Among women with type 2 diabetes, consumption of at most 1 alcoholic drink per day compared with abstention was related to better initial general cognition, but not with reduced rates of cognitive decline over four years of follow-up. Additionally, very long-term moderate intake of alcohol before and after diabetes diagnosis was positively associated with both general cognition and verbal memory at the initial interview, but there was no association with cognitive decline. Thus, there was not a clear and consistent relation between alcohol intake and cognition.
Limitations of our study should be considered. First, our findings may reflect a true null association between moderate alcohol consumption and change in cognitive function. However, it is also possible that a relation with cognitive decline may not be evident within the relatively short follow-up period (of four years) for measuring cognitive decline. This may be especially pertinent considering the fairly small sample size in this subgroup of women with type 2 diabetes, limiting our power to detect the modest changes in cognition which are generally observed in non-demented populations. In addition, information on alcohol consumption was self-reported. However, a previous validation study demonstrated high validity and reliability of self-reported alcohol intake [16
]. Furthermore, our measure of alcohol consumption is correlated with serum high density lipoprotein levels [16
] and has been associated with risks of several chronic diseases, including cognitive function and cognitive decline in the entire cohort [4
]. To address potential confounding, we conducted our study within a relatively homogeneous cohort of health professionals and performed multivariable-adjusted analyses. Also, results of age- and education-adjusted analyses were nearly identical to results of fully-adjusted analyses, suggesting that residual confounding is unlikely to explain most of the observed associations. However, the possibility of residual confounding cannot be ruled out in an observational study. Finally, our findings may not be generalizable to broader populations of adults with type 2 diabetes, including men and minorities. In addition, our participants were less likely to be regular users of insulin or oral hypoglycemic medication than adults in the general population with type 2 diabetes [19
]. Thus, our findings may not be applicable to adults with more severe or less controlled type 2 diabetes.
Among the few existing studies that have examined alcohol intake and cognitive function specifically among adults with type 2 diabetes, study populations were restricted to men and participants' alcohol intakes were measured at one time, limiting comparisons with our analyses of long-term moderate alcohol intake in women. Nonetheless, in a cross-sectional study of 119 men with type 2 diabetes, Fan et al. found that overall performance on a battery of 5 cognitive tests was significantly better in moderate alcohol drinkers, defined as those usually consuming at least 1 drink per month to no more than 2 drinks per day, compared with abstainers, after adjusting for potential confounding factors (p=0.02), similar to our cross-sectional results [20
]. Also, in a cross-sectional analysis of 187 men with cardiovascular disease or type 2 diabetes, Launer et al. reported significant decreased risks of poor cognitive function (Mini-Mental State Examination score <26) among men who drank <1 (OR 0.3, 95% CI 0.2-0.7) or 1-2 drinks per day (OR 0.2, 95% CI 0.1-0.4) compared with non-drinkers [21
]. In the same study, there was no association between alcohol intake and change in cognitive function over 3 years of follow-up. Similar to our study, power to assess a relation with cognitive decline was likely limited given the very small sample size and short follow-up period. Overall, although these findings and ours are suggestive of a cross-sectional relation between moderate drinking after diagnosis with type 2 diabetes and better cognitive function, null results from longitudinal analyses do not support a potential role for moderate alcohol consumption in preserving cognitive function in adults with type 2 diabetes. However, clearly, additional data on associations with changes in cognitive function over time are needed from studies with long follow-up and larger samples.
Several lines of evidence support our finding of a relation between moderate alcohol intake and initial cognition. For example, among adults with type 2 diabetes, moderate alcohol consumption has been associated with decreased atherosclerotic progression [22
], decreased levels of markers of inflammation and endothelial dysfunction [23
], and decreased risk of coronary heart disease [7
]. These findings suggest that moderate alcohol may help preserve vascular health in type 2 diabetes, and thus might also preserve cognitive health. More directly, several studies have reported an association between moderate alcohol consumption and improved insulin sensitivity [5
]. Maintenance of normal insulin levels in the brain may help prevent intraneuronal accumulation of amyloid beta and the formation of neurofibrillary tangles, which have been implicated in Alzheimer's disease development [25
]. Together, these data support the plausibility of an intervening role of moderate alcohol consumption on the pathway between type 2 diabetes and cognitive impairment; however, as noted above, more longitudinal data are needed from epidemiologic studies.
In conclusion, considering the inconsistencies of our findings in analyses of initial cognitive function and change in cognitive function over time, our results do not clearly support a relation between moderate alcohol consumption after diagnosis with type 2 diabetes and cognition. However, further large-scale, longitudinal studies with long follow-up are needed to assess the association between moderate alcohol consumption and cognitive decline in adults with type 2 diabetes to better understand relations.