Hydration or having enough water in the body is important because water is the body’s most critical, essential nutrient.  Illness and death will ensue from lack of water much more quickly versus the other essential nutrients.

On average, about 65% of the body is made up of water.

Water provides many vital functions:

• Virtually every chemical/metabolic reaction requires an aqueous (water-based) environment.
• Water is the principal component of the blood which is responsible for the delivery of oxygen and nutrients to cells and the removal of cellular waste products.
• Water is involved in the regulation of body temperature
• Lubrication of the joints
• Keeping the mucous membranes – nose, throat, mouth, GI tract, urogenital tract appropriately moist
• Cushions internal organs.

Bottom line – water is required for every essential function in the body.

An individual’s water requirement is based on how many calories they burn a day.

Example:  If you burn 2000 calories a day you will require 2000 mls of water (1ml per calorie)

Several external factors can influence water requirements:

1. Heat and humidity – both increase the loss of body water through sweating.  In hot, humid environments water requirements increase.  This is obviously relevant in summer.
2. Physical activity/exercise – an additional 1-2 cups per hour usually sufficient with the exception of extreme heat (may need more) or during endurance (2 or more hours) of exercise.  These situations may call for 2-3 cups an hour.
3. High altitude (8,000 feet or greater) – In high altitudes increased breathing and urination lead to increased water losses.
4. Illness – fever, diarrhea, vomiting etc – all increase losses.
5. Pregnancy/nursing

The US RDA for water is 1 ml per calorie burned – This is adjusted upwards for physical activity and hot and/or humid environments because of increased water loss through sweating.  The “average” woman needs about 2.7 liters daily.  The “average” man needs 3.7 liters.

Can’t give a single answer for each individual, as it depends on how many calories they burn and external factors like temperature and humidity.

Very important to note that water is found in all foods, especially fruits/veggies (example: watermelon 99% H2O) – Bread is 30% H2O.  So total daily needs can be provided by food in combination with any liquid (including fruit drinks, soda, juices, coffee, teas, beers, wine, etc) as all liquids are mostly made up of water.

Here are some interesting facts:

• The 8 times, 8 ounces a day is an urban myth.  Absolutely no conclusive science to support this recommendation.
• No conclusive scientific evidence that drinking large amount of water has any special health benefits.
• Coffee/tea (caffeine) in standard doses 1 – 3 cups does not have a diuretic effect, i.e. drinking a cup of coffee essentially provides same amount of hydration as a cup of water.  Only pharmacologic (big doses – 7 or more cups in one sitting) of caffeine has a diuretic effect.
• 1 – 2 drinks of alcohol do not lead to fluid loss and can contribute to hydration as they are largely comprised of water.  Excessive alcohol can lead to fluid loss, however;
• Thirst kicks in way before you are “clinically dehydrated.”  Another urban legend that “you are already significantly dehydrated by the time you are thirsty.”
  • Dehydration = 5% blood concentration
  • Thirst kicks in at 2% blood concentration
• Over-hydration (water intoxication) can be more dangerous than dehydration and although rare, does happen – especially in the context of endurance sports.  Compulsive water drinking usually is the culprit.  Generally requires several gallons consumed over a short period, but can occur more quickly in the context of stress (marathon running).  Kidneys don’t excrete water in normal fashion when body is stressed.
• Thirst is a very reliable guide to bodily needs in normal circumstances.  Elderly are slightly less sensitive to sensation of thirst and may need to be a bit more proactive.
• Pure, clean water is the healthiest beverage and if from the tap (which is fine from most municipal water supplies) cost a fraction of a cent.
• Under normal circumstances, the best guideline is to obey your thirst and natural instincts.  Drink as you normally do at meals and between meals and when you feel thirsty. 
• According to the Institute of Medicine:  “On a daily basis, people get adequate amounts of water from normal drinking behavior – consumption of beverages at meals and in other social situations – and by letting their thirst guide them.”
• Myth:  bottled water is healthier than tap H20.  Municipal water supplies are more rigorously evaluated and monitored than bottled water and in some cases is even cleaner and purer than bottled waters.
• Myth that you are dehydrated if your urine is not clear.  Dark-amber-colored urine is a good indicator that you need some hydration but having some yellow color to your urine does not signify that you necessarily need more water.
• In my opinion, current hydration craze is likely being fueled by the spin wizards of the beverage industry.  Bottled water (all types) are currently the fastest growing segment of the beverage industry and have huge profit margins.
• For the average person 20% of water provided by foods and 80% provided by liquids.

Best source of hydration –

Pure, clean 100% water is the ideal source of hydration as it is the only fluid that provides 100% of what hydrates – namely H2O.  This is especially relevant given the current obesity epidemic. (Caloric liquids, especially sugar-fortified varieties (soda, fruit drinks, sports drinks) appear to promote weight gain more readily than other calories.  (Liquid calories do not have the same appetite suppressive potential as solid food calories.  Additionally, sugar-fortified liquids have adverse effects on metabolism.)

The only exception to the above is in the context of physical activity/exercise lasting more than 1 hour when the participant is sweating, i.e. long distance running, cycling, etc.  After 1 hour of activity it’s ideal to hydrate with a sports beverage like Gatorade, that contains electrolytes that are lost during sweating.  Additional benefit to sports beverages in this specific context is that they do not quench thirst as well as water which provides greater stimulus to keep drinking.