How Much Water Do You Actually Need?

Most people have heard that they should drink eight glasses of water a day, and very few could say where the number comes from. In 2002 Heinz Valtin, a kidney physiologist at Dartmouth Medical School, went looking for the research behind it and came back empty-handed.

The closest thing to a source Valtin found was a 1945 recommendation from the US Food and Nutrition Board of about 2.5 litres of water a day for most adults. The same passage said that most of this is already contained in prepared food, and that part seems to have been forgotten. His conclusion was that for healthy adults in a temperate climate who do little physical work, there was no scientific evidence for eight glasses on top of everything else, since a large share of daily water comes from food and from other drinks.

Valtin, H. (2002). “‘Drink at least eight glasses of water a day.’ Really? Is there scientific evidence for ‘8 × 8’?” American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, 283(5), R993-R1004.

How Much Adults Actually Need

The European Food Safety Authority set reference values for water in 2010. For adult women, an adequate total intake is 2.0 litres a day, and for men 2.5 litres. That covers drinks and food together, and food typically supplies about 20–30% of it, more if your meals include plenty of fruit, vegetables and soup.

Take the food share out and you’re left with roughly 1.4–1.6 litres from drinks for a woman and 1.75–2 litres for a man. For a man that comes close to eight 250 ml glasses, which may help explain why the old rule has hung on. The difference is that coffee, tea, milk and juice all count toward it. EFSA’s figures are meant for planning across whole populations. For an individual healthy adult, drinking when thirsty and having something with meals usually gets there without any counting.

EFSA Panel on Dietetic Products, Nutrition, and Allergies (2010). “Scientific Opinion on Dietary reference values for water.” EFSA Journal, 8(3), 1459.

What a Small Shortfall Feels Like

Mild dehydration is still worth avoiding, mainly because of how it makes you feel. Lawrence Armstrong’s group at the University of Connecticut had 25 healthy young women lose about 1.36% of their body mass in fluid, brought on by exercise, with a diuretic in some sessions. For a 60 kg woman that’s a little over 800 grams. Compared with sessions where they were properly hydrated, they reported a lower mood, found tasks harder and had more trouble concentrating, and headaches were more common. Their actual scores on the thinking tasks changed little.

That was one small study in one group, with the dehydration brought on deliberately. Still, if your afternoons tend to bring a dull headache and poor focus, low fluid intake is one of the cheaper explanations to rule out.

Armstrong, L.E. et al. (2012). “Mild dehydration affects mood in healthy young women.” The Journal of Nutrition, 142(2), 382-388.

Coffee and Tea Count

Plenty of people assume coffee and tea don’t count, because caffeine makes you pass more urine. A 2016 trial looked at exactly that. Seventy-two men drank a litre of either water or another common drink, and the researchers measured how much came back out as urine over the next four hours.

Milk and oral rehydration solution were retained better than water. Coffee, tea, cola and a 4% lager were no different from water at the amounts tested, so a few cups of tea or coffee add to your intake rather than taking from it. The trial only included men and looked at one drink at a time. Nobody should read the lager result as a reason to count beer toward anything.

Maughan, R.J. et al. (2016). “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.” The American Journal of Clinical Nutrition, 103(3), 717-723.

When You Need More

Needs go up in hot weather, with exercise that makes you sweat, and during an illness with fever, vomiting or diarrhoea. Thirst tends to weaken with age, so it becomes a less reliable guide for older adults. A rough check that works for most people is urine colour: pale yellow is generally fine, and darker than that suggests drinking more. Some supplements, riboflavin in particular, turn urine bright yellow whatever you drink, so treat it as a guide rather than a test.

Drinking far beyond thirst has its own risks. During long endurance events it can dilute the sodium in your blood, which can be dangerous. Some heart and kidney conditions also come with a fluid limit set by a doctor, and if that applies to you, their advice comes first.

Before you aim at any number, it helps to know where you actually are. Lamplit’s nutrition tracker has a one-tap water log, and it’s free. Log every glass of water for three ordinary days without trying to change anything. You might find you drink more than you thought, or a good deal less, and you’ll see which days were the dry ones. Then you have a real number to set against the figures above, keeping in mind that food, tea and coffee cover part of the total.

Log your first glass of water in Lamplit. It takes one tap and it’s free.