We spend a lot of time conversing over macronutrient intakes and training principles. Recently I posted about my spin on carb manipulation. In this thread I mentioned the importance of increasing water intake during such a diet. I wanted to take the time to delve into the roles of water in the body as well as intake recommendations in order to lay a foundation for a future, related thread.

Water constitutes about 60% of an adult’s body weight and a higher percentage of a child’s. Because water makes up about ¾ of the weight of lean tissue and less than ¼ the weight of fat, a person’s body composition influences how much of the body’s weight is water. The proportion of water is generally smaller in females, obese people, and the elderly because of their smaller proportion of lean tissue.

In the body, water becomes the fluid in which all life processes occur. The water in the body fluids:

• Carries nutrients and waste products throughout the body;
• Maintains the structure of large molecules such as proteins and glycogen;
• Participates in metabolic reactions;
• Serves as a solvent for minerals, vitamins, amino acids, glucose, and many other small molecules so that they can participate in metabolic activities;
• Acts as a lubricant and cushion around joints and inside the eyes, the spinal cord, and, in pregnancy, the amniotic sac surrounding the fetus in the womb;
• Aids in the regulation of normal body temperature (in separate thread I will cover this role in more detail);
• Maintains blood volume;

To support these and other vital functions, the bod actively maintains an appropriate water balance.

Water Balance and Recommended Intakes

Every cell contains fluid of the exact composition that is best for that cell (intracellular fluid) and is bathed externally in another such fluid (interstitial fluid). Interstitial fluid is the largest component of extracellular fluid. These fluids continually lose and replace their components, yet the composition in each compartment remains remarkably constant under normal conditions. Because imbalances can be devastating, the body quickly responds by adjusting both water intake and excretion as needed. Consequently, the entire system of cells and fluids remains in a delicate but controlled state of homeostasis.

WATER INTAKE: Thirst and satiety influence water intake in response to changes sensed by the mouth, hypothalamus, and nerves. When the blood becomes concentrated (having lost water, but not the dissolved substances within it), the mouth becomes dry, and the hypothalamus initiates drinking behavior. Stretch receptors in the stomach send signals to stop drinking as do receptors in the receptors in the heart that monitor blood volume.

Thirst drives a person to seek water, but it lags behind the body’s need. When too much water is lost from the body and not replaced, dehydration occurs. A first sign of dehydration is thirst, the signal that the body has already lost some of its fluid. If a person is unable to obtain fluid or fails to perceive the thirst message, the symptoms of dehydration may progress rapidly from thirst to weakness, exhaustion, and delirium and even death if not corrected. Dehydration may easily develop with either water deprivation or excessive water losses.

WATER LOSSES: the body must excrete a minimum of about 500ml of water each day as urine – enough to carry away the waste products generated by a day’s metabolic activities. Above this amount, excretion adjusts to balance intake. If a person drinks more water, the kidneys excrete more urine, and the urine becomes more dilute. In addition to urine, water is lost from the lungs as vapor and from the skin as sweat. The amount of fluid lost from each source varies, depending on the environment (heat and humidity) and physical conditions such as exercise or fever. On average daily losses total about 1-3 quarts.

WATER RECOMMENDATIONS: because water needs vary depending on diet, activity, environmental temperature and humidity, a general water requirement is difficult to establish. In the past, recommendations were expressed in proportion to the amount of energy expended under average environmental conditions. Currently, recommendations for athletes are for a daily intake of 8-10 glasses (not cups) excluding water losses due to exercise. In terms of replacing exercise-induced losses, let’s take a second to understand first the rate of absorption in the small intestines. Water can be absorbed from the small intestine at a max rate of 8-10 oz every 20 minutes. Cold water enters the small intestine faster (the colder the better) and is therefore recommended. When preparing for exercise, anticipate a profuse sweat, simply weigh yourself prior to and after. Then ingest 16 oz of water per pound of weight lost at a rate of 8-10 oz every 20 minutes. If activity is very intense and/or lasts longer than about 60 minutes, fluids should be ingested during activity.