Dehydration may be caused by many different factors, any of which can contribute to lost fluids not being adequately replaced in the body. Most people never progress beyond mild dehydration because eating and drinking can resolve the condition. However, some people are more likely to develop more serious levels of dehydration due to certain medical conditions or situations that interfere with their normal intake of water.

Inadequate intake of fluids is one cause of dehydration and is usually responsible for mild cases. This may occur during illnesses that affect the throat (e.g., strep throat, tonsillectomy), making swallowing difficult. Some people may become dehydrated prior to medical testing or surgery, when a physician restricts liquids or food ahead of time.

People who deliberately restrict their food or fluid intake are also at higher risk of dehydration. This includes people with eating disorders (such as anorexia nervosa) and people who fast for religious or political reasons. People with certain physical disabilities may be unable to get adequate nutrition, including sufficient fluids. Others may deliberately withhold fluid intake to avoid frequent urination if going to the toilet is cumbersome due to their disability. Severe dehydration can result when people experience water and food deprivation in extreme survival conditions.

Basically, dehydration is likely to occur any time fluids are lost in excessive amounts (and not replaced). Fluid loss may be caused by a variety of factors, including:

• Gastrointestinal fluid loss:
  
  
• • Diarrhea. One of the most common causes of dehydration. Diarrhea may be caused by bacteria, viruses or parasites. Infants, children and the elderly are especially at risk for dehydration due to diarrhea because weak immune systems make them more vulnerable to illnesses that can cause diarrhea.
    
    
  • Vomiting. The rapid loss of bodily fluids that occurs with vomiting makes dehydration more likely to occur. Certain populations are at higher risk of dehydration from this cause, including infants and children, the elderly, people with eating disorders (e.g., bulimia nervosa) and anyone taking medications that can cause vomiting.
    
    
• Excessive urination:
  
  
• • Certain medications that increase urination beyond normal levels (e.g., diuretics).
    
    
  • Conditions that affect kidney function. Kidneys regulate the loss of body fluid through urine. Conditions that may affect urine production include diabetes mellitus, diabetes insipidus (water diabetes, which is characterized by frequent urination and thirst) and kidney cancer.
    
    
• Excessive fluid loss through the skin (perspiration) and lungs (breathing):
  
  
• • Heat and/or humidity. Of the two, humidity plays the greater role in dehydration because it interferes with the body’s ability to cool itself. In humid areas, sweat drips from the body rather than evaporates, so the lost fluid does not contribute toward cooling the body (the normal function of sweat). The elderly, infants and children are especially at risk for dehydration and other complications due to heat and humidity (e.g., heat exhaustion and heat stroke).
    
    
  • Physical activity. Excessive sweating can occur during sports or other vigorous physical activity and also adds to the risk of dehydration.
    
    
  • Fever or burns, including sunburn. These conditions increase body temperature, requiring more fluid for proper body functioning. Burns may inhibit the skin’s ability to draw in moisture as well as sweat, which can prevent the body from cooling down and increase the risk of dehydration.
    
    
  • Air travel and high altitudes. Commercial airplanes maintain dry air in the cabin interior, with levels as low as 1 percent humidity. Because there is less moisture in the air, additional water from other sources is needed to avoid dehydration. This is especially true on long flights or at high altitudes (8,000 feet or higher).