Health Check: what happens to your body when you're dehydrated?

Water is essential for human life. It accounts for for 50-70% of our body weight and is crucial for most bodily functions.

Any deficit in normal body water – through dehydration, sickness, exercise or heat stress – can make us feel rotten. First we feel thirsty and fatigued, and may develop a mild headache. This eventually gives way to grumpiness, and mental and physical decline.

We continually lose water via our breath, urine, faeces and skin. Most healthy people regulate their body’s water level remarkably well via eating and drinking, and are guided by appetite and thirst. But this is more difficult for infants, the sick, the elderly, athletes, and those with strenuous physical occupations, especially in the heat.

What happens when you dehydrate?

By the time you feel thirsty your body is already dehydrated; our thirst mechanism lags behind our actual level of hydration.

Research shows that as little as 1% dehydration negatively affects your mood, attention, memory and motor coordination. Data in humans is lacking and contradictory, but it appears that brain tissue fluid decreases with dehydration, thus reducing brain volume and temporarily affecting cell function.

As you “lose” body water without replacing it, your blood becomes more concentrated and, at a point, this triggers your kidneys to retain water. The result: you urinate less.

The thicker and more concentrated your blood becomes, the harder it is for your cardiovascular system to compensate by increasing heart rate to maintain blood pressure.

When your dehydrated body is “pushed” – such as when exercising or faced with heat stress – the risk of exhaustion or collapse increases. This can cause you to faint, for instance, when you stand up too quickly.

Thirsty? Joi Ito/Flickr, CC BY

Less water also hampers the body’s attempts at regulating temperature, which can cause hyperthermia (a body temperature greatly above normal).

At a cellular level, “shrinkage” occurs as water is effectively borrowed to maintain other stores, such as the blood. The brain senses this and triggers an increased sensation of thirst.

How much should I drink?

Normal water needs range drastically due to a number of factors, such as body composition, metabolism, diet, climate and clothing.

Surprisingly, the first official recommendation about water intake was made as recently as 2004. According to the Institute of Medicine, the adequate water intake for adult men and women is 3.7 and 2.7 litres per day, respectively.

Around 80% of total daily water should be obtained from any beverage (including water, caffeinated drinks and alcohol!) and the remaining 20% from food.

But of course, this is just a rough guide. Here’s how to monitor your own hydration:

1. Track your body weight and stay within 1% of your normal baseline. You can work out your baseline by averaging your weight (just out of bed, before breakfast) on three consecutive mornings.

2. Monitor your urine. You should be urinating regularly (more than three to four times per day) and it should be a pale straw or light yellow colour without strong odour. If less frequent, darker colour or too pungent, then drink more fluids.

3. Be conscious about drinking enough fluids. Your fluid consumption should prevent the perception of thirst.