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 evidence+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
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.
06
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.
07
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.
08
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 ↗