Contamination & substitution present in many herbal products, but is DNA barcoding of herbals fit for purpose?

Researchers led by Prof Steven Newmaster, botanical director of the Guelph-based Biodiversity Institute of Ontario, used DNA barcoding technology to test 44 herbal products sold by 12 companies. Their results, published in the open access journal BMC Medicine​​, indicated that only two of the companies provided authentic products without substitutions, contaminants or fillers.

Overall, nearly 60% of the herbal products contained plant species not listed on the label, and product substitution was detected in 32% of the samples.

“Contamination and substitution in herbal products present considerable health risks for consumers,”​ said Prof Newmaster. “We found contamination in several products with plants that have known toxicity, side effects and/or negatively interact with other herbs, supplements and medications.”​

Robust?​

Commenting independently on the study’s findings, Kelly Reins, VP Operations & Quality for Alkemists Labs, told us that, while the thought of using DNA to increase the safety of botanicals is intriguing, it’s still a discipline that is not fit for purpose with the current limitations.

“Currently, the DNA methods are not robust,” ​said Reins. “There are no formal validation protocols for the equipment being used. I spoke at length with several equipment manufacturers and they stated the instruments cannot be validated/qualified and they are meant for R&D use only at this time.”​

“Since there are no standards for authentication of herbal products this is interesting science at best,” ​she added. “Years of research and cross functional involvement are needed before this approach can be considered scientifically valid. Unfortunately this article has been spun in the industry and for those lacking a clear understanding of DNA testing weakness they now have a fear factor based on an incomplete picture. ​

“Once again, a little knowledge can be a dangerous thing.”​

Michael McGuffin, president of the American Herbal Product Association (AHPA), wrote to the editors of the journal, stating: "The article presents its findings as if DNA barcoding has been validated for the identification of each of the tested botanicals. But this is an emerging technology and there are still many questions about the effectiveness and limitations of this method. DNA testing has the potential to be useful in the future when it has been rigorously tested, but the article's blanket assertions about the accuracy of this novel analytical tool are premature."​

"Omitting the names of the products tested is a disservice to companies that verify the identity of the ingredients in their products using scientifically valid methods like chromatography, microscopy and organoleptic analysis by qualified experts,"​ added McGuffin. "These companies' reputations should not be tarnished and AHPA urges the authors of the paper to disclose the identity of the tested products."​

Reliable when used appropriately​​

The American Botanical Council said that DNA technology, in general, is reliable when used appropriately. Mark Blumenthal, founder and executive director of ABC, said: “We have published two articles in our peer-reviewed journal HerbalGram ​discussing the merits and benefits of DNA-based analytical methods for establishing the accurate identity of plant material, including botanical materials found in herbal teas, as well as dried powders in numerous capsules and tablets found in herbal dietary supplements and related products.​

“However,” ​added Blumenthal, “DNA-based analysis is not appropriate when used in some of the ways that the authors appear to have done so, i.e., when trying to use DNA to determine the identity of commercial herb products that may contain plant extracts. It is not clear from the DNA paper whether some of the commercial herb capsules analyzed by the Canadian researchers contained dried powdered herb extracts or not. If they did, then DNA sequencing would not reveal the identity of the labeled plant extract, and might provide confusing results based on other excipient and ‘filler’ ingredients, or contamination with other DNA that also may be in the capsule.”​

ABC has called for a retraction of the article. To read ABC's extensive critique, please click here​. 

Study details​

The study’s findings, which have generated headlines from Canada and the US to Australia, indicated that one product labeled as St. John's wort contained Senna alexandrina​, a plant with laxative properties. Other herbal products were found to contain Parthenium hysterophorus​ (feverfew), which can cause swelling and numbness in the mouth, oral ulcers, and nausea.

Prof Newmaster and his co-workers also found that one ginkgo product was contaminated with Juglans nigra​ (black walnut), which could endanger people with nut allergies.

In addition, the researchers raised concerns over the use of unlabeled fillers, including wheat, soybeans and rice, which could be a problem for people with allergies.

“It's common practice in natural products to use fillers such as these, which are mixed with the active ingredients,” ​said Prof Newmaster. “But a consumer has a right to see all of the plant species used in producing a natural product on the list of ingredients.”​

Work in progress​

The researchers wrote that their study is the first to build a partial herbal standard reference material (SRM) barcode library. Currently, they have only 100 species in their SRM library, they said, but plan to double this by the end of 2013, and to have 1,000 species by 2015.

“However, this will only add up to 55% of the 1,800 medicinal plant species that are commercially available,”​ they said. “If we want to have reliable identifications using DNA barcodes we must build an SRM herbal barcode library that has all sister species for the 1,800 known medicinal species used in commercial products. This is one of the goals of our Herb-BOL (barcode of life) research program in the next 5 years.”​

An incomplete library​

Alkemists’ Reins welcomed the acknowledgement by the authors of the current weaknesses of DNA barcoding: “They are creating an SRM herbal barcode library,”​ she said. “Their own barcode success rate was a mere 91% for combined regions. They acknowledge species resolution is not optimal. ​

“They also state that some of the DNA sequences recovered were only identified to family since their DNA barcode library is not complete. They admit that their study of herbal product contaminants as well as others has not been able to identify contaminants to species level resolution due to the incomplete library.​

“It was also noted that found rice and soybean fillers in natural plant and animal products produce mixed signals during the sequencing process, contributing to a high percentage of failed sequencing reactions in capsulated products. Herbal products contain mixtures that are difficult to barcode. Hence testing finished products can result in false failures,”​ she added.

“At least they state what we have known all along, GenBank is inappropriate since many of the DNA sequences do not have vouchers specimens archived in the herbarium. The GenBank DNA sequences could potentially be from an incorrectly identified plant. Additionally, technology is advancing. The sequencing capability of current is being replaced with the next generation of sequencing capabilities.”​

Reins said that the authors also noted that herbal products contain plant metabolites that may prevent barcode success. The complicated mixture of organic chemicals often results in PCR inhibition, she added.

Source: BMC Medicine​​
2013, 11​:222 doi:10.1186/1741-7015-11-222
“DNA barcoding detects contamination and substitution in North American herbal products”​
Authors: S.G. Newmaster, M. Grguric, D. Shanmughanandhan, S. Ramalingam, S. Ragupathy