Water balance throughout the adult life span in a German population.

Mild dehydration, defined as a 1-2% loss in body mass caused by fluid deficit, is associated with risks of functional impairments and chronic diseases. Whether water requirements change with increasing age remains unclear. Therefore, the aim of the present investigation is to quantify hydration status and its complex determining factors from young to old adulthood to analyse age-related alterations and to provide a reliable database for the derivation of dietary recommendations. Urine samples collected over a 24 h period and dietary records from 1528 German adults (18-88 years; sub-sample of the first National Food Consumption Survey) were used to calculate water intake (beverages, food and metabolic water) and water excretion parameters (non-renal water losses (NRWL), urine volume, obligatory urine volume) and to estimate hydration status (free-water-reserve) and 'adequate intake (AI)'. Median total water intake (2483 and 2054ml/d, for men and women, respectively (P<0·0001)), decreased with increasing age only in males (P=0·001). Obligatory urine volume increased in both sexes (P<0·0001) due to decreased renal concentration capacity. The latter was balanced by a decrease of NRWL (P<0·05), leaving the free-water-reserve and therefore hydration status almost unchanged. Calculated 'AI' of total water was the same for young (18-24 years) and elderly (≥65 years) adults (2910 and 2265ml/d, for men and women, respectively). The present study is the first population-based examination showing that total water requirements do not change with age although ageing affects several parameters of water metabolism. Reduced sweat loss with increasing age appears to be primarily responsible for this observation.