A microregulatory analysis of spontaneous fluid intake by humans: evidence that the amount of liquid ingested and its timing is mainly governed by feeding.

The characteristics of fluid intake in humans were investigated using a diary self-report method. Thirty-six adult humans were paid to record in a diary, for 7 consecutive days, everything that they either ate or drank, the time that they ingested it, and how thirsty and hungry they were on seven point scales. The diary entries were encoded and entered into a computer. Draughts were identified according to five different bout definitions and three different definitions of fluid amount; total fluid ingested in both solids and liquids, excess fluid ingested above digestive requirements, and total fluid ingested in "drinks." The fluid and caloric compositions of the bouts, the estimated stomach contents at the beginning and end of the bouts, and prebout and postbout intervals were calculated. These variables were then interrcorrelated with univariate and multivariate techniques. Self-rated thirst and hunger were found to be equivalent in magnitude at the beginning of the draughts but self-rated hunger was more closely associated with the prebout interval and stomach contents of food and water than was self-rated thirst. Subjective thirst was found to be negatively related to the amount in the stomach regardless of composition. The amount of fluid ingested, regardless of its definition, was found to be primarily related to the amount of food ingested in the bout, not to the estimated prebout stomach contents or the prebout interval, and only slightly with self-rated thirst. "Drinks" which occurred independent of eating were relatively rare but were strongly correlated with the degree of subjective thirst. The amount of time that would elapse before the subsequent draught, the postbout interval, was related to the amount of food ingested in the bout and not to the amount of liquid ingested regardless of definition. It was concluded that the spontaneous intake of fluid by humans, under ad lib conditions, occurs in excess of requirements, is principly determined in amount and timing by eating, and water balance is left to regulation by the kidneys.