Dehydration: 8 signs that don't lie (and the urine colour test)
by AUA Hydrate
We tend to picture dehydration as a desert problem: cracked lips, raging thirst, collapse. In reality it starts much earlier, and much more quietly: a headache at the end of the day, focus fading around 4 pm, urine darker than usual. Nothing alarming, which is exactly why it goes unnoticed. Here are the eight signs to know, the most reliable test you can do at home, and the warning signs that are anything but trivial.
⏱ The essentials in 20 seconds
- Mild dehydration starts at around 1 to 2% of body weight lost as water: about one litre for 70 kg.
- Even at this stage, controlled studies measure more fatigue, less focus and more headaches.
- The most reliable test at home: urine colour, on a scale of 1 to 8.
- Confusion, unusual drowsiness, a racing heart, barely any urine: see a doctor without delay; in an emergency, call 144.
The 8 signs, from the subtlest to the clearest
None is enough on its own: a headache has a thousand possible causes. What matters is how they add up, and the context (heat, exercise, illness, a day without drinking).
1. Dark urine
The most reliable sign. When water runs short, the brain releases vasopressin, the hormone that tells the kidneys to hold on to water. Urine becomes more concentrated and darker: pale straw means all is well; dark yellow or amber means it's time to drink.
2. You go to the toilet less often
Most adults urinate six to eight times in 24 hours, but the normal range is wide. What counts is the change from your usual pattern: only once or twice all day, and your kidneys are in saving mode.
3. Dry mouth, tight lips
Saliva production drops when the body is short of water. A sticky mouth and dry lips are among the first signs listed by the UK's National Health Service (NHS).
4. Headache
A dull, diffuse pain that sets in towards the end of the day. In a University of Connecticut study (Armstrong, 2012), mildly dehydrated women reported headaches more often. To tell it apart from a migraine, read migraine or dehydration headache.
5. Fatigue and fading focus
The most underestimated sign. In two controlled studies, losing just 1.6% of body weight in men (Ganio, 2011) and 1.4% in women (Armstrong, 2012) was enough to impair alertness, working memory and mood. The 4 pm slump isn't always about sleep.
6. Feeling dizzy when you stand up
Less water means slightly less blood volume. When you go from sitting to standing, your blood pressure struggles to keep up: mild dizziness, vision blurring for a few seconds. If it persists, see a doctor.
7. Cramps and heavy legs
They mostly appear after heavy sweating, when you have lost salt as well as water. The link is not automatic: a cramp starts in the nerve (see night leg cramps).
8. Dark circles, sunken eyes
A later sign: hollow eyes, dull complexion. By this point, the dehydration is probably no longer mild.
The urine colour test
In 1994, American physiologist Lawrence Armstrong validated an eight-colour urine scale by comparing it with laboratory measurements (urine specific gravity and osmolality). Athletes and armed forces still use it today:
well hydrated
drink more
dehydrated
Three precautions to make the test reliable:
- 🌅 Ignore your first urine of the morning: after a night without drinking, it is always darker.
- 💊 Some vitamins skew the result: vitamin B2 (riboflavin) in multivitamins turns urine bright yellow for a few hours. Beetroot and some medicines do too.
- 👵 In older people, the test is not enough: in care homes (DRIE study, 2019), neither urine colour nor the usual clinical signs correctly identified dehydration. Only a blood test settles it.
And the skin-pinch test on the back of the hand? In adults it is unreliable: with age, skin loses elasticity and the fold stays up even when you are well hydrated.
💡 AUA tip: around 4 pm, ask yourself one question. How many times have I been to the toilet since this morning, and what colour was it? Twice and dark yellow: a large glass now, then regular sips, beat a litre gulped down in one go tonight.
Why thirst sometimes comes too late
For healthy adults at rest and at normal temperatures, thirst is an honest guide. Except in three situations:
- 🧓 With age: after 24 hours without water, healthy older men felt less thirsty and drank less than young men, even though their blood was more concentrated (Phillips, New England Journal of Medicine, 1984).
- ❄️ In cold weather: thirst drops while losses continue, see why we feel less thirsty in the cold.
- 🏃 During exercise and heat: we spontaneously drink less than we sweat out.
Mild, moderate, severe: where are you?
| Level | Water lost | What you feel | What to do |
|---|---|---|---|
| Mild | 1 to 3% | Thirst, dry mouth, dark urine, slight fatigue | Drink regularly; after heavy sweating, water and salt |
| Moderate | 4 to 6% | Dizziness, cramps, irritability, faster heartbeat | Drink, rest somewhere cool; see a doctor if nothing improves |
| Severe | 7% or more | Confusion, drowsiness, barely any urine, cold skin | Emergency: 144 |
Percentage of body weight, clinical guide values for adults (StatPearls, NCBI). In children the thresholds are lower.
Warning signs that call for a doctor
- 😵 Dizziness on standing that doesn't go away.
- 💓 An unusually fast heartbeat or breathing.
- 😴 Unusual fatigue or drowsiness.
- 🚱 Very little or no urine over a whole day.
Call 144 (112 elsewhere in Europe) in case of confusion, difficulty breathing, cold, pale or blotchy skin, or if the person is hard to wake. In babies, fewer wet nappies, no tears or a sunken soft spot (fontanelle) mean calling the paediatrician straight away. With diarrhoea or vomiting, what you need is an oral rehydration solution from the pharmacy, not a shop-bought electrolyte drink.
Rehydrating, in practice
- 💧 Day to day, tap water is enough: see what Swiss tap water contains and how much water to drink per day.
- ⏱ In small, regular sips: a litre downed in one go is partly flushed out quickly by the kidneys.
- 🧂 After heavy sweating, you need to replace sodium along with water, or you dilute what is left: see why water alone is not enough.
- ☕ Coffee and tea count towards your intake; alcohol does not (coffee, tea, alcohol).
Where AUA Hydrate comes in
Let's be clear: AUA Hydrate does not treat dehydration and replaces neither water nor medical advice. It is for the days when you have sweated a lot. One stick in 500 ml of water provides 184 mg of sodium, 192 mg of potassium and 26 mg of magnesium, among 10 active ingredients (six electrolytes, two trace elements, two vitamins), for 2 kcal and zero sugar. Magnesium contributes to electrolyte balance. Formulated in Switzerland by nutritionists.
For the days you sweat
Choose from our three flavours: strawberry, peach, lemon.
Frequently asked questions
How long does it take to rehydrate?
Mild dehydration usually clears within a few hours of regular drinking. If the signs persist, see a doctor.
Is drinking lots of water always enough?
Day to day, yes. After heavy sweating, very large amounts of plain water dilute the sodium in your blood: the volume is there, the balance is not.
Does a headache mean I'm dehydrated?
Not necessarily. If it comes with dark urine after a day when you drank little, have a large glass of water and see if it eases. If not, look for another cause.
Are children and older people more at risk?
Yes. Young children can't always say they're thirsty; in older people thirst fades and the usual signs mislead. Offer them drinks regularly, without waiting to be asked.
