Water is essential for life and maintaining optimal levels of hydration is important for humans to function well. Water makes up a large proportion of our body weight (60% on average), distributed between the intracellular (inside cells) and extracellular (water in the blood and in between cells) compartments. Water is the major component of body fluids, such as blood, synovial fluid (fluid in the joints), saliva and urine, which perform vital functions in the body. The concentration of solutes (osmolality) in body fluids is closely controlled, and even very small changes in osmolality trigger a physiological response; either to increase body water by reducing urinary output and stimulating thirst; or to excrete excess water as urine. Generally, body water is maintained within narrow limits. However, if water losses are not sufficiently replaced, dehydration occurs. Extreme dehydration is very serious and can be fatal. More mild dehydration (about 2% loss of body weight) can result in headaches, fatigue and reduced physical and mental performance. It is also possible to consume too much water and in rare cases this can result in hyponatraemia (low levels of sodium in the blood).
We can get water from almost all drinks and from some foods in the diet. Food provides about 20% on average and this could vary widely depending on the types of food chosen. We also get water from all the drinks we consume, with the exception of stronger alcoholic drinks like wines and spirits. All these can contribute to dietary water, but also have other effects on health both positive and negative. The major concerns with regards to beverages are their energy content and their effect on dental health. With obesity levels continuing to increase it is important for many in the population to control their energy intake, and drinks as well as foods must be considered for their energy content. With regards to dental health, there are two concerns; dental caries and dental erosion. Dental caries are caused by a reduction in pH due to bacterial fermentation of carbohydrates, and so the frequency of consumption of drinks containing sugars is a concern for risk of caries. Dental erosion occurs at a lower pH and is caused by the consumption of acidic foods and drinks, in particular, citrus juices and soft drinks containing acids.
Individual water needs vary widely depending on many factors including body size and composition, the environment and levels of physical activity. Thus it is very difficult to make generic recommendations about the amount of water to consume.