In this large prospective cohort study, we found that 4-year changes in alcohol consumption assessed repeatedly over time were followed by subsequent changes in risk of type 2 diabetes. The lower risk associated with an increase in alcohol consumption was dependent on initial drinking levels, with no benefit associated with increased intake among men already drinking moderately. This pattern of lower risk associated with increased alcohol consumption solely among abstainers and light drinkers was further supported by associations of change in alcohol intake with total adiponectin and A1C.
Our results extend previous epidemiologic studies that have reported an inverse association between moderate alcohol consumption and the longer term risk of type 2 diabetes (1
). Recent studies have shown that alcohol consumption is associated with lower risk of type 2 diabetes even among low-risk individuals (lean, active nonsmokers) (25
) and when adjusted for multiple lifestyle factors based on BMI, physical activity level, smoking habits, and diet quality (27
). Comparisons of different beverage types generally suggest that ethanol rather than the type of alcoholic beverage is responsible for this association (23
). Furthermore, variation in the ADH1C
gene, a gene that encodes the alcohol dehydrogenase 1C enzyme that oxidizes ethanol, appears to modify the association between alcohol consumption and type 2 diabetes risk, providing further epidemiologic support for the causal nature of the relationship between alcohol consumption and diabetes risk (31
The plausibility of these observational results is supported by short-term randomized controlled trials on changes in alcohol consumption (25–30 g/day) (10
). In these studies, moderate drinking significantly improved insulin sensitivity after 6 to 8 weeks. Also, clinical trials in a variety of populations have shown that alcohol consumption increases adiponectin (11
), a hormone secreted by adipose tissue that appears to improve insulin sensitivity. Indeed, adiponectin appears to explain ~25 to 30% of the inverse association between alcohol consumption and type 2 diabetes in women (34
) Finally, longer-term randomized trials of 3 to 12 months among diabetic individuals have shown that assignment to alcohol consumption lowers fasting glucose (35
) and A1C (36
These findings from randomized trials suggest that the effects of alcohol intake on glycemia may have a short latency, as they appear within weeks of assignment to alcohol. Our results are consistent with this finding, as the more beneficial metabolic parameters and the lower subsequent risk of diabetes associated with an increase in alcohol consumption were observed in the next follow-up period. Our results further imply that the effect may be transient, as a decrease in consumption was accompanied by a modest increase in risk. Finally, our results highlight that any benefit of alcohol on glycemia and risk of diabetes is restricted to moderate drinking, and increases among those already drinking moderately confer no lower risk.
Several limitations warrant consideration. We relied on self-reported alcohol consumption. However, validation studies in these health professionals comparing self-administered questionnaire with intake assessed by detailed diet records showed correlations above 0.8 and mean and SD values almost identical by the two methods (18
). Second, we do not know why men changed their intake. However, we restricted our analysis to men with no history of diabetes and cancer and adjusted for cardiovascular disease, hypertension, and hypercholesterolemia. Third, we do not know when during each 4-year interval the change in alcohol consumption occurred, a limitation that reflects the fact that the administered FFQ specifically queries alcohol consumption in the previous year. Therefore, we cannot definitely evaluate whether the change in intake on type 2 diabetes risk is immediate. We do know, however, that the change in alcohol preceded the diagnosis of diabetes. Fourth, we performed our analysis in male health professionals, and results may therefore not be readily generalizable to other populations. However, within this homogenous group of highly educated men, potential confounding because of social economic status is substantially reduced. Fifth, we could not evaluate changes in beverage types, given the more limited use of any particular beverage compared with total alcohol use. We previously found that all beverage types were inversely associated with type 2 diabetes in this cohort (23
). Finally, as with any observational study, unaccounted factors associated with changes in alcohol consumption and risk of type 2 diabetes may introduce an unknown degree of residual confounding, despite the substantial number of potentially confounding factors we included.
In conclusion, in this cohort of male health professionals, increases in alcohol consumption over time were associated with correspondingly lower 4-year risk of type 2 diabetes, although this association was limited to rare and light drinkers at baseline. This suggests that the effect of alcohol consumption on diabetes risk may have a relatively short latency time but may also be transient and reversible. Furthermore, individuals who already consume alcohol in moderation may not further benefit from increased consumption. Although these results may suggest that some individuals should consider adopting regular and moderate intake of alcohol, our findings—even if proven to be causal—are limited to a single outcome of diabetes. Decisions and recommendations about changes in alcohol consumption should, as with alcohol consumption in general, consider the full range of risks and benefits to an individual, including the consistent harms to the individual and society of drinking that exceeds recommended limits.