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In a cross-sectional study of 4558 Australians, it was found that the proportion of subjects reporting indigestion, palpitations, tremor, headache and insomnia increased significantly with mean caffeine intake. A multiple logistic regression model was used to show that the association between the prevalence of these symptoms and usual daily caffeine consumption remained significant in both males and females for palpitations, tremor, headache and insomnia after controlling for the potential confounding factors of age, adiposity, smoking, alcohol intake and occupation. Adiposity was strongly correlated with the prevalence of indigestion and the apparent association between caffeine and indigestion disappeared when adiposity was controlled for. According to the logistic model, the relative risk of experiencing symptoms for people consuming 240 mg of caffeine (approximately 4-5 cups of coffee or tea) per day (the population average) compared with caffeine abstainers is 1.6 for palpitations, 1.3 for tremor, 1.3 for headache, and 1.4 for insomnia in males and 1.7, 1.5, 1.2 and 1.4 respectively for females. Further logistic regression analysis indicated that the associations found between caffeine intake and symptoms did not depend on the source of caffeine. In general, coffee consumption has no significant effect over and above that attributable to its caffeine content. If these associations are causal, then approximately one quarter of the reported prevalence of palpitations, tremor, headache and insomnia is attributable to caffeine consumption in this study population.
Shirlow et al. (Tue,) studied this question.