Focus on Alternative and Complementary Therapies
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Focus Alternat Complement Ther©2005 Pharmaceutical Press
Focus Altern Complement Ther 2004; 9: 184–5
I recently embarked on a systematic review of ginger for osteoarthritis and found that the first three RCTs I looked at had each tested a different extract. I do not mean that they tested different standardised extracts from the same species produced by different manufacturers. The extracts were actually from different genera. The first trial1 tested Zintona, an extract of Zingiber officinale, the species that has been subjected to most research and probably what most of us interested in herbal medicine think of as ginger. The second trial2 tested Eurovita Ext 77, a combination extract of two ginger species, Z. officinale and Alpinia galanga, Siamese ginger or greater galangal, a herb with ginger-like rhizomes and a history of traditional medicinal use. The third trial3 tested Eurovita Ext 33 from a herb identified only as ‘Chinese ginger’. Chinese ginger usually refers to Alpinia officinarum, or lesser galangal, another species with ginger-like roots that could be mistaken for Z. officinale. It is therefore clear that any search for articles relating to ‘ginger’ should at least include a search for the two genera Zingiber and Alpinia. But why stop there? The order of monocotyledenous plants Zingiberales contains eight plant families, 89 genera and about 1800 species, many of which have putative medicinal properties. The ‘ginger’ family Zingiberaceae contains 47 genera, including Zingiber and Alpinia, and some 1000 species. At least eight genera within the family have a traditional medicinal application. These are Zingiber, Alpinia, Afromomum, Amomum, Boesenbergia, Curcuma, Elettaria and Kaempferia. The genus Zingiber contains nine distinct species and the genus Alpinia contains 32 species4. It seemed worthwhile therefore to take a little time to investigate just what ‘ginger’ refers to in medical databases.
In December 2003 PubMed contained 432 articles retrievable with the search term ‘ginger’. Not one of these articles used the genus name Zingiber as a keyword and searching the database with the genus name Zingiber retrieved only 154 articles. Keywords frequently included either the order name Zingiberales or the family name Zingiberaceae but as already established above, neither of these terms is more useful than the folk name ‘ginger’ in narrowing the possible number of species referred to.
Many of the articles retrieved with the search term ‘ginger’ actually referred to species from the genus Alpinia and, less frequently, Curcuma, Asarum, Rengalmia, Renealmia or even Panax, and included both preclinical and clinical studies relevant to potential medicinal applications similar to those for Z. officinale. Unlike Zingiber, the generic names Alpinia and Curcuma were frequently included as keywords. The species could not be identified from the keywords for any article for any of the three genera. In contrast, in articles about the species Ginkgo biloba, where taxonomic confusion is highly unlikely, the full specific name often appears as a keyword. These inconsistencies defy logic and reflect a generally careless attitude to the identification of plant species used in medical research.
A combination of the three search terms ‘ginger’, Zingiber and Alpinia identified 591 articles, of which 42 were irrelevant because the term ‘ginger’ referred to something other than a plant species. Of the remaining 549 articles, the species name could be identified from the title in only 164 (30%), from the abstract in only 221 (40%) and from either the title or abstract in only 265 (48%). In 141 (26%) articles the species could be identified as Z. officinale without obtaining the full article but in a further 267 articles the plant could be identified only as ‘ginger’, which as we have seen is a term less useful even than ‘ginseng’. In 110 articles the species in question belonged to the genus Alpinia.
By coincidence 141 articles of the 549 could also be classified as clinical rather than preclinical. The former included case reports, reviews of efficacy and safety, clinical trials, clinical advice and comments on the clinical application of ‘ginger’. In only 24 (17%) of these 141 clinical articles could the species name be determined from the PubMed entry. In contrast, the species name could be identified for 241 (59%) of the 408 preclinical studies, appearing to suggest, bizarrely, that precise identification of the plant species appears to be less important when the plant is ingested by humans than when studied in animal or in vitro systems. The search included six systematic reviews and for these the species name could be determined from the PubMed entry for only one.5 Of the 40 clinical trials the species could be identified from the PubMed entry in only 16 (40%).
Of course, for many of these articles the species may be identified in the main body of the text but ascertaining this would require a considerable input of time. I did, however, try to retrieve all of the outstanding clinical trials in which the species could not be identified from the PubMed entry. Of the 40 clinical trials, 23 (57.5%) identified the species as Z. officinale, although in many cases this could still only be inferred from references to the species in the introduction or discussion rather than through a positive identification in the methods section. Worryingly, in one of these,6 the test capsules were prepared from dried ginger rhizomes purchased at the local market. Two were clinical trials of Alpinia speciosa,7,8 two were trials of zingerone extracted from ‘ginger’,9,10 one was the mysterious Ext 33 (possibly A. officinarum)3 and another was the combination product Ext 77.2 Six trials (15%) identified the source of the extract throughout the paper only as ginger311–15 and I was unable to retrieve another four articles (10%) by the time of publication. The remaining trial,16 a trial of ‘ginger’ for nausea and vomiting in pregnancy, discusses Z. officinale in the introduction but in order to qualify for the experimental group, subjects actually ingested ‘capsules, ginger tea, fresh ginger, pickled ginger, ginger cookies, ginger candy, inhaled powdered ginger, ginger crystals, and sugared ginger.’ This hardly amounts to a reliable species identification.
While the great majority of clinical trials and clinical reports still do appear to refer to the species Z. officinale, this is not always so and certainly is not the case for preclinical studies where ginger frequently refers to species from other genera. Readers should therefore exercise particular care in attributing results from in vitro or animal studies about the biological properties of ‘ginger’ extracts to Z. officinale. Any article, clinical or preclinical, that does not actively identify the species with a full taxonomic name should be treated with great caution. It would also make sense for authors to include full species names in keywords, titles and abstracts.
