Humans are mostly water. The broad spectrum, averaged proportion is 2/3 , which naturally varies according to conditions and circumstances. A higher proportion of body fat, for example, reduces the overall percentage, because fatty tissue contains much less water than other body tissues, including bone. Even so, the body has a lot of water – more than 45 liters for a 70kg person, enough for a quick shower. With the water from 3 bodies, you could wallow in a reasonably large bathtub.
It is not possible to keep the water in the body. Water must leave to carry away dissolved and potentially harmful wastes, chiefly in urine. Around 3 liters daily generally suffices for this turnover. But it’s more in hot conditions, when active, and when imbibing substances such as alcohol.
Here’s What Happens to Your Body When You’re Dehydrated
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 percent 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.
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 percent of total daily water should be obtained from any beverage (including water, caffeinated drinks and alcohol!) and the remaining 20 percent from food.
But of course, this is just a rough guide. Here’s how to monitor your own hydration:
- Track your body weight and stay within 1 percent of your normal baseline. You can work out your baseline by averaging your weight (just out of bed, before breakfast) on three consecutive mornings.
- 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.
- Be conscious about drinking enough fluids. Your fluid consumption should prevent the perception of thirst.