Water is an important nutrient in our diet since without it we cannot survive more than a few days. About 75% of body weight in infants and 55% of body weight in elderly are water. We get water through variety of sources including food and beverages like milk, soups, fruits and juices. Dehydration by all means should be prevented since water is constantly a requirement for metabolism, human performance and overall health. There are many studies on the role water plays in chronic diseases and overall health but not enough studies on determining the water requirements of individuals on daily basis. The US Dietary Recommendations for water are based on the median intakes and not on measurements of hydration status of the US population.
Thirst is a physiological control mechanism to maintain body water and fluid. Our kidneys have an amazing ability to regulate water balance by eliminating excess intake or conserving water by reducing urine excretion. Our kidneys are critical in body’s water regulation and they function better when we drink enough water and take less salt and toxins. In fact, we do not always drink because we are thirsty. We drink in social occasions and for pleasure specially when the water is not in its pure form and is a vehicle to carry alcohol or sugar. This type of drinking is mediated through the taste buds. The fluid consumption among the US adults has been increased in the past decades mainly due to consuming more high calorie beverages not just pure water.
High levels of activity and heat can cause sweating and experiencing more thirst. The cells shrink because water leaves from inside the cells to the extracellular compartments. The sodium ion also is lost through sweat and therefore people prefer drinks that are high in salt. In elderly, this can lead to hypotension, stroke or abnormal fatigue and even death. Elderly population is more at risk of dehydration since they do not experience thirst like younger counterparts. Hypo-hydration can cause loss of electrolytes, increase blood viscosity and increase the body temperature. On the other hand, children are also more at risk of dehydration stress on their body since they have higher ratio of surface area to weight and are susceptible to more heat related diseases.
Even though there is no study supporting that drinking additional water in adequately hydrated individuals is beneficial but when it comes to physical activity and athletic performance especially during endurance exercises like running, extra water intake can prevent fatigue and improve motivation. Exercise especially in hot climates is associated with hyperthermia, reduced stroke volume and cardiac output, hypotension and lower blood flow to muscles. Both children and adults should drink water above their hydration status before starting athletic activities in hot climates.
Among other impacts of dehydration on physical and mental health are delirium among older adults, constipation, stress on kidneys, impaired cognitive performance like on short-term memory, frequency and intensity of headaches, skin health, urinary tract infections and blood pressure. However there is a need for more studies to confirm associations between dehydration and other chronic diseases like bladder or colon cancer or neurological disorders. There is also no study to confirm that drinking water alone is more beneficial than water containing other components like sweeteners and or caffeine. On the other hand, studies have shown that when people drink pure water, they generally have lower caloric intake than those who drink high calorie drinks.
Given the huge variability of water requirements based on variability in metabolism, weather conditions and physical activity, an adequate intake (AI) is established for the US population. According to the Institute of medicine, an adequate intake of water for men is roughly 3 liters about 13 cups and for women is 2.2 liters about 9 cups. The recommendation of eight 8 oz-glasses of water is roughly 1.9 liters which is a close approximation.