Fundamentals of Human Nutrition/Water
Water is considered the nutrient of life as one can go longer without food than they can without water. The human body relies on adequate internal water supply to function and maintain daily processes. Water contributes to approximately 60% of the body’s weight and is found in the skin, muscles, organs and bones. The muscles contain about 75% water, and fat contains about 25% of water, thus body composition can have an impact on the amount of water within the body. Females, the elderly, and those who are obese tend to have a lower water percentage in the body. About two-thirds of the water in the body is found inside the cells and is known as intracellular fluid. Extracellular fluid, or fluid outside of the cells, is made up of plasma and interstitial fluid, which is water surrounding the tissue cells.
The body is supplied with water from drinking fluids, eating food and through body metabolism processes. It is estimated that daily intake of water is represented by:
- Liquids: 550 to 1500 mL
- Foods: 700 to 1000 mL
- Metabolic byproduct: 200 to 300 mL
Water consumption should equal the amount excreted by the body to maintain water balance. The body rids itself of water through waste disposal in the urine, vapor in the lungs, sweat from the skin, and small amounts in the feces. Though the average amount of water lost a day is 2500 milliliters, the minimum amount that the body must excrete is 500 milliliters for the disposal of waste and toxins, and any amount above this is to preserve water balance. The best source of fluids is from water itself, as it doesn’t contain any unhealthy or unneeded additives such as caffeine, alcohol, or added sugars.
Water is used in nearly every major body system. The functions of water include the following:
- Regulating body temperature: When the body rises and becomes too warm, the blood vessels dilate to release heat into the environment, and the skin releases ions and water from the skin as sweat to draw away heat. When the body temperature falls and becomes too low, the blood vessels contract and tighten to retain heat, and shivering begins.
- Maintaining moisture in the tissues and membranes
- Lubricating joints
- Protecting body tissues and organ systems
- Flushing waste products out of the body and softens stool
- Dissolving minerals and nutrients in the body allowing for easy transportation
- Carrying oxygen and nutrients to cells in the body
- Serving as a chemical reactant in body systems
- Maintaining blood volume
- Maintaining the Acid-Base Balance: the acidity of the body depends in the concentration of hydrogen ions. Water can either dilute or increase this concentration in order to maintain balance.
- Medium for Metabolism: Water can either participate in the metabolic reactions (mainly catabolic reactions), or it can act as a medium for the metabolic reaction to occur in
Water requirements are difficult to establish as they can vary by numerous factors, including but not limited to, a person’s weight, environment and activity level. Water recommendations are generally based on the amount of energy expended. For adults, 1.0 to 1.5 mL/kilocalorie expended will help the body maintain water equilibrium. This equates to two to three liters (approximately 68 to 100 ounces) for a 2,000-calorie diet. Another recommendation for water intake is one-half cup per 100 kilocalories expended, equating to 80 fluid ounces based on a 2,000-calorie diet. Adequate intake levels for males are listed at 3.7 liters per day and 2.7 liters per day for females.Beverages supply about 75-80% of the daily amount, while the remaining 20-25% comes from the water within food products. Water can also be produced in the body through metabolic reactions, but this amount is less than what is found in the food and water sources. Water should be consumed throughout the day in regular intervals, as thirst can be an unreliable method of determining the amount of water the body needs.
Dehydration happens when the body’s water content falls below adequate levels. This can happen naturally when the body expels water at higher rates than the rate at which an individual takes in water. For example, water is naturally expelled from the body through the kidneys and the gastrointestinal tract by excretion. It is also expelled through the skin as perspiration, as well as through the lungs as exhalation. Some individuals should be more conscious of their water balance, as they may at a higher risk for dehydration. This group includes the elderly, as they have a poor thirst response, as well as infants, as they are prone to frequent diarrhea and vomiting. Certain illnesses, such as the flu or a bacterial infection can cause one to expel more water than normal as a result of fever and/or diarrhea. Thus, an individual should consume more water to maintain a healthy balance of fluids in the body. The following chart outlines the effects of dehydration as water levels, represented by body weight percentage, decreases.
Body Weight Loss (%): Symptoms
- 1-2: Thirst, exhaustion, weakness, sense of discomfort, decreased appetite
- 3-4: Diminished physical performance, dry mouth, decrease in urine, reddened skin, intolerance, apathy
- 5-6: Difficulty concentrating, headache, petulance, drowsiness, compromised temperature regulation, increased respiratory rate
- 7-10: Vertigo, spastic muscles, loss of balance, disorientation, fatigue, collapse
In rare cases, a person can consume too much water. This is known as water intoxication, or hyponatraemia. Hyponatraemia occurs when sodium levels in the blood plummet to a dangerously low level. This condition can develop through a number of different ways, including excessive water consumption, decreased urine production due to kidney disease, or extreme or prolonged (longer than three hours) times of sweating without electrolyte replenishment. Symptoms of hyponatremia can consist of confusion, convulsions, and in extreme cases it can result in death.Symptoms of hyponatremia can range from mild manifestations, like fatigue, headache, or nausea; to extreme, where the cells in the brain begin to swell, which often results in hyponatremic encephalopathy. To prevent hyponatremia, individuals should drink water in limited amounts (1-1.5 liters an hour) during times of heavy sweating and exercise, eat salty foods during long durations of endurance training, and consume a healthful, balanced meal after exercise or excess sweating.
Regulation of Body Water
The body can maintain fluid balance through the detection of blood pressure, blood volume, and the concentration of sodium and other ions. When blood volume and blood pressure fall too low, or whenever the concentration of extracellular fluid has become too high, the hypothalamus sends signals to the pituitary gland to secrete the antidiuretic hormone, ADH. ADH then stimulates the kidneys to reabsorb water, and to only excrete highly concentrated waste as needed. This process also promotes the thirst mechanism. When water is then consumed or reabsorbed, the blood pressure and blood volume rise again, and the concentration of the blood solutes is diluted. Another response to dehydration and low blood pressure is the secretion of Renin by the kidneys. Renin, through a series of steps, causes the kidneys to reabsorb sodium, which, in turn, causes water reabsorption. In addition to this, Renin hydrolyzes the protein Angiotensin to Angiotensin I. Angiotensin I then remains inactive until it travels to the lungs, where it is hydrolyzed into Angiotensin II. Angiotensin II works as a potent vasoconstrictor, as it narrows the diameter of the blood vessels to raise blood pressure. Angiotensin II also stimulates the adrenal glands to release the hormone Aldosterone, which signals to the kidneys to retain sodium, and thus retain water.
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