How much water to drink per day: the 8-glasses myth

"Drink 2 litres of water a day" is probably the most repeated and least justified health advice of recent decades. The figure comes from a misreading of a 1945 US National Research Council report — which said "the average adult needs 2.5 L of water a day" and immediately clarified: "most of this amount comes from food". That nuance was lost, and decades later we still carry plastic bottles thinking we need 2 extra litres.

This article dismantles the myth with data, and gives you the real formula: how much water you actually need for your profile.

Where the 8-glasses rule actually comes from

In 1945 the US Food and Nutrition Board issued a general recommendation of 1 ml of water per calorie consumed. For a 2,000-2,500 kcal diet that's 2-2.5 L a day. And they explicitly noted:

"Most of this quantity is contained in prepared foods."
— National Research Council, 1945

Decades later, the bottled water industry and some popularisers reduced the message to "drink 8 glasses of water a day", forgetting that food already provides 20-40% of the total.

The European Food Safety Authority (EFSA) in 2010 reviewed all studies and set:

• Adult women: 2.0 L total/day (water + food)
• Adult men: 2.5 L total/day (water + food)

Of that total, food provides ~700-1,000 ml. Fruits and vegetables are 80-95% water. Soups, stews and rice also contribute a lot. Coffee, tea and infusions count as fluid (despite the myth that they "dehydrate").

The honest formula for your profile

The useful figure is 30-35 ml per kg of body weight per day, total. From that you subtract what food provides.

Figures for adult in temperate climate with moderate activity. Second column assumes a diet contributing ~700 ml/day of water (average European diet with vegetables and hot meals).

Adjustments by condition

The base figure goes up or down depending on your situation. Add or subtract from the total:

• Moderate exercise (1 h): +500 ml
• Intense exercise or heavy sweating: +1 L per hour of activity
• Summer / heat (>28 °C): +500-800 ml
• Cold winter with indoor heating: +200 ml (heating dries the air)
• Flight >3 h: +250 ml/hour of flight
• Fever or diarrhoea: +500-1,500 ml depending on severity
• Pregnancy: +300 ml/day
• Breastfeeding: +700 ml/day
• High alcohol or caffeine intake: +200-400 ml

How to tell you're hydrated — the colour test

The simplest and most reliable way is to look at the colour of your urine mid-morning or mid-afternoon:

• Pale yellow, almost clear: optimal hydration
• Light yellow: adequate hydration
• Dark yellow: start drinking more
• Amber or brown: notable dehydration, drink now

First morning urine is almost always dark — that's normal. Check from the second one on.

Thirstis a poor indicator because it arrives late: by the time you're thirsty you've already lost 1-2% of body water. Better to trust the colour.

Common myths

Myth: "Coffee and tea dehydrate"

False. Although caffeine has a mild diuretic effect, the liquid they provide far exceeds what they make you eliminate. A coffee delivers ~180 ml net. Tea and infusions deliver nearly 100%.

Myth: "Drinking lots of water makes you lose weight"

Partly true but overstated. Drinking water before meals cuts calorie intake by 5-10% (University of Birmingham studies). But it doesn't burn fat on its own. If you want to lose weight, diet does 90% of the work.

Myth: "Drinking cold water burns more calories"

Technically true but irrelevant. Heating 1 L of water from 4 °C to 37 °C consumes ~33 kcal — equivalent to 5 minutes of walking. Anecdote, not strategy.

Myth: "If you're older you need to drink more"

The opposite: with age you lose the sense of thirst and need to remember to drink, but the amount doesn't change much. Adults over 65: same 30-35 ml/kg, but with vigilance because they don't feel when they need it.

Myth: "You can't drink too much water"

False and potentially dangerous. Dilutional hyponatremia — drinking more than 5 L in a few hours — can cause coma and death. Reported cases in marathoners and absurd contests. Rule: never more than 1 L per hour for many hours in a row.

How to structure hydration through the day

1. On waking:250-500 ml (compensating overnight dehydration). The classic "glass of water on waking".
2. Before each meal: 200-300 ml (improves digestion + cuts calorie intake).
3. Every hour through the day: 150-200 ml sip without waiting to be thirsty.
4. After physical activity: 500-750 ml per 30 min of exercise.
5. 2 hours before bed:stop drinking so you don't break your sleep with bathroom trips.

What water to drink — the uncomfortable part

The water matters as much as the amount. If you're drinking 2 L/day of PET bottled, you're also ingesting 175 million microplastic particles per year (per Columbia University 2024 study). Options from best to worst:

• Tap water filtered with activated carbon — fewer microplastics, good taste, ~€70/year in filters
• Tap water straight — safe in Europe, free, but can have annoying chlorine
• Glass bottled — few microplastics but very expensive and high CO₂ footprint
• PET bottled — worst option, high microplastics and high cost

If you're outside home (office, Camino, travel), a filter bottle lets you drink from any fountain without carrying bottled. €60 solution that lasts 18-20 months.

Special situations

Exercise

Rule of thumb: 500 ml before, 150-250 ml every 20 min during, 500-1,000 ml after. If you sweat a lot, add electrolytes (one tablet per 2 L). Commercial isotonic drinks not needed for under-90-min activity.

Camino de Santiago / long trekking

1 L every 2 hours of walking in the heat. Carry at least 1.5 L in your pack, more for stretches without fountains (Meseta in summer can require 3 L between villages). A filter bottle lets you refill at any fountain without hesitation.

Office work

Sedentary, air conditioning or heating: 1.5-2 L/day spread out. 1 L bottle on the desk, refill in the afternoon. Coffee and tea count.

Older adults

Don't rely on thirst. Set reminders every 90 min. Soups and natural juices count. Watch urine colour and dehydration signs (dizziness on standing, dry skin).

Bottom line

The 2-litre rule is a simplification. The real figure depends on your weight, activity and climate. For most European adults: 1.2-2.0 L of drunk liquid per day is enough (plus what food provides).

Don't force yourself to drink more than you need — hyponatremia is a real risk. The most reliable way to know you're OK is urine colour.

And if you want to make that decision well: activated carbon filter at home to cut microplastics and cost, filter bottle for outside. Drink enough + drink well.