The diuretic effects of alcohol and caffeine and total water intake misclassification

Data from the 1994 USDA nationwide survey (CSFII) on 190 non-smoking males (aged 20-29) were used to propose a method for adjusting total water intake for the diuretic effects of caffeine and alcohol, and evaluate the potenti al for related misclassification bias. The data were processed on a per mea l basis. Under the assumption that subjects were in water balance at the st art of the survey day, water losses due to caffeine (1.17 ml/mg caffeine) a nd alcohol (10 ml/g alcohol) were subtracted from crude intake estimates. I f water intake for one meal was inadequate for excretion of the associated osmotic load at 750 mosmol/l, water losses for the subsequent meal were red uced by 32%. Unadjusted and adjusted mean total water intakes differed by 3 21.5 g. Misclassification appeared worst at higher water intakes. Linear re gression models, each with a water intake variable as an independent variab le and body mass index as the outcome, were fit to evaluate the potential f or alcohol- and caffeine-related misclassification bias. Misclassification resulted in large changes (all >10%) in linear regression estimates of effe ct. Future studies of water-disease relationships, especially those intendi ng to compare extremes of total water intake, should consider caffeine- and alcohol-related misclassification bias.