Submitted to: American Journal of Epidemiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2009
Publication Date: 5/20/2009
Citation: Imamura, F., Lichtenstein, A.H., Dallal, G.E., Meigs, J.B., Jacques, P.F. 2009. Confounding by dietary patterns of the inverse association between alcohol consumption and type 2 diabetes risk. American Journal of Epidemiology. 170(1):37-45. Interpretive Summary: Habitual intake of alcohol is associated with lower incidence of type 2 diabetes mellitus. However, it remains unclear whether the observed association is fully attributable to alcohol intake itself or is a consequence of confounding by differences in diet between those who do and do not consume alcohol. Confounding by diet is conventionally dealt with by statistical adjustment for a limited number of specific dietary factors, be it either nutrients or foods. Selection of these dietary factors is often subjective and may not adequately capture the differences in overall dietary patterns between drinkers and non-drinkers. If differences in diet are not fully controlled, then it is not possible to determine if the observed association between alcohol and diabetes is independent of alcohol-associated dietary patterns. To our knowledge, no study relating alcohol and risk of type 2 diabetes mellitus has addressed the issue of confounding by dietary patterns. We used the dietary patterns approach to adjust for potential confounding of overall diet on the association between alcohol consumption and 7-year risk of type 2 diabetes mellitus in 2,879 healthy adults enrolled in Framingham Offspring Study. The object was to determine whether the association was attributable to alcohol itself or dependent on dietary patterns related to alcohol consumption. After adjusting for standard risk factors, consumers of greater than or equal to 9.0 drinks per week had a 63% lower risk of type 2 diabetes mellitus compared to abstainers Adjustment for selected nutrients had little effect on risk, whereas adjustment for dietary patterns significantly shifted risk lower by 40%. Therefore, the observed inverse association was confounded by dietary patterns, not adjustable by individual nutrients. These data suggests alcohol intake, and not dietary patterns associated with alcohol intake, is responsible for the observed inverse association with type 2 diabetes mellitus risk.
Technical Abstract: Epidemiology of dietary components and disease risk limits interpretability due to potential residual confounding by correlated dietary components. Dietary pattern analyses by factor analysis or partial least squares may overcome this limitation. To examine confounding by dietary pattern as well as standard risk factors and selected nutrients, the longitudinal association between alcohol consumption and 7-year risk of type 2 diabetes mellitus (T2DM) in 2,879 healthy adults enrolled in Framingham Offspring Study was modeled by Cox-proportional hazard models. After adjusting for standard risk factors, consumers of >/= 9.0 drinks per week had a significantly lower risk of T2DM compared to abstainers (hazard ratio [HR]: 0.47; 95% confidence interval [CI]: 0.27, 0.81). Adjustment for selected nutrients had little effect on the HR (=0.53), whereas adjustment for dietary pattern variables by factor analysis significantly shifted the HR away from null (=0.33, 95% CI: 0.17, 0.64) by 40.0% (95% CI: 16.8, 36.4%, P=0.002). Dietary pattern variables by partial least squares showed similar results. Therefore, the observed inverse association, consistent with past studies, was confounded by dietary patterns, not adjustable by individual nutrients. These data suggests alcohol intake, and not dietary patterns associated with alcohol intake, is responsible for the observed inverse association with T2DM risk.