Why is water essential?

Amy Wood | 04 Oct, 2021

Water is essential for human health – we can't survive for more than about 3 days without it! But why exactly is this? To answer that, I'll discuss what water is used for in our body and what other factors can affect our hydration levels.

How much do I need to drink?

As our bodies have no way of storing water, we need to ensure we're replenishing our supply adequately every day. Water needs can vary based on age, sex, activity levels, the weather, and certain medical conditions. The NHS recommends we consume at least 6-8 250ml glasses/mugs of fluid per day. This includes water, low-fat milk, sugar-free drinks, tea, coffee and squash.

How do our bodies use water?

Our bodies are composed of around 60% water. We use water in all our cells for lots of important bodily functions, including:

  1. Regulating our body temperature
  2. Maintaining a healthy blood volume (our blood is around 90% water)
  3. Delivering oxygen and nutrients to our cells
  4. Keeping our joints cushioned and lubricated
  5. Protecting the brain and spinal cord from impact by acting as a shock absorber
  6. Producing saliva, aiding digestion and preventing constipation
  7. Removing toxins and waste products through urination

Our organs are largely dependent on water too. This includes our brains, which are around 70-80% water! We need water to make hormones and neurotransmitters, and to facilitate communication between brain cells. Other tissues contain a large volume of water, including the lungs (~80%) and skin (~65%). Even our bones contain water – around 30%.

Increased water intake is also associated with healthier BMI and improved weight loss, as filling our stomachs with fluids leaves less room for food, meaning we feel full on a smaller portion. Many of us often mistake thirst for hunger, so end up reaching for snacks when all we need is to rehydrate!

How do we lose water?

Our bodies are constantly losing water through breathing, sweating and going to the toilet. Most adults lose around 2 to 3 litres of water per day. However, certain factors can further increase water loss. For example, when we exercise, our breathing and sweating intensify. We also use more water when it’s hot outside, as our bodies attempt to keep us cool by sweating more than usual.

How do I know I am not drinking enough?

Often when we're dehydrated, we feel tired, sluggish, and often feel a headache coming on. Worse cases lead to dizziness and feeling faint.

If you haven't yet experienced any symptoms, a straightforward way for healthy people to gauge their hydration status is by the colour of their urine. A pale yellow colour indicates you're well-hydrated – dark urine or urine with a strong odour is a sign you need to be drinking more.

Why does alcohol dehydrate me?

Many of us will be familiar with that pounding headache following a night of drinking, and that's primarily caused by dehydration.

When we're running low on water, the electrolytes in our bodies are more concentrated than normal. Special detectors can sense this increased concentration, and subsequently call on a hormone called vasopressin, or anti-diuretic hormone (ADH) (diuresis is just a fancy word for urination!). This hormone tells our kidneys to retain water, conserving the volume of water in our bodies.

Alcohol interferes with this process by reducing the amount of ADH we produce, meaning the opposite happens: our kidneys use more water and make more urine. This is why you'll find yourself nipping to the loo more frequently than normal on a booze-fuelled night out.

What are the problems of not drinking enough?

Not drinking enough can have unpleasant effects, leading to urinary tract infections (UTIs), kidney stones and constipation. A lack of water also means our brains need to work harder to perform thinking tasks and decision making, lowering our energy levels and mood. The headaches and discomfort caused by dehydration can also make it harder to sleep, adding to fatigue. Ultimately, a severe lack of water can lead to very serious symptoms such as fainting and shock that can be life-threatening. This is why it's so crucial to maintain a good supply of fluid.

Can you drink too much?

Most healthy people don't drink enough to be at risk of overhydration. It can sometimes happen when people overestimate how much water they will need for exercise or hot weather. Drinking too much can cause the electrolytes in our cells to become too diluted, preventing them from functioning properly. A drop in sodium levels in the blood, also called hyponatremia, can result in more serious side effects, such as muscle spasms and swelling of the brain, which can be fatal. As always, the message is moderation - be aware of your hydration levels and aim for your 6-8 glasses a day.

Nutritionist Amy Wood (ANutr), MSci BSc Nutrition has a keen interest in the relationship between diet and health. Having been published in the European Journal of Nutrition, Amy is passionate about making evidence-based nutrition accessible to everyone and helping others to adopt a food-focussed approach to taking control of their health.