How does dehydration occur?
Dehydration develops when you consume less water than you lose . Dehydration means loss of water that results in loss of more than 1% of body weight, for example, more than 1.5 pounds in a 150 lbs person .
You cannot prevent water loss due to invisible perspiration, breathing, urination and defecation. This obligatory water loss is at least 1 liter of water per day in adults . Additionally, you almost always lose some water by sweating.
A healthy sedentary adult living in a moderate climate loses at least 1-2 liters of water per day . Physically active adults living in hot climates may lose as much as 10 liters of water per day [2,13]. A person who vomits repeatedly, has diarrhea or a condition with excessive urination, may lose more than 10 liters of water per day.
1. Insufficient Drinking
Common causes for insufficient drinking:
- Not being aware of thirst due to tiredness, anxiousness or preoccupation with work
- Weak sense of thirst, for example, in elderly , in cold environment  or after arriving at high altitudes  and in conditions, such as head injury, stroke and liver cirrhosis 
- Inability to drink due to painful mouth sores, strep throat, mouth or throat injuries or surgery, gastroparesis, gastric cancer, severe nausea during pregnancy (morning sickness), dementia, delirium or sedation [24,26]
- No available drinking water
- Unwillingness to drink due to an attempt to lose weight quickly (boxers before weighing), psychological conditions, such as depression, anorexia nervosa, obsessive-compulsive disorder (OCD) or psychosis [23,26]
Here is an explanation that can help you understand how much water do you need per day.
Common causes of excessive sweating include high ambient temperatures (summer, tropics, sauna, certain jobs), exercising in warm clothes (skiing in winter) and heavy physical work. Marathon runners can have the sweating rate 0.5-3 liters per hour or more, so they can lose about 1.5-10 liters of water per marathon . Miners, workers in tropics can lose as much as 10 liters water during one shift .
Another common cause of excessive sweating is fever, for example in flu, pneumonia, thyrotoxicosis or due to prolonged dancing after ecstasy use .
3. Prolonged Hyperventilation
Quite some water can be lost through breathing, especially during prolonged hyperventilation in:
- Heavy exercise
- Anxiety, hyperventilation syndrome
- Conditions, such as pneumonia, kidney disease and diabetic ketoacidosis (in poorly treated diabetes)
- High altitude 
- Poisoning with salicylates (aspirin in doses greater than 1 g)  or amphetamine 
4. Repeated Vomiting
During repeated vomiting, you can lose a large amount of gastric juice and thus water . Common causes of persistent vomiting:
- Alcohol intoxication
- Motion sickness
- Morning sickness during pregnancy (hyperemesis gravidarum)
- Infectious diseases (H. pylori infection of the stomach, bacterial meningitis)
- Kidney stones, urinary tract infection (UTI), including kidney infection (pyelonephritis) , or kidney failure (uremia)
- Hepatitis, acute pancreatitis, gallbladder attack, appendicitis, intestinal obstruction
- Cancer and chemotherapy
- Hypercalcemia, hyponatremia
- Cyclic vomiting syndrome
- Bulimia nervosa
5. Severe or Prolonged Diarrhea
Common causes of severe acute diarrhea:
- Excessive alcohol ingestion
- Food poisoning
- Gastroenteritis or “stomach flu” (mainly in infants and toddlers)
- Cholera (with water loss greater than 1 liter per hour or 20 liters per day) 
Common causes of chronic diarrhea :
- Celiac disease
- Crohn’s disease
- Intestinal parasites
- Antibiotic-associated diarrhea
- Laxatives abuse
Causes of excessive urination (an increased daily amount of urine or polyuria):
- Cold ambient temperatures, which induce the excretion of the urine (“cold-induced diuresis”) 
- Water pills (diuretics) 
- Untreated diabetes mellitus type 1 
- Kidney disease, such as polycystic kidney disease or lithium toxicity, resulting in nephrogenic diabetes insipidus 
- Hypokalemia 
- Hormonal imbalance, such as pituitary tumor (adenoma) causing central diabetes insipidus  or severe acute adrenal insufficiency in Addison’s disease 
Burns can cause damage of the skin vessels resulting in an increased water evaporation from the skin and quick water loss .
8. High Altitude
After arriving at high altitudes (over 11,500 feet or 3,500 m), unacclimated individuals often feel less thirsty . They may also lose 1-2 liters of water due to increased urination, probably due to cold-induced diuresis, and may remain dehydrated for about 14 days [8,10]. Additionally, dry air at high altitudes stimulates water loss through skin and breathing.
Do alcohol, caffeine and protein cause dehydration?
Alcohol, caffeine (in tea, coffee, cola and energy drinks)  and foods high in protein [27,28] mildly stimulate water excretion through the kidneys and transitionally increase urination, but this should not result in any significant dehydration .
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- National Academic Press Obligatory water loss
- National Academic Press Obligatory water loss in a dehydrated person
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- National Academic Press Water requirements in workers at heat
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- PubMed Central Physical effects of obsessive-compulsive disorder — OCD
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- PubMed Caffeine, body fluid-electrolyte balance and exercise performance