Dehydration is a state in which the body loses more water than it replaces, producing a deficit in total body water. It can arise from inadequate intake, excessive losses or a combination of both. Common causes include sweating, fever, vomiting, diarrhoea, prolonged physical activity, hot environments, reduced access to fluids and illnesses that increase fluid loss. Certain medicines, particularly diuretics, can also increase risk.
Dehydration is not identical to thirst: thirst is a regulatory sensation that encourages drinking, while dehydration is a physiological disturbance that can exist with or without a strong subjective feeling of thirst. The severity ranges from relatively mild fluid deficit to severe dehydration associated with circulatory instability, neurological impairment and potentially life-threatening complications.
Water losses occur through several routes. Urine is normally the largest controllable route because the kidneys adjust water excretion according to hydration status and hormonal signals. Water is also continuously lost through breathing and the skin, with respiratory and evaporative losses increasing in hot or dry environments and during physical activity. Faecal losses normally account for a smaller amount but can become substantial during diarrhoea. Vomiting can rapidly remove both water and electrolytes. Dehydration can therefore involve different combinations of fluid and electrolyte loss. This distinction matters clinically because replacing only water may not fully correct losses caused by prolonged sweating or gastrointestinal illness, particularly when sodium and other electrolytes have also been lost.
Early or moderate dehydration can cause thirst, dry mouth, darker urine, reduced urination, tiredness, weakness and dizziness. Symptoms are not perfectly specific, because dark urine can reflect concentrated urine for reasons other than clinically important dehydration, and tiredness has many possible causes. More serious dehydration can produce persistent dizziness, confusion, marked drowsiness, rapid heartbeat, low blood pressure and impaired consciousness.
Infants and young children may show fewer wet nappies, while older adults may be vulnerable because thirst sensation and the ability to access fluids can change with age. The NHS specifically identifies unusual tiredness or drowsiness, persistent dizziness on standing and reduced or dark urine as reasons to seek urgent medical advice.
Treatment depends on the cause and severity. For uncomplicated mild dehydration, drinking fluids and gradually increasing intake is usually appropriate. When nausea or vomiting makes drinking difficult, small frequent sips can be easier to tolerate. During significant vomiting or diarrhoea, oral rehydration solutions can replace water together with glucose and electrolytes lost from the gastrointestinal tract.
These solutions are designed around principles of intestinal sodium and glucose transport and are more appropriate for substantial gastrointestinal losses than simply consuming large amounts of plain water. Severe dehydration may require urgent medical treatment and, in some circumstances, intravenous fluids. Treatment should address the underlying cause as well as the fluid deficit, because continued losses will otherwise continue to worsen the problem.
Certain populations are particularly vulnerable. Babies and young children can deteriorate relatively quickly during gastrointestinal illness because their fluid reserves are smaller relative to physiological requirements. Older people may have reduced thirst, mobility limitations or dependence on others for drinks. Athletes and outdoor workers can lose large quantities of water through sweat, particularly in hot environments. People with illnesses causing fever, diarrhoea or vomiting may also become dehydrated rapidly. Individuals taking medicines that increase urine production can have additional fluid-management considerations. People with kidney, heart or endocrine disorders may require individual medical guidance because simply increasing fluid intake is not always appropriate when the body's ability to excrete water is impaired.
Dehydration is sometimes incorrectly described as simply “not drinking enough water”. This ignores fluid losses and the role of electrolytes. Someone may become dehydrated despite drinking some water if vomiting, diarrhoea or heavy sweating exceeds intake. Conversely, drinking excessive quantities of plain water can produce a different problem, namely dilutional hyponatraemia, particularly during prolonged endurance activity. Alcohol can contribute to fluid loss, while moderate caffeine-containing drinks still contribute to overall fluid intake despite caffeine's diuretic effects being frequently overstated. Therefore, dehydration prevention involves maintaining an appropriate balance between intake and losses rather than following a universal rule that one specific beverage must be consumed in one fixed quantity.
Prevention depends on anticipating circumstances that increase fluid loss. Regular drinking is particularly important during hot weather, prolonged physical activity and illness. The NHS recommends drinking regularly and increasing fluid intake when sweating or unwell, with water being a suitable option for replacing ordinary fluid losses. During gastrointestinal illness, oral rehydration solutions may be preferable when substantial water and electrolyte losses occur.
Food also contributes to total water intake, so hydration cannot be assessed solely from the volume of beverages consumed. Monitoring urine colour can provide a practical everyday indicator, although it is not a substitute for clinical assessment when severe symptoms are present. Prevention is especially important in people who may have difficulty recognising thirst or independently obtaining fluids.
The medical significance of Dehydration arises from the consequences of reduced body water and disturbed electrolyte balance. Falling circulating volume can impair blood pressure and tissue perfusion, while changes in electrolyte concentration can affect nerve and muscle function. Severe cases can progress to shock, acute kidney injury, altered consciousness and other complications. Dehydration should therefore be distinguished from ordinary short-term thirst or from the marketing concept of being “optimally hydrated”.
The relevant clinical issue is whether fluid balance has become sufficiently disturbed to impair normal physiology. Recognising the cause, severity and associated electrolyte losses is essential because treatment varies accordingly. Prompt assessment is particularly important when dehydration occurs in infants, older adults, people with significant chronic disease or anyone experiencing persistent vomiting, diarrhoea, confusion or reduced consciousness.
