Feeling parched? You’re already mildly dehydrated. Photo / Getty Images
Dehydration can creep up on you and play havoc with your health. Here’s how to spot the warning signs.
When did you last have a decent drink of water? We all know we should down more H₂O, yet the average person in the United Kingdom sips less than half the recommended six to eight glasses a day, shows recent research by PureGym. This doesn’t just leave us feeling parched, it can put us at risk of dehydration and long-term health issues.
“Water is essential for life and plays a vital role in nearly every body process,” says Stavros A. Kavouras, a professor of nutrition and director of the Hydration Science Lab at Arizona State University. “It helps regulate your temperature, transport nutrients, remove waste and lubricate your joints. Proper hydration aids digestion, supports kidney and heart health, and enhances physical performance.”
The human body is around 60% water, but we constantly lose fluid – about 2 to 2.5 litres a day – through our sweat, urine, stools and breath. Replacing that fluid regularly is crucial.
“Many of us are mildly dehydrated or ‘under-hydrated’ without realising it,” says Kavouras. “This can subtly, yet significantly, impact how we feel and function. More importantly, it’s linked to chronic health conditions such as hypertension, diabetes and dementia, which in turn means a reduced life expectancy.”
In hot weather, dehydration can escalate quickly, potentially leading to collapse or even death. The risk rises if you’re older, taking multiple medications or have sickness and diarrhoea. Dehydration is one of the most common reasons for A&E admissions in the elderly.
Here are the telltale symptoms to watch out for:
Feeling parched? You’re already mildly dehydrated. “Thirst is the first stage of dehydration,” says professor Dileep Lobo, a professor of gastrointestinal surgery at University of Nottingham, who researches fluid and electrolyte balance.
“Once you’ve lost 2% of your body weight in fluid (1.4 litres for a 70kg person), it triggers receptors in the brain which make you feel thirsty. Mild dehydration can happen easily if you’re out in the sun or working in a warm office and not drinking much. You lose quite a lot of water through your sweat, especially if you’re not acclimatised.” At this point, dehydration is easily reversed by drinking fluids.
“If you replace the water you’ve lost, you’ll feel better immediately,” says Lobo. “The fluid enters your bloodstream and corrects the imbalance.” Our sense of thirst declines with age, so sip water throughout the day.
Check the colour of your urine. If it’s honey coloured, you need to drink more. If it’s darker brown, you may be seriously dehydrated. “Once thirst occurs, your body starts holding on to water,” explains Lobo. “Your kidneys send less water to the bladder, making your urine more concentrated.” This is a major risk factor for kidney stones.
“If you don’t drink enough, your kidneys have to work really hard and this raises your risk of certain diseases, including urinary tract infections, especially if you’re a woman,” adds Dr Lewis James, a reader in human nutrition at Loughborough University. Kidney function declines with age, making dehydration more likely, so keep an eye on your urine output. Ideally, you’ll be passing urine about seven times a day, and it should be transparent yellow.
The brain is 73% water so your brain cells can struggle to function once you’re dehydrated. “Even a low level of dehydration can reduce cognitive skills such as memory, attention and motor coordination,” says Dr James. “This may affect your performance whether you’re working in an office, driving your car or deciding if it’s safe to cross the road.”
A study in the Journal of Clinical Nutrition found 45% of doctors and nurses were dehydrated by the end of their shifts, and their short-term memory was impaired. Just drinking a 300ml glass of water has been shown to improve memory recall. As dehydration progresses, you can become confused and disoriented.
Dehydration is a common trigger for headaches. As fluid leaves your brain, it can put pressure on the pain receptors and nerves in the brain’s lining. The pain can worsen when you bend and turn your head or move around. Most dehydration headaches improve within an hour or two, once you’ve rehydrated and rested. Stay out of the sun and try putting a cool compress on your forehead.
A dehydration headache can turn into a migraine, so try to drink fluids even if you feel nauseous – small sips are best. “Drinking enough water can help reduce risk of a migraine attack through dehydration,” says Pippa Coulter, health information manager for The Migraine Trust. A study in the Journal of Clinical Neuroscience found migraine pain, duration and frequency was significantly lower in patients who drank more water.
Dehydration is behind one in 10 GP consultations for fatigue, according to a study of 300 GPs by the Natural Source Water Association. Regularly sipping water keeps your body and brain energised, especially if you’re active. During hot weather, you can lose 1.5 to 2 litres of water an hour through sweat whether you’re hiking or playing a round of golf. “If you don’t replace these fluids, your blood thickens, and your heart has to work harder to maintain your blood pressure and oxygen levels,” warns Lobo.
As we age, dehydration can make daily life exhausting. “An 80-year-old who weighs 40kg and loses bodyweight through fluids is more likely to feel the effects,” says Lobo. “Our research shows that older adults, admitted to hospital with dehydration, stay in for longer and have a 6% higher mortality rate.”
“Once you’ve lost 4% of your bodyweight in fluid, sweating becomes difficult and your temperature rises, putting you at risk of overheating,” says Lobo. “Blood pressure drops, and you can become dizzy or faint.” This is “moderate dehydration” and you should seek medical advice. Oral electrolyte solutions can help but sometimes an IV drip is needed. Without treatment, dehydration can progress to “severe” (a 10% loss of bodyweight in fluid) within 24 hours. It’s now a medical emergency. “The body reduces blood flow to non-vital organs, like your kidneys,” says Lobo. “Cellular waste builds up, organ damage can occur and, without intervention, you may eventually suffer seizures or collapse.” Multi-organ failure, coma and even death can follow. Hot weather can accelerate the process, so always carry water and seek shade if you feel tired or light-headed.
How much water should I drink a day?
“Your body needs around two litres of fluid a day,” says Johanna Hignett, consultant nutritionist. “Some of that comes from food, such as fruit and vegetables, so aim to drink 1.5 to 2 litres of water (about six to eight 250 ml glasses). If you’re very active or it’s hot, your fluid needs rise.”
Tea and coffee do count. “The diuretic effect of the caffeine is offset by the fluid content – research shows up to four cups of coffee (400mg caffeine) a day is ok,” says Hignett. “Milk counts too – it’s over 80% water – as does fruit juice, but plain water is best because it’s absorbed directly and contains no calories or sugar.”
Is it better to drink cold or room temperature water?
“It doesn’t matter what temperature your drinking water is,” says Hignett. “It warms up in your stomach to body temperature.” Drinking warm or hot water is said to aid digestion and circulation. Chilled water feels refreshing so may quench your thirst and cool you more in summer.
Should I drink water before, after or during exercise?
“If you’re exercising for up to an hour, you can be guided by thirst,” says Dr James. “Ensure you’re hydrated before you start, then a drink afterwards may be sufficient.” For longer sessions, you may need to top up with water or a sports drink while you exercise. “You can calculate your fluid losses by weighing yourself before and after your session,” says James. “If you’ve lost half a kilo and drunk half a litre during your workout, you’ve lost about a litre of sweat, so aim to replace that fluid over the day, on top of what you’d normally drink.”
Is it possible to drink too much water?
“It’s rare, but it can happen,” says Hignett. “If you drink large amounts in a short time, your cells become too dilute (hyponatremia) meaning essential minerals, such as sodium and potassium, can’t perform their functions. Your kidneys struggle, your cells swell and you can get confusion, cramps, vomiting, seizures and, in extreme cases, coma or death. This is why it’s important to drink little and often.” Our kidneys can process about a litre of water an hour, so avoid exceeding that.
When’s the best time to drink water?
“Drinking little and often keeps your fluid levels steady, rather than the balance going one way, then the other,” says Hignett. “Waiting until you’re thirsty, then drinking lots of water, makes your body think it’s well hydrated, so it excretes more. This happens when you have a cup of tea before bed, then wake up in the night needing the bathroom.” Don’t avoid drinking for fear of needing the loo – concentrated urine aggravates the bladder, exacerbating the problem. “Just drink regularly so your body gets used to it,” says Hignett. Research shows that drinking 500ml water before a meal can aid weight loss by preventing overeating. It’s also thought to help digestion.
Can food count towards your fluid intake?
“Up to a third of the water we consume comes from food,” says Lobo. Fruit and vegetables are good sources of water, as are soups, stews and porridge cooked with milk or water. Cucumbers, tomatoes and broccoli are over 90% water. “Look for squishy fruits, such as melon and strawberries,” says Hignett. “Anything that doesn’t freeze well will have a high-water content.”
Does alcohol cause dehydration?
Although alcohol contains water, it’s a diuretic, making you need the loo more and so you’ll lose extra fluid. “If you want an alcoholic drink, a spritz is a sensible choice,” says Hignett. “Adding sparkling water can help hydrate you, and it means you’ll drink a bit less alcohol, fewer calories and less sugar. Beer with less than 4% alcohol can hydrate you, but I don’t recommend it! Try to alternate each alcoholic drink with water.”
Can medication cause dehydration?
“Some antidepressants (selective serotonin reuptake inhibitors, like Prozac) and weight-loss drugs (GLP-1 receptor agonists, like Ozempic) can suppress thirst, potentially increasing risk of dehydration,” says professor Kavouras.
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