What are the causes and symptoms of dehydration?

Water fulfills many functions that are essential to maintaining homeostasis and life. Because the adult body is made up of 60% of water but cannot store it, water is vital for humans. Therefore, it is essential to ensure water intake to avoid dehydration, which can have disastrous effects.

Water circulates through the body in an uninterrupted cycle and is never stagnant. What is eliminated must be compensated. On average, humans lose two liters of water per day: 0.5 liters through perspiration (diffusion of water vapor through the skin), 0.5 liters through breathing, and one liter through urine. To compensate for this water loss, it is often recommended to absorb an average of two liters of water each day: 1.5 liter by drinking and 0.5 liter by eating. 

However, these figures are averages that depend on many factors, such as body size, age and physical activity. If water represents 60% of an adult's body weight, this average rises to 70% for a child and 55% for an older person. It will also be influenced by the body's composition in terms of fat or muscle. If adipose tissue is 10% water, muscle is 75% water. Thus, the more muscular a person is, the greater the proportion of water in his or her body weight and the greater his or her water needs.

Dehydration: an imbalance between water loss and intake 

If the balance is disturbed, i.e. if water losses exceed water intake, the body becomes dehydrated. If a person loses 2% of their water, they feel the need to drink. At 10% loss, the skin shrinks and the dehydrated person may experience hallucinations. Dehydration can be fatal if water loss reaches 15%.

Dehydration results from an imbalance between water intake and loss: in other words, the dehydrated person does not drink enough or eliminates too much water or both at the same time. The causes of excessive water loss can be many: diarrhea, excessive sweating, burning, taking diuretic drugs (which increase urine production) or laxatives, alcohol intake (which has diuretic effects), or certain chronic diseases.

Symptoms of dehydration moderate or severe

The first symptom of dehydration is obviously thirst. It can be accompanied by a sensation of dry mouth. In order to limit water loss as much as possible, the body will decrease its perspiration and urine production. The symptoms may worsen if the water loss is not quickly compensated and dehydration continues.

When the body's water volume decreases, so does blood volume, causing blood pressure to drop. To compensate for this problem, the water in the cells will move into the extracellular compartment and into the blood to maintain as normal a volume as possible. The downside is that the cells will then curl up and dry out. And when this system is no longer sufficient to maintain the blood volume, drops in blood pressure can occur, leading to fainting.

When dehydration becomes severe, clinical signs include headaches, dark circles, skin that lacks suppleness (a fold appears on the skin when pinched), and damage to the internal organs, which receive too little blood. These lesions can become irreversible if dehydration persists. A severely dehydrated person may experience confusion, hallucinations, intense fatigue or agitation. The most severe cases may result in coma or death.

The elderly and babies are more vulnerable to dehydration

Older people are more at risk of dehydration because they feel thirsty later in life. As for babies, water represents a more significant proportion of their bodies (about 70%). A water loss of equal volume will be more significant for an infant's body weight. 

It is therefore vital to ensure that people at risk are well hydrated - especially in hot weather when perspiration and therefore water loss are more important.

Mild dehydration can be treated by drinking enough water. If the water loss is due to diarrhea, vomiting or excessive sweating. In that case, the dehydrated person will also be deficient in mineral salts which must be compensated for. Hydration solutions (sold in pharmacies), sports drinks, vegetable broth or sugar water, can address this problem. 

More severe dehydration is an emergency and will require prompt consultation to avoid complications.