Ask most people how much alcohol they drank last week, month or year and chances are they’ll have trouble giving you an exact answer – or possibly would prefer not to. Keeping track of consumption is notoriously tricky, drinking habits change over the course of a lifetime, and notions of what constitutes a standard drink vary wildly. These factors pose significant challenges for researchers wanting to measure and assess the effects of alcohol on people’s health and wellbeing, as Dr Sue Brennan and colleagues from Cochrane Australia highlight in their recent review on the long-term effects of alcohol consumption on cognitive function.
‘This review was commissioned by the National Health and Medical Research Council (NHMRC) to inform the latest update of the Australian Guidelines to Reduce Health Risks from Drinking Alcohol,’ Sue explains. ‘The revision of the 2009 guidelines involved a comprehensive analysis of many studies, systematic reviews and mathematical modelling on the health effects of alcohol on different groups and conditions. For example, colleagues here at Cochrane Australia and at SAHMRI collaborated on a review on alcohol consumption during pregnancy and breastfeeding health outcomes. Other teams of researchers looked at alcohol and mental health, and modelled mortality and morbidity risks. Our latest review is a small piece in this much larger puzzle, with a specific focus on the cumulative effects of lower levels of alcohol exposure on cognitive function over time.’
‘Our primary aim was to determine if there was a dose-response relationship between levels of alcohol consumption and long-term cognitive effects. These effects may be most evident later in life, but there’s also a need to look at the potential for long-term effects on cognition to arise from drinking alcohol earlier in life, up to the age of 25. This is because of concerns that exposure to alcohol during this period of brain development may bring an increased risk of cognitive impairment. Unfortunately we didn’t find any studies with the kind of data we really need for a detailed analysis of the cognitive effects of alcohol consumption among young people.’
Sue and her team read the full text of over 200 studies, and included 27 in the review. While many of the cohort studies and randomized trials have run for decades and provide a long-term view, only a small proportion were set up to investigate questions about alcohol specifically. Most included alcohol consumption among a series of unrelated questions, and use different measurements and methods across countries and time periods. This makes analysis a very complex undertaking.
‘We found the reporting of key elements of study design was generally problematic, with many studies reporting ambiguous information about the timing of data collection for alcohol exposure and cognition outcomes – or omitting it altogether,’ Sue says. ‘This is one of the many challenges with alcohol-related research. Studies might measure consumption at a certain point in time and ask participants to talk about their average behaviour over a month or a year. But one of the things that we know happens is that if people get sick, they stop drinking. Those people are then counted as non-drinkers because 10 years or 20 years earlier, they stopped drinking and the investigators didn't really measure their drinking patterns prior to that.’
‘Then there’s the question around what’s considered a ‘standard drink’ and how this is defined. In some countries, a standard drink has 8 grams of alcohol, in others it has 14 grams. Different studies use different standards or they might just say ‘one drink a day’ and not specify a type or quantity – was it a small glass of wine, a pint of beer, a large shot of tequila? The unit of measurements and methods are often not clear or consistent.’
'Recognising this, we've done a very careful assessment of individual study methods to try to tease out what the problems were in terms of alcohol measurement and whether or not those results were likely to be overestimating any potential benefit of having alcohol. I think that’s what makes this review quite different from previous research in this area. We looked very closely at study methods - probably in a way that hasn't been done before - with the help of new tools developed within Cochrane that really enable us to analyse study methods and the extent to which the results were likely to be accurate.’
‘Ultimately our overall findings need to be understood in the context of the layers of complexity inherent in this field. Overall, we found there’s currently very low certainty evidence that shows little difference in cognition at levels of alcohol consumption at or below those indicated as lower risk for women and men in the 2009 Australian Guidelines (i.e. two standard drinks per day). At high levels of alcohol consumption, our results indicated detrimental effects on cognition. Importantly, this uncertainty and the evidence gaps we highlight illustrate that if we’re to find clearer answers to questions about the effects of alcohol over time, it’s got to happen through designing the original studies very differently.’
Published: 23 June 2020