Water plays several vital roles in the body. It allows nutrient transportation to the cells and waste removal from body tissues. It acts as a shock absorber in joints, lubricates the digestive and respiratory tracts, and helps body temperature regulation. In addition, water is the perfect medium for several chemical reactions. You can find water both inside and outside body cells. Some are obvious such as blood, tears, and saliva; others, not so much. Did you know that muscle tissue is composed of 75% water? Even bones (20%) and teeth (10%) contain significant amounts of water.
So yes, water is the greatest component of the human body, comprising 50% to 70% of one’s total body weight. This means that we carry approximately 10 gallons of water. However, water is not well stored in the body and must be replenished constantly. During the day, water is lost not only through urine and feces, but also as we breath and sweat. Adults excrete between 1 ½ to 3 quarts of water per day.
A person may survive around eight weeks without food, but only a few days without water. When fluids are lost but not replaced, dehydration occurs, causing dry mouth and skin, tiredness, headache, dizziness, and constipation. However, severe dehydration is dangerous and is considered a medical emergency. Usually, losing 1% to 2% of body weight in fluids provokes a thirst response. A 4% loss leads to muscle weakness, while 10% to 12% loss will impair heat tolerance. If a 20% reduction occurs, coma and death may arise.
As you can see, relying only on thirst for fluid replacement may not be your best option, because once you are feeling thirsty, chances are you are already dehydrated. Furthermore, athletes and exercise enthusiasts should carefully monitor their fluid status because of water-loss through sweating. Athletes are usually advised to weigh themselves before and after training sessions and to consume 2 ½ to 3 cups of fluids for every pound lost. A simpler way to see if your fluid intake is adequate is by checking the color of your urine. If it is clear to pale-yellow, you are well-hydrated. If it is dark-yellow, you need more fluids.
The Adequate Intake (AI) for adult women is around 9 cups of fluids per day. This may include several types of beverages such as fruit juice, coffee, tea, soft drinks, and water. However, keep in mind that according to the Beverage Guidance System, fluids should not contribute a significant number of calories in a healthy diet. This means that in a 2,200 kcal diet, beverages should provide 10% or fewer of the total calories (around 220 kcal). In this case, plain water, which is calorie-free, may be your best option most of the time.
Another aspect to consider is that caffeine has a diuretic[1] effect, meaning that relying solely on caffeinated beverages (coffee, teas, soft drinks, and energy drinks) for fluid replacement may be counterproductive. For people who exercise for long periods (more than 60 minutes) or who sweat heavily, sports drinks may be a good hydration option. This is because these beverages replace fluids and electrolytes[2] lost through sweat and also provide energy.
References
Boyle, M. and Long, S. Personal Nutrition. Belmont, CA: Wadsworth Cengage Learning. 2010.
Powers, S. and Howley, E. Exercise Physiology. Theory and Application to Fitness and Performance. New York: McGraw-Hill. 2007.
Wardlaw, G. and Smith, A. Contemporary Nutrition. New York: McGraw-Hill. 2009.