journal BMC Medicine.

Undisclosed Wheat, Nuts, and Flowers

Newmaster said contamination and substitution in these products present considerable health risks.

“We found contamination in several products with plants that have known toxicity, side effects and/or negatively interact with other herbs, supplements, and medications,” he said.

Among the findings, researchers discovered:

  • St. John's wort contained Senna alexandrina, a plant not intended for prolonged use that is linked to chronic diarrhea, liver damage, and colon problems.
  • Several products contained the flower feverfew, which can cause swelling and numbness in the mouth, oral ulcers, and nausea. It also causes negative reactions when combined with medications metabolized by the liver.
  • One ginkgo product was contaminated with black walnut, which is dangerous for people with nut allergies.

Only two of the 12 sampled companies—none of whom were named in the study—provided products without contaminants, substitutions, or fillers.

Regulating a Global Industry

With more than 1,000 companies across the globe making more than 29,000 herbal products, medicinal herbs are now the fastest-growing segment of the North American alternative medicine market, valued at $60 billion a year.

“There is a need to protect consumers from the economic and health risks associated with herbal product fraud. Currently, there are no standards for authentication of herbal products,” Newmaster said. “The industry suffers from unethical activities by some of the manufacturers.”

In the U.S., the Food and Drug Administration (FDA), which continues to operate at limited capacity during the government shutdown, has a separate set of guidelines for nutritional and herbal supplements, as opposed to prescription drugs.

“Generally, manufacturers do not need to register their products with FDA or get FDA approval before producing or selling dietary supplements. Manufacturers must make sure that product label information is truthful and not misleading,” according to the agency's website.

The Canadian Food Inspection Agency, however, has regulated natural health products since 2004. Still, thousands of products on the market lack a full product license, as regulators face a backlog of license applications.

Beware of the 'Healthy Halo'

While many people take supplements in an effort to be healthier, it's easy to fall victim to the "healthy halo."

Research from Cornell University’s Food and Brand Lab shows that people view “healthy” food labels differently. Researchers asked 115 shoppers to assess snack foods based on their labels.

People thought foods labeled “organic” were more nutritious, lower in fat, and higher in fiber than the “regular” foods, and were willing to pay up to 23.4 percent more for the food labeled “organic.” The difference was in the labels only: all the study participants were eating the same foods.

Learn More About How Food Labels Can Suck You Into the “Healthy Halo.”