Dehydration Effects and Complications

By , August 30th 2013. Last reviewed 16th June 2016.

Dehydration Dangers (Complications)

Possible side effects of severe dehydration:

  • Seizures
  • Blood clots
  • Heat stroke
  • Hypovolemic shock
  • Acute kidney failure
  • Heart attack
  • Coma
  • Death

Other effects of dehydration:

  • Fainting upon standing
  • Decreased physical performance
  • Impaired short-term memory, decreased attention
  • Worsening of asthma and allergies

Dehydration and Physical Performance

CHART 1. Effect of dehydration, ambient temperature and altitude on aerobic exercise performance

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

Delayed Gastric Emptying

  • In two studies, moderate dehydration (loss of 3-4% 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 3.

Impaired Thermoregulation

Core Body Temperature

Dehydration results in a reduced skin blood flow and hence less sweating and therefore lower ability to lose heat by sweat (impaired thermoregulation) 1. Dehydration-related RISE of body core temperature occurs:

  • During exercise in heat, each percent of dehydration can result in elevation of the body core temperature by 0.1-0.2 °C 4. Except in extreme conditions, the body core temperature remains within 2-3 °C of the normal 37°C 1.
  • In heat exhaustion and heat stroke
  • In certain underlying causes of dehydration, such as infections and hyperthyroidism.

Dehydration-related DROP of body core temperature can occur in hypovolemic shock.

Skin Temperature

  • 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 sun, in heat stroke, hyperthyroidism and in certain infections.

Heat Injury: Cramps, Exhaustion and Heat Stroke

In hot, humid weather (during heat waves), you might not be able to lose heat efficiently, so your body temperature may rise. Causes and risk factors for heat illness 18:

  • Hot, humid weather, direct exposure to sun, excessive or tight clothing. People have suffered from heat stroke at ambient temperatures as low as 86 °F (30 °C) 32.
  • Dehydration
  • Obesity
  • 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 phenothiazines (for schizophrenia) 18

General Symptoms of Heat-Related Illness

  • Increased body temperature, fatigue, weakness, nausea, headache.

Heat Cramps Symptoms

  • Painful cramps in the legs, abdomen or arms that occur during or after exercise in heat 18,19.
  • Increase of body temperature up to 102.5 °F (39 °C) 19.

Heat Exhaustion — Symptoms and Signs

  • Increase of body temperature: 98.6-104 °F (37-40 °C) 19
  • Profuse sweating, cool, pale skin

Heat Stroke — Symptoms and Signs

  • Body temperature greater than 106 °F (41.1 °C), or up to 115 °F [46 °C] or more
  • Dehydration-related heat stroke: Hot, flushed, dry skin (minimal sweating can be present)
  • Exertional heat stroke: Hot, flushed, moist skin
  • Severely increased heart rate: >130/min
  • Irritability, tremor, delirium, hallucinations, seizures, coma, death
  • References: 7,8,18,19

First Aid in Heat Injury

Dehydration and Blood

Dehydration can result in:

  • Decreased blood volume — hypovolemia, but sometimes even in hypervolemia (in hypertonic dehydration, when blood becomes hypertonic, so it attracts water from the cells) 27
  • Increased hematocrit (HCT), hemoglobin (Hb) and blood viscosity (Hb) 25,30
  • Increased risk of deep venous thrombosis and blood clotting 26.

Blood Pressure

Dehydration by itself does not necessarily significantly affect blood pressure.

Orthostatic Hypotension

Dehydration can cause orthostatic hypotension, which means a drop of the systolic (upper) blood pressure for at least 20 mm Hg upon raising 21. Symptoms, which may appear within 3 minutes upon raising:

  • Dizziness, blurred vision, nausea, fatigue, excessive sweating (diaphoresis), fainting 21.

An individual may prevent orthostatic hypotension in the morning by drinking 300-500 mL water 15 minutes before getting up 24.

Hypovolemic Shock

Severe dehydration may lead to hypovolemic shock with decreased blood volume and consequent inadequate perfusion of the tissues.

Symptoms and Signs:

  • Excessive sweating, pale, cool skin
  • Increased heart rate
  • Weak pulse
  • Low blood pressure. This is a late sign of shock. An individual may be already in the first phases of shock but have normal blood pressure.

The Heart

Heart Rate

Even mild dehydration (1-2% loss of body weight) can slightly increase your heart rate 23.

Cardiac Output

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 22, cyclists who were moderately dehydrated (4% loss of body weight) had lower stroke volume and the higher heart rate so they managed to maintain the same cardiac output than when they were normally hydrated. When they were both dehydrated and hyperthermic (increase of the esophageal temperature for 1 °C), their cardiac output has dropped by 13%.

Heart Attack

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 37. This is more likely the “end of life” rather than everyday situation. Dehydration may also trigger heart attack in individuals with various heart diseases, such as hardening of the heart arteries (coronary artery disease) or impaired heart valves. 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).

Brain Function

Dehydration with hypernatremia can result in brain shrinkage, and dehydration with hyponatremia in brain swelling, which can both result in brain damage 29.

  • Mild dehydration can cause anxiety and other mood changes.
  • Severe dehydration can cause dizziness, numbness and tingling in fingertips, fainting, confusion, delirium, hallucinations or coma 38.

Decreased Cognitive Performance

Even mild dehydration (1-2% loss of body weight) can result in 1,4,5,20:

  • 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) 4.


Hypernatremia  means increased blood sodium levels (>145 mmol/L); it develops when relatively more water than sodium is lost from the body 9. Hypernatremia can be associated with the following causes of dehydration 9,10:

  • Water deprivation, when water is not available or in disabled elderly persons in nursing homes or hospitals who have decreased sense of thirst 9
  • Excessive sweating without water replacement (marathon runners)
  • Diarrhea in young infants
  • Repeated vomiting
  • Excessive urination (polyuria) in untreated diabetes mellitus or diabetes insipidus
  • Diuretics use

Hypernatremia increases blood tonicity, which results in movement of water from the brain into the blood and brain shrinkage. Symptoms: general weakness, lethargy, confusion, seizures and symptoms of dehydration (decreased urination) 10. Death usually occurs when blood osmolality exceeds 350 mmol/L 4. Treatment includes low-sodium diet and intravenous fluid infusion 11.


Hyponatremia means decreased blood sodium levels (<135 mmol/L); it develops when relatively more sodium than water is lost from the body. Hyponatremia can be associated with the following causes:

  • Drinking of fluids with low or no sodium during prolonged exercise with profuse sweating and hence salt loss (usually during marathon or military training).
  • Chronic malnutrition
  • Gastrointestinal obstruction, ileus
  • Burns
  • Diuretics use

Hyponatremia decreases blood tonicity, which results in the movement of water from the blood into the cells, including brain cells, which can lead to brain swelling (cerebral edema). Symptoms of hyponatremia usually appear only when blood sodium levels drop under 120-125 mmol/L and may include: restlessness, agitation, nausea, vomiting, confusion, headache, seizures, coma and, eventually, death 12. Treatment of hyponatremia usually includes isotonic or hypertonic (3% saline) intravenous infusion 13.

Impaired Kidney Function (Failure)

  • In dehydration, mucous membranes in the tubular organs dry up. This increases the risk of urinary tract infections (UTI).
  • Severe dehydration can result in acute kidney failure 28, especially when it is associated with muscle disintegration (rhabdomyolysis), for example in marathon runners 31.
  • Chronic dehydration increases the risk of kidney stones 24.

Gastrointestinal Tract

  • When you are dehydrated, you secrete less digestive juices (saliva, gastric juice, pancreatic juice), which may slow down digestion.
  • Dehydration can cause constipation 24.

Other Conditions

Dehydration can worsen symptoms in the following conditions:

  • Acute mountain sickness 6
  • Allergies 39
  • Asthma (exercise-related) 24
  • Depression 39
  • Hyperglycemia in diabetes 24
  • Restless leg syndrome (according to some patients’ experiences 41)
  • Heart failure 40
  • Vein thrombosis and deep vein thromboembolism 24
  • Tendinitis, such as Achilles tendinitis 33
  • Yeast infection in the mouth (oral thrush, candidiasis, moniliasis) 34
  • Slow wound healing 35
  • Pressure ulcers (bed sores) 36

There is lack of evidence about 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.