Your body is mostly water. In fact, 55% to 60% of an adult's weight is. How much water each body tissue contains varies:
- Blood (80-85% water)
- Muscle (70-75% water)
- Bone (20-25% water)
- Fat cells (8-10% water)
Water is also essential to health: if you stopped consuming water in any form today, you'd die in about 5 days. Greater than 15% dehydration is considered fatal.
There is also some scientific evidence that consuming lots of water is beneficial to health, although all scientists do not agree. Our aim here is not to review the scientific literature, so we'll move straight to the conclusions: consuming plenty of water seems to be salutary, whereas drinking "too much" isn't a concern.
ACTION STEP 24
With this "risks vs. benefits" analysis in mind, we recommend you drink 1.5 liters of water (6 cups) of water each day. In addition, if you do cardiovascular exercise, drink 500 mL (2 cups) 30 minutes before your work-out and another 1 liter (4 cups) right after.

German scientist F. Manz reviewed the evidence in "Hydration and disease", published in 2007 in the Journal of the American College of Nutrition, and reports that mild dehydration may be one of the factors causing oligohydramnios, prolonged labor, cystic fibrosis, hypertonic dehydration, and renal toxicity of xenobiotica. It also "plays a critical role" in the development of pulmonary disorders like exercise asthma and cystic fibrosis. As well, staying well hydrated would be beneficial to urolithiasis, urinary tract infection, constipation, hypertension, venous thromboembolism, fatal coronary heart disease, stroke, dental disease, hyperosmolar hyperglycemic diabetic ketoacidosis, gallstone disease, mitral valve prolapse, and glaucoma.