Hydration is important. In temperatures like those we’re increasingly seeing in much of the world, sweating can be the only way for our bodies to cool down, and our thirst isn’t always the best indicator of how much water we’ve lost or need. The consequences of not being sufficiently hydrated as temperatures creep towards the 40s can be severe, and can kick in much faster than most people realise. The good news is that remembering to drink plenty of water at regular intervals throughout the day will be enough for most people to avoid the worst. But if you’d like to understand why dehydration is so dangerous, whether you really need extra electrolytes, or if a cup of tea really can cool you down, read on.

To start with, it’s helpful to understand that our bodies are producing heat – and therefore losing water – all the time. “All the cells in our body are constantly using fuel for energy for various different processes, whether that’s movement or just staying alive,” says Dr Lewis James, a lecturer in sport, exercise and health sciences at Loughborough University. “About 75 to 80% of the energy that we use appears as heat.” If we didn’t have any way of dissipating this heat, then even lying on the couch would see your body temperature rise about 1.3C in a single hour (already enough to make you noticeably feverish) – but of course, we do. Normally, we lose a decent amount of heat through a combination of convection and radiation: the blood vessels in our skin dilate, allowing the blood to be cooled by the outside air. The problem is that when the external temperature goes up, this process becomes less effective and eventually stops working altogether. At this point, our main way of losing heat is through sweating: our bodies produce tiny droplets of warm water mixed with trace minerals, which (usually) evaporate on contact with the air, drawing heat away from the skin in the process. And as we rely more on sweating, it’s increasingly important to replace the fluids our bodies are losing.

“During a normal day, a normal individual who doesn’t exercise loses very little water through sweat, probably less than 500ml a day,” says James. “But if the outside air temperature is hotter than our skin, sweating is one of the only ways that we can lose that heat. Typically, it doesn’t get that hot in the UK, but certainly when it’s above, say, 35, 36C, that’s where it becomes a concern.”

Over the short term, even mild dehydration can impair how your body and brain function. “Typical symptoms include headaches, fatigue, reduced concentration and poorer physical performance,” says Bridget Benelam, a nutrition scientist at the British Nutrition Foundation.

“The body responds [to dehydration] by conserving water: processes like temperature regulation and circulation become less efficient. Over longer periods or at more severe levels, dehydration puts real strain on the body.” This can, she says, lead to impaired cognitive and physical performance, “increase the risk of issues like constipation, urinary infections and falls, and in extreme cases be life-threatening”. At the most severe end, dehydration is fatal, but the problems start to compound before that: you’ll be thinking too slowly to look after yourself.

So how can you tell when you’re dehydrated? Don’t wait until you feel thirsty, as this tends to happen after you’ve already lost 1 to 2% of your body mass in fluid, meaning that you’re already suffering from mild dehydration. The simplest – and, for most people, best – way remains the pee test, says James. Subtle changes in colour can alert you before thirst kicks in. “If you’re producing very small amounts of urine and it’s very dark in colour – from dark yellow to brown – then that can indicate dehydration,” he says. “It’s not, as you might read in some places, an absolute indication – your kidneys are marvellous at concentrating water when your body thinks it’s needed, so you’ll sometimes have concentrated urine even when you’re not really dehydrated – but it’s a fairly reliable signal. And the opposite is also true: if you’re producing pale-coloured urine, you’re probably well hydrated.”

Realistically, of course, you want to stay ahead of this, which is why it’s sensible to make a conscious effort to drink more water at regular intervals. “People often wake up already dehydrated because our bodies lose significant water overnight through respiration and sweating in warm bedrooms,” says the University of Brighton’s Dr Neil Maxwell, a specialist in applied environmental physiology. “I would imagine 70-80% of the population will start the day on the back foot by being dehydrated after a hot night during a heatwave. Starting the morning with 300-500ml of water sets a stable baseline before daytime heat spikes.”

Just in case you’re considering it, though, there’s no real upside to glugging down a day’s worth of water first thing in the morning: you’ll just excrete anything that can’t be immediately used by your tissues or processed by your kidneys. In general, it’s better to drink moderate amounts reasonably frequently. “If you have three meals a day, then try to drink a large glass of water with your meals,” says James. “If you’re drinking a pint of water with every meal, and then you have the odd drink in between meals, you will most likely have an adequate amount of water every day. Drinking with meals also slows down the rate at which the water passes through your system, and might help you to absorb it better.” If you’re exercising, you’ll want to step this up a bit: the standard advice is to aim for about 500 to 750ml of water for every hour of intense training, depending on how heavily you sweat. But it also helps to stay ahead of the game: if you’re heading to the gym or out on the road, drink about 500ml of water a couple of hours beforehand, giving your kidneys time to process the fluid, balance your blood volume, and excrete anything it can’t handle, so you’re not sloshing around unnecessarily.

Does it matter what you’re drinking? As long as you’re keeping it non-alcoholic (and relatively low-calorie), only in the sense that some drinks might muddy your body’s signals. “Sparkling water and diet soft drinks can provide effective hydration, but the carbonation can cause a feeling of fullness or bloating that may prematurely signal the brain satiety centres, causing a person to stop drinking before they are fully rehydrated,” says Maxwell. “Regular consumption of highly acidic diet soft drinks or flavoured waters containing citric acid can contribute to dental erosion, a factor to balance against their hydration benefits – though not a reason to avoid them entirely. Tap water or filtered water remains the gold standard for daily hydration during a heatwave, but the best drink is ultimately the one you will actually consume.”

Fruit juice and milk (or plant-based milk alternatives) are fine for hydration, but come with the caveat that it’s easy to overconsume calories if they’re your go-to option. “Milk is hydrating, provides some nutrients, including protein, calcium and vitamins,” says Benelam. “Fruit juice and smoothies hydrate and contain vitamins but also free sugars. The recommended limit is 150ml per day – one small glass.” If you’re making your own homemade smoothies, of course, you can always cut down on sugar by going heavier on the veg than the fruit: celery, spinach and cucumber all help with hydration while also adding important minerals.

When it comes to tea and coffee, you might have heard that caffeine acts as a diuretic, which causes you to produce and excrete more urine. This isn’t really true in any way that most people need worry about: the effect is so minuscule that you’d need to down around five espresso shots at once to notice a real diuretic effect. “Any diuretic effect you get is massively outweighed by the positive water intake you get from the drink, especially if you’re the sort of person who doesn’t drink much water otherwise,” says James. Caffeine might mask the symptoms of thirst slightly, and so – once again – you can’t rely on the fact that you feel satisfied after you drink it. Otherwise, it’s fine.

Booze is a bit trickier. Technically, one low-ABV drink of about 4% can have a net-hydrating effect: though alcohol is a much more potent diuretic than caffeine, a pint of lager or a fruit cocktail can contain enough water to offset the effect. This effect doesn’t last, though: as you move to drink two or three, the higher alcohol concentration in your blood starts to suppress the body’s anti-diuretic hormone vasopressin, leading to what some people call “breaking the seal”. One pint might be lovely and refreshing, but leave it there.

You might imagine that glugging hot mugs of tea when it’s over 30C is counterproductive, but the truth is a bit more nuanced. “The first thing to say is that personal preference matters,” says James. “Most people, if they’re not drinking tea, like drinks that are cooler than room temperature more – so in that sense, you’re likely to drink more [cold drinks] and so better defend against dehydration. But when we’re talking about body temperature, there’s some truth to the idea that tea can actually cool you down in certain situations.”

If you have a cup of tea when it’s already fairly warm outside, your body’s heat receptors detect a rise in internal temperature and send signals to your brain to cool off. As a result, blood vessels near the surface of the skin widen, and your sweat rate goes up – a cooling response that’s usually disproportionate to the amount of heat you’ve taken on board. In contrast, if you have a cold drink, the opposite happens – the cold fluid in your stomach pulls blood away from the skin, which means the drink is not cooling you down as much as you think. “The warming effect of a warm drink is cancelled out by the increased heat loss, while the cooling effect of a cold drink is cancelled out by the reduced heat loss, except for in very specific scenarios that tend to depend on environment,” says James. “If you’re in very humid conditions or you’re exercising, you’re not able to lose much heat to the environment because the sweat you produce can’t evaporate fast enough.” A good rule of thumb, then, is that if the sweat is already dripping off you, it’s probably too hot to be relying on tea for hydration.

What about electrolytes? There’s been a huge marketing push for these in recent years, with supplement and sports drink companies galore suggesting that topping up your levels of potassium, magnesium and (especially) sodium is a necessary part of hydration for all – not just the athletic or very sweaty. But the evidence for that is, at best, mixed – especially since the majority of people in the UK already take in too much salt. “For the majority of people, you’ll get enough electrolytes from food,” says Brunel University’s Prof José González-Alonso. “If you’re having a banana or an apple during the day, you’re already getting electrolytes – there’s nothing magical about sports drinks.” If you’re exercising intensely or for a decent amount of time (more than 60 minutes, in hot weather), sweating much more than normal, or simply unable to snack regularly, then it can be helpful to top your levels up. If you’re drinking a lot of water, it helps to stop your kidneys from immediately flushing it out. It also helps to balance the risk of hyponatremia, or diluting your blood sodium levels to dangerously low levels (the clue’s in the name: hypo means low, natrium is the Latin word for sodium, and emia means in the blood). “Even recreational exercisers and avid gardeners are at risk of hyponatremia if they aggressively overdrink plain water out of a fear of dehydration,” says Maxwell. “When you lose salt through sweat and flood your system with massive amounts of water, you dilute your blood’s sodium levels, which can cause cells to swell and become dangerous.”

This sounds scary, but it’s only likely to occur if you’re drinking large volumes of water very quickly, with no added salt in between. It bears repeating that, if you’re not very physically active and eating and drinking normally, you should be fine. If you’re doing something more strenuous – enough to make you sweat a lot – add a couple of salty snacks and consider supplementing with electrolytes. Other good dietary sources include avocados, sweet potatoes and spinach, while plenty of foods have a decent enough water content to top up your levels: cucumber, watermelon, strawberries and mango are all more than 80% water by weight.

So while drinks manufacturers make hydration sound highly technical, best practice is reasonably simple: drink a big glass of water every couple of hours, preferably with food, and add in more if it’s hot, you’re physically active, or planning to be so. “The nice thing with water is that if you have a little bit too much, that’s not a bad thing,” says James. “If you’re thinking about energy intake, for example, just having a bit too much carbohydrate on a regular basis can lead to problems down the road, like fat gain or insulin resistance. Whereas if you drink a bit too much water, all you’ll do is pee it back out. And so it’s fairly easy to make sure you’re drinking an amount of water that will mean that you are generally defended from dehydration from most scenarios in your life, with no downside.”