Although water is an important nutrient, there are no recommended intake values. Here, water intake, energy intake, physical activity and water loss was measured over 1 week in summer and in winter. Subjects were healthy volunteers, forty-two women and ten men, mean age of 29 (sd 7) years and mean BMI 21·8 (sd 2·2) kg/m2. Water intake was measured with a 7 d food and water record. Physical activity level (PAL) was observed as the ratio of total energy expenditure, as measured with doubly labelled water, to resting energy expenditure as measured in a respiration chamber. Water loss was measured with the deuterium elimination method. Water loss was highly reproducible and ranged from 0·20 to 0·35 l/MJ, independent of season and activity level, with higher values in women. Water loss was related to water and energy intake in summer (r 0·96, P<0·0001 and r 0·68, P<0·001, respectively) as well as in winter (r 0·98, P<0·0001 and r 0·63, P<0·01, respectively). Water loss was, for men, higher in subjects with a higher physical activity in summer (r 0·94, P<0·0001) and in winter (r 0·70, P<0·05). Normalizing water loss for differences in energy expenditure by expressing water loss in litres per MJ resulted in the same value for men in summer and winter. For women, physical activity-adjusted values of water loss were higher, especially in summer. In men, water turnover was determined by energy intake and physical activity, while seasonal effects appeared through energy expenditure. Women showed a higher water turnover that was unrelated to physical activity.