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How much water to drink

Reference intakes for water are adequate intakes derived from what healthy populations usually consume, and they count total water from plain water, other drinks and the moisture in food. A review of the popular advice to drink eight glasses a day found no studies supporting it for healthy, largely sedentary adults in temperate climates. Randomised evidence suggests coffee and tea hydrate much like water, dehydration beyond about 2% of body mass impairs attention and coordination in experiments, and one trial in people with a history of urinary stones found fewer recurrences with higher water intake. Drinking far beyond needs can cause dangerous low blood sodium.

Evidence retrieved up to

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6 research claims

Marks

What the research shows

Australian adequate intakes for water are set for total water, which counts plain water, other drinks and food moisture, and are derived from median intakes in a national survey; in national survey data, other beverages supplied as much of the total water of adults as plain water, and food moisture about a quarter.

No known concerns at usual amountsUnclear

Who this is about

adults in Australia

Details and sources

Practical note

The literature reports NHMRC adequate intakes for adults of 3.4 L a day of total water for men and 2.8 L for women including food moisture, or 2.6 L and 2.1 L of fluids excluding food. These are median intakes of healthy people rather than minimum requirements. Graded low: descriptive cross-sectional data from two-day recalls.

Vegan note

In the same survey, higher total water intake went with higher fruit and vegetable intake, consistent with water-rich plant foods contributing to total water.

Safety note

Older adults are described as being at higher risk of dehydration because thirst perception declines with age, alongside other factors such as kidney function and some medicines.

Sources

For healthy adults in a temperate climate who are largely sedentary, a review found no scientific studies supporting the popular advice to drink at least eight glasses of water a day, and reported that drinks containing caffeine can count toward the daily total.

No known concerns at usual amountsNo clear effect

Who this is about

healthy, largely sedentary adults in a temperate climate

Details and sources

Practical note

Graded low: a single narrative review based on surveys and expert consultation, without a systematic search or pooled outcomes. The eight-glasses figure is about 1.9 L of drinks a day.

Safety note

The same review notes that larger fluid intakes are called for during vigorous work and exercise, especially in hot climates.

Sources

In randomised cross-over trials in men, moderate coffee intake by habitual coffee drinkers produced hydration markers similar to water, and urine output after coffee, tea and several other common drinks did not differ from water.

No known concerns at usual amountsNo clear effect

Who this is about

healthy adult men, including habitual coffee drinkers

Details and sources

Practical note

Trials gave four 200 mL servings of coffee a day providing 4 mg of caffeine per kg body weight over three days, or 1 L of a single drink with urine collected over four hours. Graded low: two small trials in men only with short-term surrogate markers, downgraded for indirectness and imprecision.

Sources

In experimental studies in adults, dehydration caused small but measurable impairments in cognitive performance, particularly attention, executive function and motor coordination, with larger effects when water loss exceeded 2% of body mass.

No known concerns at usual amountsRisk

Who this is about

adults in experimental dehydration studies

Details and sources

Practical note

Based on a 2018 meta-analysis of 33 studies with 413 participants and dehydration of 1-6% of body mass, overall effect size -0.21. Graded moderate: controlled experiments, downgraded one level for heterogeneity and indirectness, since dehydration was induced by exercise, heat or fluid restriction rather than arising from everyday drinking habits.

Sources

In people with a history of urinary stones, increased water intake, as part of a healthy lifestyle, may help reduce the risk of urinary stone recurrence; no randomised trials have tested it in people without previous stones.

No known concerns at usual amountsBenefit

Who this is about

adults with a history of urinary stone disease

Details and sources

Practical note

The single trial defined increased intake as drinking enough water to reach a urine volume of at least 2.0 L a day, and found fewer recurrences over five years, risk ratio 0.45. Graded low, as the Cochrane authors did: one trial, downgraded for study limitations and imprecision.

Safety note

People with kidney, heart or other conditions that affect fluid balance may have specific fluid guidance from their health professional.

Sources

Drinking far more water than the body needs can lower blood sodium to dangerous levels; published cases of water intoxication in adults involved intakes well above population recommendations, and exercise was the reason for drinking in about one in eight cases.

Caution: see the safety noteRisk

Who this is about

adults with very high water intake, including during endurance exercise

Safety note

Severe cases in the review presented with seizures or coma. Overdrinking during long endurance events is a recognised cause of exercise-associated hyponatraemia; confusion or severe headache after heavy drinking during exercise needs urgent medical care.

Details and sources

Practical note

In the reviewed cases the median intake was 8 L a day. Graded very low: almost 90% of the 177 included studies were case reports, which show that the harm occurs but not how often.

Sources