Coffee Counts, Food Counts, and 8 Glasses Was Never Proven

“Drink eight glasses of water a day.” Most of us have heard it. Some of us carry a giant bottle to make sure.

So is it true? Here’s the short version: nobody has ever proven it.

Your needs depend on your body, your day and your weather. For most healthy people, thirst does a good job.

But for a few groups, drinking more has real, measured benefits.

How much water should you drink a day?

There’s no single number that fits everyone. Health bodies give rough guides, not strict targets.

The US National Academies set “adequate intake” levels of about 2.7 litres (91 ounces) a day for women and 3.7 litres (125 ounces) for men. That’s total water, from all drinks and food.

About 80% usually comes from drinks, including caffeinated ones. The other 20% comes from food like fruit, vegetables and soup.

The panel said the vast majority of healthy people meet their needs by letting thirst guide them. It deliberately gave no glasses-a-day rule.

In the UK, the NHS suggests 6 to 8 cups or glasses of fluid a day, as a guide. A simpler check: aim for pale yellow pee.

You may need more if you’re very active, in hot weather, pregnant or breastfeeding, or unwell.

What the research shows

Where “8 glasses” came from

In 2002, kidney physiologist Heinz Valtin went looking for the evidence behind “8 x 8”. That means eight 8-ounce glasses, a little under 2 litres.

His conclusion was blunt: “No scientific studies were found in support of 8 x 8.”

He also found that caffeinated drinks can count toward the daily total. The old idea that coffee “doesn’t count” didn’t hold up.

Even the official numbers aren’t requirements

A 2018 review in Nutrients by hydration researchers Lawrence Armstrong and Evan Johnson explains a subtle point.

The US figures are “adequate intakes”.

They describe what healthy, well-hydrated people typically consume. They are not proven minimum requirements.

The authors note there’s no widespread agreement on human water requirements. The body’s water system is too dynamic for one number.

Your body’s real water use varies a lot

The biggest study of daily water use was published in Science in 2022. Researchers tracked water turnover in 5,604 people, aged 8 days to 96 years, across 23 countries.

They used a precise isotope method, tracing labelled water through the body. No guessing from diaries.

Water use varied with age, body size, physical activity, athletic status and pregnancy. It also varied with climate: latitude, altitude, temperature and humidity.

People in less developed countries used more water than people in wealthier ones.

That’s exactly why one number can’t fit everyone.

When drinking more clearly helps

For most healthy people, extra water beyond thirst has no proven benefit. But for some groups, trials show a real difference.

Recurring bladder infections

In a 2018 randomised trial, 140 women with frequent bladder infections who drank under 1.5 litres a day were split into two groups. One group added 1.5 litres of water daily for a year.

The water group averaged 1.7 bladder infections over 12 months. The control group averaged 3.2.

That’s roughly half as many infections, without extra antibiotics.

Kidney stones that come back

A 2020 Cochrane review looked at water for preventing urinary stones. For people who’ve already had stones, drinking enough to make at least 2 litres of urine a day may cut the chance of another.

The evidence is limited, mostly from one trial of 220 people. But the advice is consistent across guidelines.

Can you drink too much water?

Yes, though it’s rare in everyday life.

Drinking a lot very fast can dilute the sodium in your blood. This is called hyponatraemia.

As the US National Library of Medicine explains, cells then swell with too much water. Brain cells are especially sensitive, which causes many of the symptoms.

Low sodium that develops in under 48 hours is more dangerous than slow-developing low sodium.

What earlier research found

Thirst is a decent guide

The 2004 National Academies report found most healthy people meet their needs through normal drinking at meals and in social situations, plus thirst.

Food water is real water

The same report estimated about 20% of total water intake comes from food. A diet rich in fruit, vegetables and soups adds meaningfully.

Coffee counts too

Many people still believe coffee dehydrates you. In a 2014 trial, 50 men who drank four mugs of coffee a day for 3 days stayed as hydrated as when they drank water.

We covered that trial in detail in our coffee and dehydration explainer.

Low drinkers may be running a little dry

Armstrong and Johnson’s review describes studies of healthy young women who drank 0.7 to 1.6 litres a day. Their hormone signals suggested the body was working to conserve water.

That doesn’t prove harm. But it suggests very low intakes aren’t ideal.

How it fits together

The evidence points away from rigid rules and toward signals: thirst, pee colour, activity and weather. Targets matter most for people with specific conditions.

How much should you trust this?

Promising. The broad picture is consistent, but there’s no trial-tested daily requirement for healthy people.

What makes it convincing

• Official intake levels come from large national surveys of healthy people.
• The 2022 Science study measured real water use with a precise method in over 5,600 people.
• The bladder infection finding comes from a randomised trial.

What makes me cautious

• Adequate intakes describe typical intakes, not proven needs.
• The kidney stone evidence rests mainly on one small trial.
• The bladder infection trial included only women who drank little to begin with.
• Several key papers here were only available to me as abstracts.

What this means for you

• Drink when you’re thirsty. For most healthy adults, that’s enough.
• Check your pee. Pale yellow is a good sign; dark yellow suggests drinking more.
• Count everything. Tea, coffee, milk and watery foods all count.
• Drink more in heat, exercise or illness. Your needs rise quickly.
• If you get repeat bladder infections or kidney stones, ask your doctor whether a daily target makes sense.
• Don’t force huge amounts. Gulping litres quickly can be dangerous.

The NHS guide to water, drinks and your health explains daily fluid needs and signs of dehydration.

In this video from SELF, physician Mike Varshavski answers the daily water question:

What we still don’t know

1. What’s the true minimum? No one has set a proven daily requirement.
2. Do healthy people benefit from drinking extra? Trials are lacking.
3. Does more water help weight loss? The evidence is mixed and was beyond this review.
4. How should older adults drink? Specific guidance is thin.
5. Can we predict personal needs? The 2022 study built equations, but they aren’t everyday tools yet.

My take: you can put the giant bottle down

What I like about this research is how freeing it is. You don’t need to hit a magic number.

I’m less convinced by apps that nag you to finish litres a day. The evidence doesn’t back that for healthy people.

But I wouldn’t swing to “ignore water” either. If you get bladder infections or have had a kidney stone, drinking more is one of the simplest things that works.

For everyone else, a glass with meals, a drink when thirsty and an eye on pee colour will do.