As the UK braces for a scorching bank holiday weekend with temperatures exceeding 30°C, health experts are urging the public to rethink their water intake. While the traditional advice of eight glasses a day has long been a benchmark, new research suggests individual needs vary significantly—especially during extreme heat.
The Rising Threat of Heat-Related Illness
Heat-related deaths in Europe have surged by approximately 30% over the past two decades. In 2023 alone, nearly 48,000 people succumbed to heat-related causes, a sharp increase attributed to the human body's struggle to cope with heat stress. Alex Ruani, a researcher in nutrition science at University College London (UCL), warns that in extreme heat, dehydration can set in much faster than usual. "Generally, humans die after a few days without fluid intake," she explains. "But in extreme heat, where fluid and electrolyte losses are greater, death from dehydration comes a lot sooner."
Chronic dehydration can lead to urinary tract infections, kidney stones, reduced physical and mental performance, constipation, delirium, seizures, and low blood pressure. These conditions can exacerbate heat exhaustion and heat stroke, making proper hydration critical.
How Much Water Do We Actually Need?
The human body is about 50-70% water, and every cell requires it to function. Despite this, a 2023 study by AirUp found that 45% of British people drink the equivalent of just one glass of water per day, far below the NHS guideline of six to eight glasses. During hot weather, exercise, or pregnancy and breastfeeding, this requirement increases.
Ruani emphasizes that staying hydrated has numerous benefits: "Proper hydration enables your brain to work properly, regulates body temperature, maintains steady hormone production, promotes cell longevity, aids digestion, and helps eliminate toxins." A 2023 study from the National Institutes of Health (NIH) involving 11,255 adults found that those with higher fluid intake had fewer chronic illnesses and slower biological ageing.
Individual Variation and Thirst Cues
The "eight glasses" rule may be a myth, according to a 2022 study from the University of Wisconsin-Madison. Researchers measured water intake of over 5,000 people across 26 countries and found that needs vary widely. Professor Tim Noakes, author of Waterlogged, advises: "The body will tell you if you need to drink more and when. If you're thirsty, drink." He notes that men generally need more fluid than women, and factors like menstruation increase requirements.
However, older adults may have a reduced sense of thirst, putting them at higher risk. "As we get older, our sense of thirst is reduced," Ruani adds. "Some elderly adults may also be on medications that increase urine output."
Signs of Dehydration and How to Monitor
Ruani suggests checking urine colour: it should be light and clear, like pale straw, throughout the day. A "skin turgor test"—pinching the skin on the back of your hand—can also indicate dehydration if the skin doesn't snap back immediately. Ideally, you should urinate five to six times daily; less may signal insufficient fluid intake.
Nutritionist Sam Rice notes that tea, coffee, and milk count toward hydration, but sugary soft drinks should be avoided as they can deplete fluids. Water-rich foods like watermelon, strawberries, cucumber, and lettuce also contribute. "A simple rule is to drink a glass of water with every meal in addition to what you usually drink," Rice says.
Can You Drink Too Much?
While rare, overhydration can cause hyponatremia (water poisoning), where sodium levels drop dangerously low. A 2021 study in the British Medical Journal suggested that drinking 5.3 litres over four hours could be harmful. Ruani advises sipping water throughout the day rather than gulping large amounts at once.
As the heatwave continues, experts stress that listening to your body is key. "Proper hydration should be given the same priority as sleep and exercise," Ruani concludes.
