http://www.water.org.uk/home/water-for-health/ask-about/adults The British are really wordy (like me). Anyway, the middle two paragraphs hold the gist of the it. Basically you don't actually need to drink 6 - 8 cups of water a day. You need about that much including water in food, so you're actually needing to drink (guessing) 3 to 5, depending on what and how much you eat, and how much you exercise. I had read about this 6, 7 years ago after hearing about it on the radio, but I'm pretty sure I had read it a little wrong (superficially). It doesn't hurt to have a little extra water, though.
Daily water requirements
The normal daily turnover of water is approximately 4% of total body weight in adults.(iv) The amount of water required by the body is that which is necessary to balance the insensible losses. This can vary markedly, depending, for example on climatic conditions. Other factors determining water loss include the amount of water required by the kidneys to excrete waste products (the solute load). This may vary with dietary composition and other factors. In view of these limitations, in 1989, the National Research Council (US) noted that the estimation of water requirement was complex and highly variable and concluded that it was impossible to set a general water requirement. Nevertheless, for practical purposes they recommended that the water requirement for adults could be estimated as "1ml per kcal energy expenditure for adults living under average conditions of energy expenditure and environmental exposure" - thus a person eating 2900kcal would require 2900ml of water.(v)
In 2002, Valtin (v) carried out an extensive review into the origins of advice regarding daily water requirements. He noted that an earlier, similar, recommendation from the US National Research Council may have been the source of the frequently exhorted advice to "drink at least eight glasses of water a day". In 1945, the Food and Nutrition Board of the National Research Council had advised: "A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." Valtin proposed that the last sentence may not have been heeded, and the recommendation was therefore erroneously interpreted as eight glasses of water to be drunk each day. (1 glass/cup - approx. 8 ounces = 240ml; therefore 8 glasses = approx. 1920ml). It is interesting to note that the advice from the National Research Council in 1989 did not comment on the source of the 1ml/kcal water required. This may have perpetuated the misconception that all of this water requirement must be in the form of drink.(v) Whereas, in accordance with the table above, if an adult gains in the region of 1 litre of their 2500 ml water requirement from food and metabolic water contributes a further 250ml, this leaves only 1250ml to be consumed in the form of drink.
In spite of this, some authorities(vi,vii) still advise that we should drink "six to eight glasses" or "2500-3000ml" of water per day. However, this guidance may have been influenced by the additional recommendation made by the Food and Nutrition Board of the National Research Council in 1989, that as "there is so seldom a risk of water intoxication … the specified requirement for water is often increased to 1.5ml/kcal to cover variations in activity level, sweating, and solute load."(v)
In 2004 the National Academies, Institute of Medicine, Food and Nutrition Board issued revised recommendations on the Dietary Reference Intakes for Electrolytes and Water.(viii) The Panel on Dietary Reference Intakes undertook an extensive review of the literature in addition to conducting surveys as part of their review of this subject. They commented that “although a low intake of total water (which includes a combination of drinking water, water in beverages, and water that is part of food) has been associated with some chronic diseases, the evidence is insufficient to establish water intake recommendations as a means to reduce the risk of chronic diseases. Instead, an Adequate Intake for total water is set to prevent deleterious, primarily acute, effects of dehydration, which include metabolic and functional abnormalities.” Based upon their review of water balance studies for inactive adults in temperate climates, the panel recommended that the minimal water requirement should be approximately 1 to 3.1 litres per day to replace respiratory, urinary, faecal and insensible fluid losses. Since individual water requirements can vary greatly, even on a day-to-day basis, because of differences in physical activity, climate and diet, they recommended the following Adequate Intake for water: