Effects of time of day, gender, and menstrual cycle phase on the human response to a water load

Estrogen and progesterone interference with renal actions of arginine vasop ressin (AVP) has been shown. Thus we hypothesized that women will have a hi gher water turnover than men and that the greatest difference will be durin g the luteal phase of the menstrual cycle. Seven men (32 +/- 3 yr) and six women (33 +/- 2 yr) drank 12 ml water/kg lean body mass on different days a t 0800 and at 2000 following 10 h of fast and a standardized meal at 0600 a nd 1800. Women participated on days 4-11 and 19-25 of the menstrual cycle. Initial urine and plasma osmolalities and urine flow rates were similar in all experiments. The cumulative urine voided over 3 h following the morning drink was less in men (73 +/- 12% of the water load) compared with women i n either the follicular (100 +/- 3%) or luteal phases (102 +/- 10%) of the menstrual cycle. Nighttime values (30-43% of the water load) were lower in all experiments and were not different between sexes or menstrual cycle pha ses. Plasma AVP was higher at night and may contribute to this diurnal resp onse. The data are generally consistent with the stated hypothesis; however , possibly owing to the greatly reduced urine flow in both sexes at night, a difference between sexes was not observed at that time.