The effect of xanthines on fluid balance.

To determine whether drinks containing caffeine and theophylline cause more fluid loss than equivalent amounts of non-xanthine drinks, a convenience sample of 30 adults between the ages of 57 and 81 was selected. Subjects were picked for 2 groups: those who had developed tolerance to these xanthines and used them freely (the X Group) and those who habitually omitted drinks containing xanthines from their diets (the NO X Group). Both groups consumed approximately 2 liters of fluid daily for 5 days. When overall means were compared by analysis of variance, the two groups did not differ on serum osmolality for Day 5, change in serum osmolality (from Day 0 to Day 5), adjusted I/O ratio, percent weight change from Day 0, or percent weight change from previous day. Analysis of the same variables on a daily basis indicated that the X group was better hydrated on Day 4.