2023eBioMedicine
Dmitrieva et al.
11,255 US adults, 25 years
Middle-aged adults with blood sodium above 142 mmol/L were biologically older and had higher risk of heart failure, stroke, diabetes and early death.
Find on PubMed ↗Research library · Everyday body care
Hydration: how much water you really need
Water needs vary hugely with body size, climate and activity, so 'eight glasses a day' is a myth. But staying well hydrated over years may matter: adults whose blood is more concentrated, a sign of low fluid intake, age faster.
Good evidenceUpdated Oct 2026 · 3 new studies+39%risk of chronic disease in adults with blood sodium above 142, a sign of too little fluidDmitrieva 2023, eBioMedicine, 11,255 adults
2%loss of body weight from sweat is enough to lower endurance performanceACSM position stand, Sawka 2007
4.2 Laverage daily water turnover in men aged 20–35 (women about 3.3 L), including water from foodYamada 2022, Science, 5,604 people
01
The basics
Your body's water balance
About 60% of your body is water. It carries nutrients, cushions joints, controls temperature through sweat and lets your kidneys clear waste. Your brain watches the concentration of your blood and triggers thirst and a hormone (vasopressin) that tells your kidneys to hold on to water.
You get about a fifth of your water from food, and tea, coffee and milk all count. Needs rise with heat, exercise, altitude, pregnancy and breastfeeding. Older adults feel thirst less strongly, so they're more likely to run low.
Drinking far too much is also risky. Endurance athletes who drink well beyond thirst can dilute their blood sodium (hyponatraemia), which can be dangerous.
Your daily water balance
Rough shares for a typical adult on a mild day. Heat and exercise can double the sweat side.
02
How it works
Where water goes
1
You drink and eat
Drinks, plus water in fruit, vegetables, soups and yoghurt.
2
Absorbed
Most water is absorbed in the small intestine within about 20 minutes.
3
Blood stays balanced
Thirst and vasopressin keep blood concentration in a narrow range.
4
Kidneys fine-tune
They filter about 180 litres of fluid a day and keep only what you need.
03
Measure it
Simple hydration checks
- Urine colour: pale straw is the target. Dark yellow means drink more. (B vitamins can turn it bright yellow.)
- Weigh before and after exercise: each kilogram lost is about one litre of sweat. Losing more than 2% means drink more next time.
- Blood test: a sodium level in the upper end of normal (above 142 mmol/L) may hint that you usually drink too little.
- Thirst: for most healthy people at rest, drinking when thirsty is enough.
Try your number
Well hydratedDrink a bit moreDehydrated
18
Based on the urine colour chart (Armstrong 1994).
04
What the research says
The key studies
2022Science
Yamada et al.
5,604 people, 23 countries
Daily water turnover ranged widely, from about 1 to 6+ litres, depending on size, activity, climate and age. One rule doesn't fit everyone.
Find on PubMed ↗2007Med Sci Sports Exerc
Sawka et al. (ACSM)
Position stand
Losing more than 2% of body weight in sweat impairs endurance exercise, especially in heat. Overdrinking should also be avoided.
Find on PubMed ↗1996Journal of Urology
Borghi et al.
199 people after a first kidney stone
Drinking enough to make 2+ litres of urine a day halved the chance of another stone in 5 years (12% vs 27%).
Find on PubMed ↗2010Obesity
Dennis et al.
48 adults, 12 weeks
Drinking 500 mL of water before each meal on a diet led to about 2 kg more weight loss.
Find on PubMed ↗2005NEJM
Almond et al.
488 Boston Marathon runners
13% finished with low blood sodium, mostly from drinking too much during the race.
Find on PubMed ↗05
New in 2025–26
What the newest research adds
Drinking too little may make your body react more strongly to stress, and steady intake of water, tea and coffee is linked to longer life. But a large 2026 trial found that coaching people to drink more did not prevent repeat kidney stones, so 'just drink more' is not a cure-all.
The new studies, in order
Every study added to this page since January 2025. Green rings are the strongest evidence: large trials or meta-analyses.
Smart bottles and coaching didn't stop kidney stones
PUSH trial, 2026 (Lancet), 1,658 people with past stones. Share who had another stone within 2 years.
NewMar 2026Lancet
Desai et al. (PUSH)
1,658 people with past kidney stones, 2 years
A programme of smart bottles, coaching and rewards raised urine output a little, but repeat stones were almost the same: 19% versus 20%. Getting people to drink enough every day is much harder than advising it.
Strong evidenceRead the source ↗
NewSep 2025J Appl Physiol
Kashi, Walsh et al.
healthy adults aged 18 to 35, one week plus stress test
People who usually drank under 1.5 litres a day had a cortisol (stress hormone) response over 50% higher during a stress test than those meeting the 2 to 2.5 litre advice, even though they felt equally anxious.
Early evidenceRead the source ↗
NewSep 2025Br J Nutr
Shi et al.
182,770 UK Biobank adults, 13 years
People drinking 7 to 8 cups a day of water, tea and coffee combined had a 28% lower risk of death than those having fewer than 4. Swapping some water for tea or coffee looked helpful only once people drank more than 4 cups a day.
Good evidenceRead the source ↗
What this changes for you
Drink to thirst and keep your urine pale; water, tea and coffee all count. Around 7 to 8 cups a day of all drinks combined looks best in long-term data. Extra gadgets and coaching to force more water haven't been shown to help.
06
Take action
Easy hydration habits
Let thirst and urine colour guide you, then adjust for heat and exercise.
- 1
Drink with every meal
A glass of water with meals and snacks covers most of a normal day.
- 2
Start the day with water
You lose water overnight through breathing. A glass on waking is an easy anchor.
- 3
Weigh in for long workouts
Replace about 1.25–1.5 litres for every kilogram lost after exercise.
- 4
Hot days: add salt
For more than an hour of heavy sweating, use a drink or snack with sodium.
- 5
Eat water-rich foods
Cucumber, melon, oranges, soups and yoghurt all count.
- 6
Older adults: drink on a schedule
Thirst fades with age, so keep a bottle in sight and drink regularly.
07
Myth or fact
Common myths, checked
✕ MythEveryone needs eight glasses a day.
✓ FactNeeds vary from about 1.5 to over 4 litres; food and other drinks count too.
✕ MythCoffee dehydrates you.
✓ FactIn a study of regular coffee drinkers, moderate coffee hydrated as well as water.
✕ MythYou can't drink too much water.
✓ FactOverdrinking during long events can dangerously lower blood sodium.
✕ MythBy the time you're thirsty, you're already dehydrated.
✓ FactThirst switches on early, at small fluid losses, and works well for most healthy adults.
08
Questions
Frequently asked
Do I need sports drinks?
Only for long or very sweaty sessions. Otherwise water is fine.
Does sparkling water count?
Yes, it hydrates just as well as still water.
Is alkaline water healthier?
No good evidence. Your body tightly controls its own acidity.
09
Test yourself
True, partly true or myth?
Read each claim and pick an answer: the card flips to show the verdict and why. Not sure? Use “Just reveal”.
Card № 01True or myth?
Everyone needs exactly eight glasses of water a day.
Myth
Everyone needs exactly eight glasses of water a day.
Needs vary with size, heat and activity; food and other drinks provide a lot of fluid too.
Card № 02True or myth?
Coffee and tea count towards your daily fluid intake.
True
Coffee and tea count towards your daily fluid intake.
At normal amounts their mild diuretic effect is small; they still hydrate overall.
Card № 03True or myth?
Pale straw-coloured urine is a simple sign of good hydration.
True
Pale straw-coloured urine is a simple sign of good hydration.
Dark yellow suggests you need more; completely clear can mean you're drinking more than needed.
Card № 04True or myth?
Thirst comes too late to be useful.
Partly true
Thirst comes too late to be useful.
For most healthy people thirst works well. Older adults, people in heat and hard exercisers may need to drink before thirst.
Card № 05True or myth?
Drinking too much water can be dangerous.
True
Drinking too much water can be dangerous.
Very large amounts in a short time dilute blood sodium (hyponatraemia), which has killed marathon runners.
Card № 06True or myth?
Older people feel less thirsty and are more prone to dehydration.
True
Older people feel less thirsty and are more prone to dehydration.
The thirst signal weakens with age, so a regular routine of drinking helps.
Card № 07True or myth?
Sports drinks are needed for any workout.
Myth
Sports drinks are needed for any workout.
For under an hour of exercise, water is enough. Electrolyte drinks help in long, hot or very sweaty sessions.
Card № 08True or myth?
Even mild dehydration (1–2% of body weight) can affect focus and mood.
True
Even mild dehydration (1–2% of body weight) can affect focus and mood.
Studies show worse attention and more fatigue at this level, especially in heat.
Card № 09True or myth?
Drinking more water helps you lose weight.
Partly true
Drinking more water helps you lose weight.
Water before meals helped a little in some trials, mostly by replacing sugary drinks and adding fullness.
Card № 10True or myth?
Alkaline water is healthier than ordinary water.
Myth
Alkaline water is healthier than ordinary water.
Your stomach acid and kidneys control pH; there's no good evidence it improves health.
Nothing here yet. Try another filter.
10
Sources
References and related topics
- Dmitrieva et al. (2023). eBioMedicine. 11,255 US adults, 25 years. PubMed ↗
- Yamada et al. (2022). Science. 5,604 people, 23 countries. PubMed ↗
- Sawka et al. (ACSM) (2007). Med Sci Sports Exerc. Position stand. PubMed ↗
- Borghi et al. (1996). Journal of Urology. 199 people after a first kidney stone. PubMed ↗
- Dennis et al. (2010). Obesity. 48 adults, 12 weeks. PubMed ↗
- Almond et al. (2005). NEJM. 488 Boston Marathon runners. PubMed ↗
- Desai et al. (PUSH) (2026). Lancet. 1,658 people with past kidney stones, 2 years. Source ↗
- Kashi, Walsh et al. (2025). J Appl Physiol. healthy adults aged 18 to 35, one week plus stress test. Source ↗
- Shi et al. (2025). Br J Nutr. 182,770 UK Biobank adults, 13 years. Source ↗