The hydration equation: One simple, science-backed way to tell if you’re drinking enough water
No, you don't necessarily have to drink eight glasses of water a day.
In the UK, the NHS recommends six to eight glasses, or up to 1.2 litres of fluids, a day, pointing out that we also get water from food. Harvard Medical School recommends four to six cups a day. But it’s the more extreme two litres of water a day advice that has won the internet.
In 2016, the idea that getting most of your liquids from water is more beneficial was debunked by Dr Stuart Galloway, an associate professor in physiology, exercise and nutrition at the University of Stirling. His study showed a range of drinks, including diuretics like lager and instant coffee, didn’t stimulate any additional fluid loss than water when drunk in normal quantities.
But nobody can really say how much everyone needs to drink, as we all have different bodies, diets and activity levels, not to mention varying environments (hot, dry, humid, etc). Most people can tell if they need more water because they feel thirsty, although this urge diminishes in old age. If in doubt, for the majority of adults, the number of trips to the loo can be a potentially useful guide to adequate hydration, says Galloway.
“It accounts for differences in fluid losses due to environment, or activity level, as well as variations in fluid intake. A rough rule of thumb would be four to six visits to the toilet to pee during a typical day, if adequately meeting your water requirements.”
More than six pees means you’re overdoing it, while less than four means you probably need to drink more.
“This approach can have some shortcomings, such as impacts of any alteration in kidney function with age, certain medications, or different beverage compositions that all affect urine concentration and volume,” he warns. “So it is a rough rule of thumb rather than an accurate guide.”
Urine colour is also useful, with similar caveats, he says. “To get the best idea, don’t rely on a single marker but evaluate using a combination.”
About our expert, Dr Stuart Galloway
Dr Galloway is a professor of exercise physiology at the University of Sterling. He is also group lead for the University’s Physiology, Exercise and Nutrition Research Group, and has over 90 articles published peer-reviewed research articles, review articles, and book chapters. His research focuses on human nutrition and exercise metabolism, and on fluid and electrolyte balance.
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