A recent study indicates that without water, the body's physiology causes a person to involuntarily reduce what he or she eats. One way water deprivation does this is by causing dry mouth, making eating more difficult. But evidence indicates too that there are receptors in the gastrointestinal tract that can detect the water level there; when water in the gut is low, the receptors respond by terminating eating earlier than normal. Once the water returns to a normal level, a body will resume eating as normal.
This physiological response to water deprivation doesn't just affect humans. Many animals, including rats, dogs, cows, and camels, reduce their food intake during water deprivation. The reduction in eating serves the purpose of allowing the body to absorb what little water is in the gut. And with less food to digest, less water is lost through urine.
This is a topic with significant relevance in the world today because a number of African countries now face water scarcity, and water scarcity is a problem that is spreading. Though these countries' food supplies may be low too, these findings suggest that the response of international relief agencies to send food to these people may not actually benefit them as much without an adequate water supply as well.