What Is Dehydration?

Water makes up roughly two-thirds of the human body and is involved in almost everything it does, from regulating temperature to carrying nutrients and removing waste. When fluid runs low, things start to go wrong surprisingly quickly. Dehydration is common, usually easy to put right, and just as easy to overlook. Here is how to recognise it and respond. This is general information, not medical advice — for your own symptoms speak to your GP, pharmacist or NHS 111.

What dehydration is

Dehydration is the state of losing more fluid than you take in, so your body does not have enough water to function as it should. It is not a disease in itself but a shortfall, and it ranges from mild and barely noticeable to severe and dangerous.

Your body loses water constantly and unavoidably — through urine, sweat, breathing and even in the moisture you exhale. Normally you replace those losses by drinking and through the food you eat. Dehydration sets in when that balance tips: either losses rise (through sweating, illness or heat) or intake falls (you simply do not drink enough), or both at once.

Because water is the medium in which so many bodily processes happen, being short of it affects energy, concentration, temperature control, digestion and circulation. That is why even mild dehydration can leave you feeling below par without an obvious reason.

The signs to watch for

Dehydration often announces itself before it becomes serious, if you know the signals. In adults and older children, common signs include:

• Feeling thirsty — a simple but reliable cue.
• Dark yellow, strong-smelling urine, and going to the toilet less often than usual.
• A dry mouth, lips and eyes.
• Tiredness and difficulty concentrating.
• Dizziness or light-headedness, especially on standing.
• A headache. Mild dehydration is a well-recognised trigger, and our guide to tension headaches lists it among the everyday causes.

One of the most practical home checks is the colour of your urine.

Pale, straw-coloured urine generally means you are well hydrated. Dark yellow or amber urine is a common sign you need to drink more.

In babies and young children, the signs differ and matter more, because they dehydrate faster. Look for fewer wet nappies, few or no tears when crying, a sunken soft spot (fontanelle) on a baby's head, drowsiness, and sunken eyes.

What causes it

Anything that increases fluid loss or reduces intake can lead to dehydration. The usual causes are:

• Not drinking enough, particularly when busy, distracted or simply not feeling thirsty — which is more common in older adults.
• Hot weather and exercise, where sweating ramps up fluid loss. Heavy or prolonged activity also loses electrolytes — minerals such as sodium and potassium that the body needs alongside water.
• Illness, especially with vomiting, diarrhoea, fever or sweating. Stomach bugs are a leading cause, particularly in children.
• Drinking a lot of alcohol, which makes you pass more urine.
• Some medicines, such as certain diuretics ("water tablets").
• Uncontrolled diabetes, which can cause excessive urination.

Who is most at risk

While anyone can become dehydrated, some groups need extra care:

• Babies and young children, whose smaller bodies lose fluid quickly and who cannot tell you they are thirsty.
• Older adults, whose sense of thirst can dull with age and who may drink less to avoid trips to the toilet.
• People who are ill, particularly with vomiting or diarrhoea.
• Very active people and outdoor workers, especially in heat.
• People with certain long-term conditions, including diabetes and kidney problems.

For these groups, it is wise to encourage regular drinking rather than waiting for obvious thirst.

How much should you drink?

There is no single magic number, but the NHS suggests most adults aim for around 6 to 8 cups or glasses of fluid a day. That total includes water, lower-fat milk and sugar-free drinks — and yes, tea and coffee count towards it, despite the common myth that they are purely dehydrating. Food contributes too, since many fruits and vegetables are largely water.

You will need more in hot weather, when exercising, and when you are unwell with a fever or sickness. A sensible approach is to drink regularly through the day, have a glass with each meal, and use thirst and urine colour as your guides rather than fixating on an exact figure.

How to rehydrate

For mild dehydration, the fix is straightforward: drink more fluid. Take small, frequent sips rather than gulping a large amount at once, especially if you feel unwell. Water is fine for everyday rehydration.

When dehydration follows heavy sweating, vomiting or diarrhoea, you lose salts as well as water. In these cases oral rehydration solutions — sachets available from any pharmacy that you dissolve in water — can be more effective than water alone, as they replace both fluid and electrolytes in balanced amounts. A pharmacist can advise on the right product, particularly for children and older adults. Staying hydrated also supports other body systems; for example, even mild fluid shortfalls can nudge up your resting heart rate as the body works to maintain circulation.

When to get help

Most dehydration is easily reversed at home, but severe dehydration is a medical emergency and needs urgent treatment, sometimes with fluids given directly into a vein.

Seek urgent medical help if you or someone else:

• Is unusually drowsy, confused or difficult to wake.
• Is not passing urine, or a baby has had few or no wet nappies.
• Has sunken eyes, a sunken soft spot in a baby, a fast heartbeat or rapid breathing.
• Cannot keep any fluids down because of repeated vomiting.

Call NHS 111 for advice if you are unsure, if symptoms are not improving with home rehydration, or if you are worried about a baby, young child or older relative.

The bottom line

Dehydration is simply losing more fluid than you take in, leaving the body short of the water it relies on. The early signs — thirst, dark urine, tiredness, dizziness and headache — are easy to spot once you know them, and urine colour is a handy daily check. Most cases are quickly fixed by drinking fluids, with rehydration solutions useful after sickness or heavy sweating. The groups most at risk are the very young, older adults and anyone who is ill or very active. Drink regularly, watch for the warning signs, and treat severe dehydration as the emergency it is.