What dehydration does to your body, and 3 ways to tell you’re dehydrated

Your body is made up of about 60% water, so dehydration affects all body systems in some way. However, it's subtle and can creep up on you.

Discover all about electrolytes and how to replenish them, how to stay hydrated, and more health content in our health & nutrition hub.

3 foolproof ways to tell that you’re dehydrated

1. Headache

If you get a headache suddenly, it's a sign that your brain is shrinking due to lack of fluid. Your pounding headache is therefore a sign of thirst.

2. Skin ‘pinch test’

Pinch the skin on the back of your hand. Does is snap back? If so, that’s a good sign. If it stays tented, you’re dehydrated – it’s time for a drink of water!

3. Bad breath

If your mouth is like a desert and your breath smelly, that’s your third clue. Dehydration dries up saliva, and without water, it can make breath smell bad.

What happens in your body when you’re dehydrated?

Your brain actually shrinks

Water supports brain function, so even mild dehydration can impair brain function. When you’re dehydrated, brain tissue shrinks slightly, which can cause headaches and fatigue. Also, as dehydration decreases blood volume, blood flow is reduced – this limits the oxygen and nutrients your brain receives, making it harder to think clearly. Water is also critical for making neurotransmitters – the brain chemicals responsible for mood, concentration and memory.

All-round skin deterioration

Water keeps our skin looking healthy, and without it, the skin’s structure and function are disrupted. The loss of moisture makes the skin appear dull, dry or flaky. The elasticity and protective barrier of the skin becomes compromised, which can delay wound healing. The repercussions include fine lines, rough texture and increased risk of skin disorders, such as eczema and acne.

You suffer stiff joints

Synovial fluid, which lubricates joints, is mostly made of water. Dehydration can reduce the volume of this fluid, increasing friction at the joint and triggering inflammation and joint stiffness or pain.

You feel bloated and constipated

Water is essential for nearly every step of digestion, from saliva production and nutrient absorption, to waste elimination. Dehydration causes a dry mouth, making chewing and swallowing more difficult. Water also helps move food through the gut. When you’re dehydrated, stools become harder and more difficult to pass. This can leave you feeling uncomfortable, bloated, constipated and generally sluggish.

You get irritable

Unsurprisingly, dehydration can have a powerful and immediate effect on mood and emotional regulation, even before you feel thirsty. The brain is highly sensitive to changes in fluid balance. Dehydration can raise levels of cortisol, the stress hormone, and this can amplify feelings of anxiety, irritability and low patience.

When is dehydration dangerous?

Severe dehydration can leave a lasting imprint on multiple body systems. For example:

• Blood flow to the kidneys is reduced, forcing them to work harder. Prolonged dehydration can increase the risk of kidney disease, kidney stones and in extreme cases, kidney failure
• Severe dehydration can cause brain cell damage and lead to cognitive decline and memory issues, and possibly increase the risk of neurodegenerative diseases in older adults
• It can strain the cardiovascular system and may increase the risk of hypertension and other heart-related issues
• Chronic constipation and acid reflux caused by severe dehydration can increase the risk of gastrointestinal disorders like ulcers or diverticulitis
• Chronic dehydration may accelerate skin ageing and worsen joint disorders like arthritis

Are there repercussions to frequent low-level dehydration?

Yes, frequent low-level dehydration can have significant cumulative effects on health and performance too, even if it doesn’t cause immediate or obvious symptoms. Over time, the body and brain operate under a constant strain. Even 1-2% fluid loss can reduce focus, processing speed and short-term memory. It's also linked to increased fatigue, irritability and low mood, reduced physical performance and accelerated skin ageing.

When is dehydration fatal?

Dehydration can become fatal when your body continues to lose more fluids than it needs to function and you don’t replace them. As a general rule, you can survive without water for three days – after that, the body cannot function properly and will stop working. Babies, toddlers and the elderly are most at risk, as they dehydrate fast and can’t always tell you they are thirsty.

How much water do you really need to drink?

The amount of water you need each day can vary depending on age, gender, activity levels, climate and overall health. A general guideline from the UK Government is six to eight glasses of water per day, which is around 1.2 litres. This includes water, but also tea, coffee, milk and low-sugar beverages.

Tips for getting more high-fluid foods in your diet:

• Eat more fruit and veg: top hydrating options are cucumbers, watermelon, strawberries, oranges, lettuce and celery. Add them to meals and snacks
• Include soups and broths, and avoid excess salt
• Have smoothies or blended drinks made with water-rich fruits and a base of water or coconut water
• Include yogurt or cottage cheese in your diet, which are hydrating dairy products
• Make ice lollies with 100% fruit juice, or try freezing grapes
• Cook grains like rice or oats with slightly more water than usual

In conclusion, your body needs water – not energy drinks or coffee, but boring old water. Your brain thrives on it and your skin glows from it, so fill your water bottle and keep it with you, especially on hot days.

Discover more:

15 best foods to help you stay hydrated
Top 5 health benefits of drinking water
How much water should your child drink in a day?
Top benefits of lemon water
The health benefits of coconut water

Jo is a registered nutritionist (RNutr) with the Association for Nutrition with a specialism in public health. Since graduating from the University of Westminster in 2010, Jo has worked in a variety of public and private contexts, delivering weight management programmes, community cookery projects, and corporate wellness packages.

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