Dehydration does not only cause thirst—it affects the brain, heart, kidneys, and even the skin, leading to fatigue, dizziness, constipation, and reduced performance. Staying properly hydrated is essential for daily health.

Water makes up about 60–70% of an adult’s body weight and plays a role in every vital function: regulating temperature, transporting nutrients, eliminating toxins, and maintaining the activity of the brain, heart, liver, and kidneys.

However, when the body loses just 2% of its water, physical and mental performance already begins to decline noticeably. If dehydration persists, many organs may suffer serious damage, potentially becoming life-threatening.

Mechanisms of dehydration and common causes

According to the WHO, dehydration occurs when the amount of water lost exceeds the amount taken in. Common causes include:

Not drinking enough water during the day, especially in hot weather or during heavy physical work.
Excessive sweating from exercise, high fever, or hot, humid weather.
Vomiting, diarrhea, or viral fever, which rapidly deplete water and electrolytes.
Excessive intake of coffee, alcohol, or energy drinks, which have diuretic effects and worsen water loss.
Chronic conditions such as diabetes, kidney disease, adrenal insufficiency, or use of diuretics.

Signs that the body is dehydrated

When dehydrated, the body sends several “warning signals,” such as:

Frequent thirst — the earliest sign that the body needs water.
Dark, concentrated, foul-smelling urine — indicating the kidneys are conserving water.
Dry mouth, cracked lips, dry tongue.
Reduced skin elasticity, dry and rough skin.
Fatigue, dizziness, headache, poor concentration.
Rapid heartbeat and low blood pressure in severe dehydration.
In children: irritability, sunken eyes, sunken fontanelle, reduced or absent urination.

Prolonged dehydration makes the blood thicker, forcing the heart to work harder to pump it. This can cause low blood pressure, arrhythmias, and even stroke due to reduced blood circulation.

What happens inside the body when dehydrated?

Effects on the brain and mental function
A 2023 Harvard study shows that losing just 1–2% of body water can reduce concentration and short-term memory by up to 20%. Dehydration reduces blood flow to the brain, causing headaches, fatigue, drowsiness, and irritability.

Thermoregulation disorders
Water is the body’s natural “coolant.” Without enough water, the body cannot produce sufficient sweat to lower temperature, increasing the risk of heat exhaustion or heatstroke—especially in outdoor workers.

Effects on the cardiovascular system
As water levels drop, blood volume also decreases. The heart must beat faster to maintain blood pressure and deliver oxygen to cells. Over time, this strain can lead to arrhythmias or increase the risk of heart attack in people with cardiovascular disease.

Kidney and liver damage
The kidneys need water to filter toxins and excrete them through urine. Dehydration causes toxins to accumulate, increasing the risk of kidney stones, nephritis, or acute kidney failure. The liver is also stressed as it must process waste in a more concentrated environment, which may raise liver enzyme levels.

Effects on digestion and skin
Water supports digestive fluids and lubricates the intestines. Lack of water causes constipation, bloating, and indigestion. Dehydration also makes the skin dry, dull, less elastic, and prone to early wrinkles—one of the factors contributing to accelerated aging.

Severe dehydration: a medical emergency

Losing more than 10% of the body’s water may lead to:

Low blood pressure, rapid heartbeat, and rapid breathing.
Seizures, confusion, or fainting.
High body temperature.

Severe dehydration can cause shock and become life-threatening if not treated promptly.

How much water should you drink per day?

According to the National Institute of Nutrition, average daily water needs are:

Adults: 35–40 ml/kg body weight per day (equivalent to 2–2.5 liters for someone weighing 50–60 kg).
Children: 1–1.5 liters/day depending on age.
Pregnant or breastfeeding women: an additional 0.5–1 liter per day.
Athletes or people working outdoors: 200–250 ml every 20 minutes of activity.

Note: Do not drink too much at once—divide intake into 8–10 times throughout the day. Hydration can also come from broth, vegetables, fruits, or electrolyte drinks (if sweating heavily).

When should you see a doctor?

Seek medical care immediately if experiencing:

No urination for 8 hours, or very dark urine.
Severe fatigue, dizziness, rapid heartbeat, cold skin.
High fever, prolonged diarrhea, or persistent vomiting.
Confusion or loss of consciousness.

These signs indicate severe dehydration requiring IV fluids and urgent medical treatment.

Dehydration does not only cause discomfort—it silently damages vital organs. Build the habit of drinking water regularly each day, especially in hot weather, when working outdoors, or when ill. Sometimes, a glass of water at the right moment is the best “medicine” for your health.