Water
H₂OToday
3-day window. Water-soluble and not stored, so a short run of days is the signal.
What it is
Water makes up roughly 60% of your body and is the medium every biochemical reaction happens in. The commonly quoted target of about 3.7 L a day for men and 2.7 L for women is total water — around a fifth of it comes from food, so the amount you actually need to drink is meaningfully lower. There is no need to count it precisely: thirst is a reliable guide for healthy adults, and pale yellow urine is a better check than any number.
Reference intake
Your daily target, personalised to your age and sex.
- AI
- 3700 mLmale_19_plus
- Upper limit
- none established
What the evidence says
Wording reflects how strong the evidence actually is.
Prevents dehydration and maintains cardiovascular and thermoregulatory function
Reduces recurrence of kidney stones
May aids weight loss when drunk before meals, though the evidence is thin
Has been associated with: everyone needs eight glasses of water a day
Deficiency
dehydration
Common and usually mild. Clinically meaningful dehydration is frequent in older adults, in whom the thirst response is blunted.
Early signs
- thirst
- dark yellow urine and reduced output
- dry mouth
- headache
- fatigue and reduced concentration
Higher risk
- older adults, whose thirst response declines with age
- endurance athletes and outdoor workers in heat
- people with vomiting, diarrhoea or fever
- people on diuretics
- people with poorly controlled diabetes
Excess
hyponatraemia
No upper limit has been set, because the healthy kidney excretes a surplus readily — up to roughly 0.8–1.0 L per hour.
From supplements
- frequent urination and disturbed sleep
- dilutional hyponatraemia when intake exceeds renal clearance
At extreme doses
Drinking far beyond renal clearance dilutes plasma sodium and can cause cerebral oedema, seizures and death. This is rare but real, and is documented in endurance events and in water-drinking contests. Sustained intake above roughly 1 L/hour is where the risk begins.
Absorption
Essentially complete, mostly in the small intestine
Helps absorption
- small amounts of sodium and glucose, which drive co-transport
- drinking steadily rather than in large single volumes
- high-water foods, which arrive with electrolytes and are absorbed steadily
Hinders absorption
- very high osmolality drinks, which can draw water into the gut
- alcohol, which suppresses vasopressin and increases urinary loss
- large single volumes, much of which are simply excreted
Top sources
Per 100 g, in mL. FoodData Central IDs are resolved by the importer, not written by hand.
- 95.2 mL
Cucumber, raw
104 g (1 cup sliced)
- 94.5 mL
Tomato, raw
123 g (1 medium)
- 91.5 mL
Watermelon, raw
152 g (1 cup)
- 89.2 mL
Milk, 2% fat
244 g (1 cup)
- 86.8 mL
Orange, raw
131 g (1 medium)
- 85.1 mL
Greek yogurt, plain, nonfat
170 g
Ranking these by what you already eat needs the real log — see docs/STATUS.md.
Sources
Last reviewed 2026-08-11