As the UK bakes in a heatwave, staying hydrated is vital. But how can you spot the signs of dehydration? A sports nutritionist and water hydration expert has shared what to watch out for13:17, 24 Jun 2026Britain is currently experiencing sweltering temperatures, with large parts of the nation under a red heat warning. The thermometer could climb as high as 40°C in certain areas, and residents are being advised to remain sensible amid the scorching conditions.One of the most crucial things to bear in mind during a heatwave is maintaining proper hydration. If you're perspiring more heavily in the warm weather, you must replace that fluid to prevent becoming dehydrated.But how can you distinguish between dehydration and ordinary thirst?James Morehen, sports nutritionist and hydration specialist at the electrolyte brand Vidrate, has identified the subtle signs of dehydration to monitor, and how to recognise when it's become serious.How to tell if you're dehydratedDetermining whether you're dehydrated isn't straightforward. The indicators can be rather subtle, making it essential to stay vigilant.James said: "Dehydration signs aren't always obvious, they can show up as a headache or difficulty concentrating."This can be mistaken for tiredness or screen fatigue, but if you experience either during hot weather, it can be an early sign your body needs more fluids."Particular cravings can also indicate dehydration. If you catch yourself wanting salty or sugary snacks, it might suggest your body is under strain.James explained: "When fluid levels drop, it can lead to fatigue, which the body may interpret as hunger."The third indicator can prove uncomfortable both for you and those nearby. James explained: "Persistent bad breath can sometimes be an overlooked sign of dehydration. When you're not drinking enough fluids, your body produces less saliva, which plays a key role in washing away bacteria in the mouth."As bacteria build up, it can lead to an unpleasant odour, so staying well hydrated can help support better oral health as well as overall hydration."What's the difference between thirst and dehydration?Thirst is actually a late indicator of dehydration. By the time you begin feeling thirsty, it's already taken hold.That's why it's crucial to regularly sip water throughout the day to prevent yourself from becoming too dehydrated.James said: "Drinking a full glass of water when you wake up and before you go to bed is one of the best things you can do during hot weather."Your morning glass helps to rehydrate after a night of sweating, and a glass before bed can help regulate body temperature - just leave a gap of 30 to 45 minutes before sleep to avoid a disrupted night."Keeping up your water intake consistently throughout the day is equally important.How much water should I be drinking?James said: "It is recommended to drink six to eight glasses of water a day but during hot weather, but in hot weather this can increase to around three litres, especially if you're active or spending time outdoors."A simple way to check if you're hydrated enough is by checking your urine colour. You should aim for a pale-yellow colour, if it's darker than that, you likely need to consume more water."