The amount of solid food eaten by humans in spontaneously ingested bouts is the most important determinant of the amount and timing of fluid ingestion. In order to investigate whether this relationship occured as a result of the osmotic and volumetric effects of the ingested nutrients, analyses were performed on the data obtained from 219 adult humans. They were paid to maintain diaries for 7 days of everything they ingested, the timing and conditions present at the bout, and pre- and postbout self-ratings of subjective thirst. Carbohydrate and protein intake were found to be the dietary constituents that were most highly related to fluid intake and subjective thrist while sodium and fat were found to be either not at all or only weakly related. Carbohydrate and protein intake were found to be positively related to the amount ingested of total fluid, fluid in excess of digestive requirements, and fluid in the form of "drinks," for the amounts ingested in individual bouts, over the entire day, and over the entire week. In addition, carbohydrate and protein intakes were found to be positively related to the reduction in the subjective state of thirst, while negatively related to the level of thirst self-reported at the end of the bout. The results indicate that fluid intake and subjective thirst are influenced by the repleting characteristics of ingested nutrients and not by their depleting effects, suggesting that fluid intake occurs in response to and as an adjunct of food intake, not fluid homeostasis.
