Dehydration Dangers (Complications)
In otherwise healthy individuals, mild and moderate dehydration is usually not dangerous.
Possible effects of mild and moderate dehydration:
- Fainting upon standing
- Decreased physical performance
- Impaired short-term memory, decreased attention
- Worsening of asthma and allergies
Severe dehydration, especially in individuals with underlying chronic diseases, can result in:
- Blood clots
- Heat stroke
- Hypovolemic shock
- Acute kidney failure
- Heart attack
CHART 1. Dehydration and Physical Performance
|ENVIRONMENT||DROP OF EXERCISE PERFORMANCE|
|PROPER HYDRATION||DEHYDRATION (>2% loss of BW)|
|Cold (36-50 °F; 2-10 °C)||—||3%|
|Temperate (68 °F; 20 °C)||—||5-7%|
|Warm (86 °F; 30 °C)||~8%||12%|
|Hot (104 °F; 40°C)||~17%||23%|
|Altitude (9,843 feet; 3,000 m)||~11-15%||33%|
Chart 1, reference: 4
Core Body Temperature
Dehydration results in a reduced skin blood flow and hence less sweating and therefore lower ability to lose heat by sweating (impaired thermoregulation) .
Dehydration-related RISE of body core temperature occurs:
- During exercise in the heat, each percent of dehydration can result in elevation of the body core temperature by 0.1-0.2 °C 4. Except in the extreme conditions, the body core temperature remains within 2-3 °C of the normal 37 °C 1.
- In heat exhaustion and heat stroke
- In infections and hyperthyroidism
Dehydration-related DROP of body core temperature can occur in hypovolemic shock.
- Dehydration-related DROP of the skin temperature occurs in heat exhaustion and hypovolemia.
- Dehydration-related RISE of the skin temperature occurs during direct exposure to the sun, in heat stroke, hyperthyroidism and certain infections.
Heat Injury: Cramps, Exhaustion and Heat Stroke
In hot, humid weather (during heat waves), you might not be able to sweat and thus lose heat efficiently, so your body temperature may rise. Causes and risk factors for heat illness :
- Hot, humid weather with temperatures as low as 86 °F (30 °C) 
- Young age (infants) or old age
- Fever from an underlying disease
- Heart disease, high blood pressure
- Medications for Parkinson’s disease (they may inhibit perspiration) and schizophrenia (phenothiazines) 
General Symptoms of Heat Injury
- Increased body temperature, fatigue, weakness, nausea, headache
- Painful cramps in the legs, abdomen or arms that occur during or after exercise in heat [18,19]
Heat Exhaustion — Symptoms and Signs
- Increase of body temperature: 98.6-104 °F (37-40 °C) 
- Profuse sweating, cool, pale skin
Heat Stroke — Symptoms and Signs
- Body temperature greater than 106-115 °F (41-46 °C)
- Hot, flushed, dry skin
- Heart rate >130/min
- Irritability, tremor, delirium, hallucinations, seizures, coma, death
- References: 7,8,19
First Aid in Heat Injury
- Move a person to a cool place, remove excessive clothes, enable a cool shower or bath. NOTE: in high ambient temperature fans may not be very effective.
- Give a person a cool beverage, preferably water.
- When the above cooling methods do not provide relief within 30 minutes, call 911 (in the U.S.) [emergency phone numbers for other countries], because a person may need an intravenous fluid infusion.
- References: 17,19
Hypernatremia means increased blood sodium levels (>145 mmol/L); it develops when relatively more water than sodium is lost from the body . Hypernatremia can develop in [9,10]:
- Water deprivation
- Excessive sweating (marathon runners)
- Diarrhea in infants
- Repeated vomiting
- Excessive urination (polyuria) in untreated diabetes mellitus or diabetes insipidus
- Diuretics use
Hypernatremia increases blood tonicity, which results in the movement of water from the brain cells into the extracellular space and thus in brain shrinkage. Symptoms include general weakness, lethargy, confusion, seizures and symptoms of dehydration (decreased urination) . Death usually occurs when blood osmolality exceeds 350 mmol/L . Treatment includes low-sodium diet and intravenous fluid infusion .
Effects on Gastrointestinal Tract
Dehydration can cause constipation .
When you are dehydrated, you secrete less digestive juices (saliva, gastric juice, pancreatic juice), which may slow down digestion.
In two studies, moderate dehydration (loss of 4-6% body weight) during exercise in trained athletes resulted in delayed gastric emptying of fluids and gastrointestinal complaints, such as nausea and upper abdominal cramps, and increased rectal temperature [2,3]. Dehydration did not affect glucose absorption .
Dehydration can result in:
- Increased hematocrit (HCT), hemoglobin (Hb) and blood viscosity [25,30]
- Increased risk of deep venous thrombosis 
Effects on Heart and Blood Pressure
Dehydration by itself does not necessarily affect blood pressure.
Even mild dehydration (1-2% loss of body weight) can slightly increase your heart rate .
The cardiac output means the amount of blood expelled from the heart to the arterial system. Moderate dehydration (4% loss of body weight) by itself does not necessarily affect the cardiac output. In one study, cyclists who were moderately dehydrated (4% loss of body weight) had lower stroke volume and higher heart rate, so they managed to maintain the same cardiac output than when they were normally hydrated . When they were both dehydrated and had increased esophageal temperature by 1 °C, their cardiac output dropped by 13%.
Dehydration can cause orthostatic hypotension, which means a drop of the systolic (upper) blood pressure for at least 20 mm Hg upon raising . Symptoms, which may appear within 3 minutes upon raising, include dizziness, blurred vision, nausea, fatigue, excessive sweating (diaphoresis), fainting .
You can prevent orthostatic hypotension in the morning by drinking 300-500 mL water 15 minutes before getting up .
Severe dehydration may lead to a drop of blood volume and consequently inadequate perfusion of the tissues (hypovolemic shock).
Symptoms and signs of hypovolemic shock:
- Excessive sweating, pale, cool skin
- Increased heart rate
- Weak pulse
- Low blood pressure (a late sign)
It is not likely that an otherwise healthy person, when moderately dehydrated, would suffer from heart attack. Dehydration in heart patients can also trigger chest pain (angina pectoris) or irregular heart rhythm (arrhythmia).
When dehydration is so severe that it leads to a drop of blood volume of such an extent that causes hypovolemic shock, it may trigger heart attack . This is more likely the “end of life” than the everyday situation. Dehydration may also trigger a heart attack in individuals with coronary artery disease or impaired heart valves.
Even mild dehydration (1-2% loss of body weight) can result in [1,4,5,20]:
- Anxiety and mood changes
- Decreased alertness
- Short-term memory loss may persist for up to two hours after rehydration
- Slower decision making
- Impaired visual motor tracking
- Prolonged reaction time (23% prolongation in 4% dehydration i one study) .
Severe dehydration (>6% loss of body weight) can cause dizziness, numbness and tingling in fingertips, fainting, confusion, delirium, hallucinations, seizures or coma .
Impaired Kidney Function (Failure)
In dehydration, mucous membranes in the tubular organs, such as ureters, urinary bladder and urethra, dry up, which increases the risk of urinary tract infections (UTI).
Severe dehydration can result in acute kidney failure , especially when it is associated with muscle disintegration (rhabdomyolysis), for example, in marathon runners .
Chronic dehydration increases the risk of kidney stones .
Effect f Dehydration on Other Conditions
Dehydration can worsen symptoms in the following conditions:
- Acute mountain sickness 
- Asthma (exercise-related) 
- Hyperglycemia in diabetes 
- Restless leg syndrome [anecdotal report]
- Heart failure 
- Deep vein thrombosis 
- Yeast infection in the mouth (oral thrush, candidiasis) 
- Slow wound healing 
- Pressure ulcers (bed sores) 
There is a lack of evidence about the association of dehydration with fibromyalgia.
Dehydration and Death
An otherwise healthy person who has lost more than 10% of body weight due to dehydration can die. Here you can read how long you can survive without water.
- Physical performance (nature.com)
- Gastric emptying (nih.gov/pubmed)
- Gastrointestinal symptoms in athletes (nih.gov/pubmed)
- Evaluation (dtic.mil)
- Mood (uconn.edu)
- Acute mountain sickness (nih.gov/pubmed)
- Heat stroke (emedicine.medscape.com)
- Heat stroke causes and symptoms (emedicine.medscape.com)
- Hypernatremia mechanism (emedicine.medscape.com)
- Hypernatremia causes and symptoms (emedicine.medscape.com)
- Hypernatremia treatment (emedicine.medscape.com)
- Hyponatremia causes and symptoms (emedicine.medscape.com)
- Hyponatremia treatment (emedicine.medscape.com)
- Heat exhaustion symptoms (mayoclinic.com)
- Heat exhaustion causes (mayoclinic.com)
- Heat exhaustion treatment (mayoclinic.com)
- Heat illness risk factors FPNotebook
- Heat injury (childrenshospital.org)
- Cognitive functions (nih.gov/pubmed)
- Lanier JB et al, 2011, Evaluation and Management of Orthostatic Hypotension American Family Physician
- Hyperthermia and heart function (nih.gov/pubmed)
- Symptoms and signs (emedicine.medscape.com)
- Physiological consequences (nih.gov/pmc)
- Blood viscosity (nih.gov/pubmed)
- Kelly J et al, 2004, Dehydration and venous thromboembolism after acute stroke PubMed
- Hypovolemia and hypervolemia (nih.gov/pubmed)
- Kidney failure (mayoclinic.com)
- Dehydration can affect brain structure and function in adolescents (nih.gov.pubmed)
- Lab tests (stat.unc.edu)
- Rhabdomyolysis (emedicine.medscape.com)
- Heat-related illness (lni.wa.gov)
- Heart failure (nih.gov/pmc)
- Oral thrush (nursingtimes.net)
- Wound healing (woundandhyperbaric.org)
- Pressure ulcers causes NHS.uk
- Heart attack (iama.edu)