Hydration Advice: Why Individual Needs Differ

“Drink eight glasses a day” is easy to remember, which is why it has lasted. But the bodies that set nutrition guidance do not agree on one figure, and several say outright that an exact requirement for an individual cannot be given. Looking at why they differ is more useful than picking a favourite number.

Three published figures, three different things

The figures look as if they contradict each other. Mostly they do not, because they are counting different things.

Why the numbers do not match

Some include food, some do not

The National Academies figures are for total water – everything you drink plus the water in what you eat. Its report estimated that about 80% of people’s total water comes from drinks and about 20% from food. The British Dietetic Association (BDA) gives a similar split: drinks supply roughly 70 to 80% of water needs, and the rest comes from foods such as soup, stews, fruit and vegetables. A figure for “total water” will always be larger than a figure for “drinks”.

They describe populations, not you

The National Academies figures are based on what adults who appeared adequately hydrated typically consumed. EFSA says of its dietary reference values in general that they are not nutrient goals or recommendations for individuals, and that they are intended for healthy people. The BDA calls its quoted figures averages and says individual needs vary. The NHS calls 6 to 8 cups a guide.

The panel that set the US figures gave no glasses-a-day rule

The National Academies announcement said that most healthy people meet their hydration needs by letting thirst guide them, and that the panel did not set a specific number of glasses, because water comes from many drinks and foods.

What changes how much a person needs

The sources agree on the main factors:

• Heat and dry air. The BDA mentions hot days and also air-conditioned offices.
• Physical activity. The NHS says people who are active for long periods may need more. The National Academies noted that very active people in hot climates may need far more than the general figures.
• Pregnancy and breastfeeding. The NHS lists both as times when more fluid may be needed. The BDA’s table, sourced to EFSA, adds about 300 ml a day in pregnancy and 600 to 700 ml when breastfeeding.
• Illness. The NHS includes being ill or recovering from illness. The BDA mentions fever, diarrhoea and vomiting.
• What you eat. A day of soup, fruit and stew supplies more water than a day of dry foods.

Two people of the same age can therefore have quite different needs on the same day, and one person’s needs change between a desk day in winter and a walking day in summer.

Signals that are more useful than a target

Because no fixed number fits everyone, the practical guidance leans on signs.

Urine colour. The NHS suggests most people should drink enough that their urine is a clear, pale yellow. The BDA describes pale straw as a sign of good hydration and dark yellow as often a sign of dehydration, while noting that dark urine can occasionally have other causes.

Thirst and a dry mouth. The BDA lists increased thirst, a dry or sticky mouth and darker urine as early signs, with headaches, tiredness and poor concentration as possible effects.

A limit to these signals. The BDA cautions that thirst and urine colour are not reliable indicators in older adults, who may not notice they are becoming dehydrated. Its advice is to drink regularly through the day rather than waiting for thirst. The same applies if you care for someone who depends on others for drinks.

Which drinks count

The NHS counts water, lower-fat milk and lower-sugar drinks, including tea and coffee. The BDA says caffeine has a small dehydrating effect, but that the water in a cup of tea or coffee means the body still benefits overall from the fluid. Both bodies suggest keeping fruit juice and smoothies to one small glass a day because of their free sugars; the NHS puts that at 150 ml. If caffeine later in the day affects your nights, see caffeine and sleep timing.

Tap water, where it is safe to drink, costs almost nothing. You do not need bottled water or special drinks to be well hydrated.

When general guidance does not apply

Everything above is written for generally healthy adults. For some people, the right amount is set by a clinician and may be lower than the general figures. The US National Institute of Diabetes and Digestive and Kidney Diseases notes that people with chronic kidney disease may need to limit how much liquid they have, because damaged kidneys cannot remove extra fluid. The US National Heart, Lung, and Blood Institute says people living with heart failure may be asked to limit salt and fluids to reduce fluid build-up. The BDA also lists taking certain medicines among the things that affect the risk of dehydration.

If any of that applies to you, your own care team’s figure is the one to follow, and “drink more” advice from an article, an app or a friend is not a reason to change it.

A short list of your medicines makes that conversation quicker; here is how to keep one. And when a headline announces a new magic number for water, it is worth running it through the questions we use for any nutrition claim.