Limitations, including the misclassification of former drinkers

Roizen, Fillmore, Chikritzhs, and Stockwell (2013) provide a clear summary of the systematic review literature relating to the relationship between consumption of alcohol and later dementia or cognitive impairment. They focus on one of the limitations present in the studies to date in this area, the issue of former drinkers, i.e. those who have a history of drinking but who do not fall into the current drinker categories at the time of study data collection. The authors highlight the level and lack of overlap between the systematic reviews and in doing so, remind us that each such review will include only those papers that meet the authors’ inclusion and exclusion criteria and that any subsequent meta-analysis is likely to include still fewer studies. This does highlight the need for the reader to be aware of the respective research questions when critically appraising systematic reviews. In this case the key issue discussed is the risk of misclassifying former drinkers as non-drinkers. Assuming that former drinkers are at increased risk, this in turn may then potentially increase the chance of seeing a J-shaped relationship with both the non-drinker and heavy drinker categories at an increased risk of dementia compared to the light drinkers. This debate is not new and has arisen within the alcohol and cardiovascular and mortality outcomes literature, as discussed previously by Filmore, Kerr, Stockwell, Chikritzhs, and Bostrom (2006) and Klatsky (2007), amongst others. In actual fact, the risk of misclassification is much wider than this. Not only is there a risk that the non-drinker category may include former drinkers, it may also include a mixture of other factors for which full adjustment may not be in place or possible (Lee et al., 2009), for example, differing cultural groups, different levels of health status and be subject to cohort effects for example, older women may have been less likely to drink than their male counterparts. Former drinkers themselves may also include those who may or may not have been heavy drinkers in earlier life. In fact, as pointed out by Roizen et al., each category of consumption may overinclude or exclude individuals. Different studies also choose differing cut off points to classify light/moderate and heavy drinking. Misclassification, however, is also just one of a number of limitations in this area. As a randomised clinical trial in this area would be extremely difficult and ethically impossible, we must rely on observational data coming from a wide range of studies usually with a primary focus that is neither alcohol consumption nor dementia. Further issues include the classification of dementia outcomes with different studies using different screening and diagnostic tools at different times, different age groups, lengths of follow up, type and pattern of alcohol consumption, for example, heavy consumption of beer and spirits at the weekend versus moderate wine consumption throughout the week (Marques-Videl et al., 2000), the role of alcohol in the wider diet, influence of co-morbid disease and type of data collection, for example accuracy of self-report when eliciting historical consumption data. As highlighted by Roizen et al. selection of a clear reference group is neither straightforward, nor is taking account of change over time or taking full account of all limitations when examining the current literature. Future research needs to start to separate out some of these areas taking a primary focus on risk factors for dementia with sufficient participants to allow exploration of subgroups and the possibility of using alcohol consumption as a continuous variable. There may need to be learning from the addiction literature and wider clinical trials and epidemiological literature regarding the classification of alcohol consumption, the impact of other risk factors, intensity and duration of exposure and the need for a life course approach taking account of time to develop dementia. This is particularly important given the ageing population and the possibility that the role of risk factors may differ in older adults. Certainly, there has been some suggestion that the J-shaped relationship may be more likely in older adults whereas younger groups may show a more linear relationship with higher consumption associated with increased risk (Klatsky, 2007; Lobo et al., 2010; Weyerer et al., 2011). Whether this is due to something other than the greater potential for misclassification of former drinkers in older groups is unclear. Even so, despite all the caveats associated with research in this area, there is some physiological evidence that gives tentative support to the possibility of a J-shape relationship between alcohol intake and dementia (Bau, Bau, Rosito, Manfroi, & Fuchs, 2007) and it clearly behoves us to carry out further robust cohort studies in this area before we can conclude that light drinking is beneficial or that less is better with