Whether mild "under-hydration" has long-term effects

Some research links habitually low fluid intake to health markers, but whether nudging up intake in healthy people changes outcomes is unsettled.

The exact ideal for each person

Needs are individual; there’s no single correct daily number.

Hydration, honestly — the “8 glasses a day” myth

Hydration is important and endlessly over-complicated. The honest headline: the famous "8 glasses a day" rule has no solid scientific basis, your needs vary, much of your water comes from food and other drinks, and thirst is a reasonable guide for healthy people.

Where the rule came from

The "drink eight 8-ounce glasses of water a day" rule is a catchy number with no strong scientific origin. A kidney physiologist’s well-known review found no evidence supporting it for healthy adults in normal climates.

That doesn’t mean hydration is unimportant — it means the specific rule is a myth, and the real answer is "it depends".

What the evidence supports

Needs vary; the body is good at this.

The “8×8” rule isn’t science — There’s no good evidence behind the eight-glasses rule for healthy people in temperate climates. It’s a well-meaning slogan, not a physiological requirement. (hype)
Food and all drinks count — A large share of daily water comes from food (fruit, vegetables, soups) and from coffee, tea, milk and other drinks — not just plain water. Coffee and tea count toward, not against, your fluids. (established)
Thirst and urine colour are decent guides — For most healthy people, drinking to thirst and aiming for pale-yellow urine works well. Needs rise with heat, exercise, illness, pregnancy and body size. (emerging)

When to be more deliberate

Older adults, athletes, hot climates, illness — Thirst blunts with age, and heavy sweating, heat, fever or vomiting raise needs — situations where consciously drinking more (and replacing electrolytes when losses are large) matters. (note)
Overdrinking is a real, if rare, risk — Forcing large volumes far beyond thirst can, in extreme cases, dangerously dilute blood sodium. More is not automatically better. (note)

What we still don’t know

Whether mild "under-hydration" has long-term effects Some research links habitually low fluid intake to health markers, but whether nudging up intake in healthy people changes outcomes is unsettled.
The exact ideal for each person Needs are individual; there’s no single correct daily number.

Key terms

Hydration Your body’s water balance; well-regulated in healthy people by thirst and the kidneys.
Electrolytes Minerals like sodium and potassium lost in sweat; worth replacing during heavy or prolonged sweating.
Hyponatraemia Dangerously low blood sodium, which can result from drinking far too much water.