Water is a colorless, tasteless, odorless substance essential to all known forms of life. It is formed by a combination of two parts hydrogen and one part oxygen (H₂0). Water is the nutrient required in the largest amount by our bodies, more than carbohydrates, protein, vitamins or minerals. Water is considered a macronutrient because it is essential in large amounts each day. However, it cannot be stored in the body like other nutrients. Daily replenishment of water is necessary to maintain body function and to avoid dehydration (the excess loss of water and electrolytes).

The body is made up of approximately 60 percent water. Water composition depends on factors such as age, gender and body composition (amount of lean muscle versus fat tissue). Muscle holds more water compared to body fat. Infants’ and children’s bodies have a higher percentage of water. On average, females, the elderly and obese people have less water in their bodies.

A majority of the water in the body is found inside body cells (intracellular fluid). The remaining amount is known as extracellular fluid. This water outside the cells helps to make the blood and bodily secretions (e.g., saliva, tears, urine). This extracellular fluid carries electrolytes and can move in and out of the semi-permeable membranes of body cells (a process known as osmosis).

If fluid imbalances occur, the body automatically responds to restore the balance. For example, if there is too much of a mineral (e.g., sodium) inside the cell and not enough fluid, the body will send water inside the cell to restore the proper balance. It can also send water outside the cell if there is too much fluid and too little of a mineral inside. The body continually makes quick, automatic adjustments to achieve a balance of fluids inside and outside body cells. The natural tendency of the body to maintain stability in such a way is called homeostasis.

Water is essential to life because it:

• Regulates body temperature through perspiration
  
  
• Helps the body absorb nutrients and convert food into energy
  
  
• Transports nutrients and oxygen to cells
  
  
• Helps to moisten body tissues (e.g., in the mouth, eyes, nose and throat)
  
  
• Lubricates joints and protects body organs
  
  
• Helps to prevent constipation by softening stool
  
  
• Helps carry waste out of the body
  
  
• Is the main component in every body fluid (e.g., blood, gastric juices, urine, saliva, amniotic fluid for a developing infant)

A person cannot survive for long without drinking water. Although people can survive for several weeks without consuming any food, they cannot live more than a few days without water.

The National Academy of Science’s Food and Nutrition Board defines the daily adequate intake levels of water (from beverages and food) for healthy adults as:

• 2.7 liters (about 11 cups or 91 ounces) for women
  
  
• 3.7 liters (about 16 cups or 125 ounces) for men

Most experts agree that a good rule of thumb is to drink at least eight 8-ounce glasses of water a day. However, individual hydration needs vary. Most healthy adults need about 1 to 1.5 liters of water per 1,000 calories of energy they consume. Thus, people who consume 2,000 calories per day will need approximately 8 cups of water (about 2 liters) per day.

People get most of their water needs met through drinking, which constitutes about 80 percent of a person’s total fluid intake. Drinking water is the easiest way to get adequate quantities each day. Other beverages are a good source of water (e.g., juice, milk, tea, soda), although they may also include substances that encourage water loss. For example, the caffeine commonly found in coffee, tea and soft drinks acts as a diuretic, increasing the amount of fluid expelled from the body in urine.

Food also contains water and is usually responsible for about 20 percent of a person’s total fluid intake. Many fruits, vegetables, meat and legumes derive more than half of their weight from water. Foods with an extremely high water content include Iceberg lettuce, watermelon, cucumbers, broccoli, squash, grapefruit and oranges. Even so-called dry foods have some water content (e.g., bread, nuts). Oils do not contain any water.

In addition to water supplied by the diet, a small amount of the body’s water is produced inside body cells. This occurs during the body processes of digestion and metabolism.

Waiting to drink until the sensation of thirst appears is not a good indicator of proper hydration. Thirst is actually a symptom of mild dehydration. People who are well hydrated are rarely thirsty, and have urine that is colorless or only slightly yellow.

On average, adults lose approximately 10 cups (2.4 liters) of water each day, which must be replaced. Reasons for water loss in healthy adults include:

• Urine. This is the greatest source of water loss, with about 4 to 6 cups (0.95 to 1.42 liters) daily. If more or less water is taken in it can change the amount of urine excreted.
  
  
• Sweat. Perspiration accounts for about 2 to 4 cups (0.47 to 0.95 liters) of water lost per day. Variations in this amount depend on factors such as heat, humidity and level of physical activity.
  
  
• Breathing out (exhalation). This is known as insensible water loss because people are essentially unaware of this process. Breathing can result in the loss of about 1.5 cups (0.35 liters) of water per day.
  
  
• Stool output. Bowel movements account for the loss of approximately two-thirds of a cup (0.16 liters) of water each day. When a person has diarrhea, this amount significantly increases.

Vomiting is also responsible for fluid loss. It may occur during certain illnesses or in people with certain conditions (e.g., bulimia nervosa). When it occurs, vomiting can account for a significant loss of fluids.

No tolerable upper intake levels have been established for water. Most people do not drink excessive amounts of water, and healthy kidneys can usually handle more than 24 ounces of fluid per hour. However, a rare condition called dilutional hyponatremia (inadequate sodium levels due to excess extracellular fluid) may occur when people drink too much water. When water moves into body cells to achieve fluid balance, the cells may begin to swell. Most cells can handle extra water, but brain cells cannot because the brain is confined by the skull. Swelling of the brain is a life-threatening complication of this condition.