Cochrane promotes itself as ‘an international, non-profit, independent organization, established to ensure that up-to-date, accurate information about the effects of healthcare interventions is readily available worldwide’. Cochrane reviews are considered to be the most reliable source of scientific evidence there is worldwide. I used to think this is true but now my head is full of doubt, and after reading this post yours might be, too.
Last spring, a Cochrane review by Riley et al. (2015) ended up drawing a lot of confusion among laypersons, and especially oral health professionals and members of scientific communities. Based on the results of the review, the authors stated that ‘There was insufficient evidence, from single studies (mostly with small sample sizes), to determine a difference in caries between the following groups, and thus the uncertainty associated with the effect estimates resulted in them being compatible with both a reduction and an increase in caries associated with xylitol’.
In Finland, researchers and experts in the field of dentistry were very surprised by the results of the review and quite soon a statement by docents Eva Söderling and Kaisu Pienihäkkinen from University of Turku (Finland) was released in both the journal and website of the Finnish Dental Journal (text only available in Finnish, I’m afraid). In their statement, Söderling and Pienihäkkinen, both renowned researchers with decades of experience in xylitol research, criticize a number of things about the review. Due to the lack of an English version of the statement, I’m explaining the main points of the response below to save you from crappy online translators leaving you with nothing but just more questions.
First of all, Söderling and Pienihäkkinen question the fact that only ten studies were regarded as eligible for the review when there are more than five hundred published studies on the effects of xylitol. According to the docents, the review authors had also ignored the fact that the effects of xylitol depend highly on the dosage and frequency of the xylitol intake per day. The duo reminds that xylitol is not an antimicrobial compound and that it appears to have plaque-reducing and caries preventing effects only when the minimum daily intake is five grams. They point out that in half out of the ten selected studies included in the review, the given dosage of xylitol was less than five grams. Söderling and Pienihäkkinen state that the Costa Rica study that is included in the review, is very hard to access and that the study does not even reveal the dosage of xylitol that was given to the test subjects. According to the docents, the Lycksele study included in the review was conducted on small children and the dosage of xylitol was tiny. The inclusion of a probiotics study by Teemu Taipale also raises questions, for in his study, xylitol was used for a few hundred milligrams merely as a bulk agent in probiotic tablets given to study subjects.
As for the rest of the five studies, one of them was conducted on children with excellent state of oral health and thus the results of the xylitol intake were impossible to prove even in theory. All in all, the five studies were not even comparable to one another as the form of xylitol varied from pastilles and syrup to somekinds of ‘wipes’, and the age of the study subjects varied from children of different age to adults. In addition, the other one of the two xylitol pastille studies included in the review was regarded as having a ‘high overall risk of bias’. Söderling and Pienihäkkinen remind that there are dozens of studies on xylitol chewing gums that have been accepted as the basis of recommendations of EFSA and the International Association of Paediatric Dentistry. The docents do admit that there is still need for new, carefully designed studies examining the effects of xylitol with high enough intake, and this is about the only thing that both the review authors and the docents seem to agree on. In the final words of their response, Söderling and Pienihäkkinen state that they cannot but wonder about the intentions of the authors. After all, they conclude, news headlines are based on a half a page summary and few people pay closer attention to the matter at hand and assess the report itself.
Interested in learning more about these mysterious xylitol studies? Here’s one list of xylitol studies mainly conducted by Finnish researchers but you might as well type ‘xylitol’ in Google Scholar or any other scientific database search engine (while writing this post I tested Google Scholar and got 69,500 hits). As an oral health professional and an academic I can only note that all that glitters is not gold – and that applies to Cochrane reviews as well.
Still believe that Cochrane reviews automatically equal truth and nothing but the truth?