Thirst is a sensation that is best described as the desire to drink. The reason for drinking may not be directly involved with a physiological need for water intake, but it can be prompted by habit, ritual, taste, nutrients, craving for alcohol, caffeine, or other drug in a beverage, or a desire to consume a fluid that will give a warming or cooling sensation. Much of the perception of thirst is a learned or conditioned process, with signals such as dryness of the mouth or throat initiating drinking, whereas a feeling of fullness of the stomach can stop ingestion before a fluid deficit has been restored.
The thirst response is thought to be regulated by neural modulators that operate as a reward mechanism, integrating the effective requirement for water intake with the sensations of taste and pleasantness of the fluid ingested. Thus, when the individual is hypohydrated multiple areas of the brain are activated, promoting the intensity of the thirst sensation. As the water deficit is restored the feeling of thirst diminishes and this subjective sensation correlates well with a reduction in neural activation. However, areas of the brain associated with taste that are activated by water when thirsty remain as active following drinking to satiety when water is ingested.
Although it is true that thirst in man is a poor indicator of acute hydration status and that daily fluid intake is normally in excess of obligatory water loss, the preservation of TBW volume under a variety of environmental and nutritional stresses is remarkably robust and is mainly due to the drive to drink, which the sensation of thirst chronically provokes.
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