European isolation and confinement study. Water and salt turnover.

Intake and output of water were studied in six male subjects from six European countries during 28 days of isolation and confinement in order to assess whether the observed reactions can be compared with those observed during space travel. On the average, the subjects drank 17.5 ml/kg/day fluids. An additional 25 ml/kg/day was recruited from food intake and metabolism. The lowest fluid intake of 11.3 and 12.1 ml/kg/day was shown by two subjects who concurrently lost 3 to 4% of body weight. Three subjects maintained body weight, and one subject gained. A linear correlation existed between the total water output and the fluid intake by drinking. The time series of fluid intake, urine output, and sodium excretion revealed a weekly pattern for these parameters, characterized by low values on Sundays and high values on Fridays. This pattern was most apparent for sodium excretion. Low water turnover rates were seen in the first week of isolation; in the last week the values were above average. In conclusion, it can be said that with respect to the water balance, strong individual differences and time dependent variations (day to day, week to week) of the respective parameters have to be taken into account for the data analysis during such long-term studies.