The extremes of the British summer are back, with temperatures tipping beyond 30 degrees this week. We all know that keeping hydrated is vital – with some groups at particular risk. Last week, Carrie Johnson, wife of Boris Johnson, who is breastfeeding their new baby, revealed she was hospitalised with severe dehydration. So what really works when it comes to hydration, and what is a myth? Is your iced coffee as refreshing as it feels? Will fizzy water top your fluid levels up as much as still? And should you choose wine over beer – or ditch booze completely? Here’s what the experts advise:
Dehydration occurs when the body loses more fluid than it takes in, and can cause symptoms including dizziness, dryness of the mouth, sunken eyes and tiredness. Fluid needs vary based on age, size, activity level and the temperature, but 2-2.5l of water is recommended as a daily average, according to dietician Nichola Ludlam-Raine, with an extra litre on top during a heatwave. If in doubt, the colour of your urine can indicate hydration levels (lighter is better).
Water plays a series of vital functions within the body, from temperature regulation to cognitive function and digestive support. “Drink regularly throughout the day, not just when you’re thirsty,” says Ludlam-Raine, as our thirst impulse often lags behind hydration needs. “Sip fluids steadily, aiming for a 250 ml glass every hour or so”. Carry a reusable water bottle to help you to remember to keep sipping, and set reminders if you tend to forget to top up.
You don’t need electrolyte-infused water – milk is much better
Electrolytes are minerals – namely sodium, potassium, chloride, magnesium and calcium – that support several key functions in the body, including muscle contraction, nerve transmission and fluid balance, explains Dr Nidia Rodriguez-Sanchez, lecturer in physiology and nutrition at the University of Stirling. While millennial-friendly bottles boasting of boosted electrolytes may have shelf appeal, “for most people, plain water is enough to stay hydrated in daily life… these types of drinks are particularly useful for athletes [as electrolytes are lost through sweat], but they are not essential for everyone.” There is no one catch-all electrolyte-rich food but bananas, avocados, spinach and nuts are among the best sources.
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If you like milk it can be a great option during a heatwave, according to Ludlam-Raine, as it contains water, electrolytes (like potassium and sodium), and natural sugars, “making it effective at replenishing fluids. Studies – including one from St Andrews University – have shown that both skimmed and full-fat milk can be more hydrating than water after exercise, due to their electrolyte and protein content, which slows gastric emptying and helps retain fluid for longer.”
Studies remain mixed on whether hot or ice-cold drinks are better for cooling during a heatwave. A 2023 paper from the University of Ottawa, for instance, showed that warm beverages worked well, reducing the amount of heat held within the body and triggering a sweat response, with those excretions then cooling the skin down. (This works only in non-humid environments.)
On the other hand, “cool water – but not ice-cold – may feel more refreshing and can support thermoregulation,” says Dr Rodriguez-Sanchez. “However, the most important factor is to drink water (or non-sugary drinks) regularly, regardless of the temperature.”
Fizzy drinks high in sugar are not recommended, due to their dehydrating effects.
There is no major difference in the hydration quality of still versus sparkling water either, she says, beyond the fact that “some people may drink less sparkling water because it can cause bloating.”
Coffee and tea do count – don’t ditch caffeine
Moderate caffeine intake doesn’t cause dehydration, though excess consumption can act as a mild diuretic (causing more frequent urination). Provided you consume no more than 400mg a day or less (or 200mg for women who are pregnant or breastfeeding) – and ensure water is your main beverage – there are plenty of health benefits to be had, from the antioxidants such as flavonoids in black, green and herbal tea, and the heart and brain-boosting polyphenols in coffee. Both tea and coffee can be hydrating and count towards your daily fluid intake.
Meal-skippers, beware: limiting your food intake means cutting down on its water content, too, which can up the chance of dehydration. Wearing black or heavy, non-breathable clothing can also lead to excess fluid loss; ditto spending long periods in the sun without a break as it promotes sweating. While the dry weather may make it tempting to prune the garden, this unwitting exertion can be a risk for dehydration. Provided you’re keeping well hydrated these activities needn’t be avoided – but proceed with caution.
Cooling down beneath the blast of air con may feel amazing, but “spending long hours in air-conditioned environments can also lead to fluid loss,” Dr Rodriguez-Sanchez says
If you’re drinking alcohol, beer is best
Tempting though it might be to rehydrate with rosé, alcohol is a diuretic. “The alcohol content and volume both matter when we look at how quickly you will dehydrate,” says GP Dr Helen Wall, with alcohol suppressing vasopressin (or ADH, which helps regulate water balance within the body), leading to a need to urinate more frequently. The general rule is that the higher the alcohol content, the more dehydrating it is – making wine worse than beer, and spirits worst of all, as “they dehydrate more unless consumed with good amounts of water or mixers.” Ludlam-Raine adds that “regardless of the type, all alcoholic drinks can contribute to fluid loss, so it’s important to alternate with water and limit intake in hot weather.”
It’s important to keep an eye on what you eat as well as drink when it is very hot. “Salty products, such as chips and processed meats, can increase fluid loss as your body tries to balance sodium levels in the kidneys,” says Dr Wall. “This is why they often make us feel more thirsty.”
Around 20 per cent of our daily water intake comes from what we eat, with some foods more hydrating than others. Cucumber, for example, is 96 per cent water; tomatoes follow at 95 per cent, while apples are 86 per cent water, and blueberries 85 per cent. So hydrating foods like watermelon, cucumber, strawberries, tomatoes, and watery soups are important during a heatwave, says Dr Wall.
It’s worth knowing also that high protein diets such as keto or paleo lead to the production of more urea (a waste product); the body’s physiological response to protein (the nitrogen in protein forms ammonia which the liver then converts to urea). “This takes more water to get rid of it from the kidneys,” says Dr Wall. This potentially leaves us feeling more dehydrated.
Sugary foods such as pastries or chocolate can also increase fluid loss, in part due to its stimulation of the gut to release water and electrolytes, loosening the bowels. Dr Wall advises avoiding them where possible during a heatwave.
Avoid high-intensity exercise sessions
It is best to avoid exercising in very hot conditions, says Dr Rodriguez-Sanchez, “as this can raise body temperature above normal and increase the risk of heat exhaustion or even heat stroke. High-intensity sessions, such as running or heavy gym workouts, should ideally be avoided during the hottest hours of the day,” she adds, with morning or evening exercise (when temperatures have cooled) being safest. The more vigorously you exercise the more aware you need to be of staying hydrated.
“It is also important to be aware of the signs of dehydration, such as feeling dizzy or lightheaded, tiredness, weakness, headache, or a dry mouth, and to seek medical advice if these symptoms worsen.”
Additional reporting by Hannah Bentley
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