Eight glasses of water a day. Says who?

Cross-sectional study. 5,604 people. single measurement period per person (isotope elimination over days); no health follow-up.

One snapshot in time. Nobody was told what to do. A clue, not proof.

Who: 5,604 people (3,729 female, 1,875 male) aged 8 days to 96 years from 23 countries (published abstract; author manuscript and press releases say 26), from industrialised populations to hunter-gatherers and subsistence farmers

• Young men≈4.3 La day
• Young women≈3.4 La day

Water turnover ranged from ~1 to ~6 L a day. Authors: one size does not fit all

Not all that water came from drinks.

More to know, from our read of the paper:

• Water turnover is NOT the same as how much you need to drink: ~10% is metabolic water, 2-3% each from breathed-in vapour and skin; intake from food + drinks is ~85% of turnover
• The study did NOT measure how much water came from food vs drinks; the authors say no study has adequately addressed this (self-report surveys suggest 20-50% from food, likely underestimated)
• Did not assess hydration status: cannot say whether participants were adequately hydrated
• Equation explains only 47.1% of the variation in turnover; values are averages under normal conditions, not for illness (e.g. diarrhoea)
• Does not show that drinking less (or more) changes health; it does not recommend drinking less

Scientists tracked how much water moves through the body each day (in and out) in 5,604 people aged 8 days to 96, using a harmless water tracer instead of asking people (a measurement study). It ranged from about 1 to 6 litres a day: about 4.3 L for men and 3.4 L for women in their 20s, and about 3 L over 70. Heat and sport each added about 1 litre. Not all of that water comes from drinks (some comes from food, and the body makes a little), so this does not say how much you must drink. The authors say 8 glasses a day for everyone is not backed by objective evidence: one size does not fit all.

We read the abstract and the full text of this paper and checked every number on this page against it (18 facts, checked on 2026-10-01). The study design is named above. Cohort and other observational studies are described as linked, not proven. Found an error? Email theis@onestudyaday.com.