At the risk of stating the obvious, hydration is thirsty work right now. Perhaps you’ve noticed the endless TikTok videos on the subject. Or the sports drinks flooding the market, promising scientifically engineered liquids that increase your ‘performance’ like the oil in a car.
Then there are the hydro homies. A subpopulation of the wellness bro, hydro homies are people who take their fluids extremely seriously.
They add salt to their tap water, track their liquid intake via phone apps, and walk around with water bottles the size of modest ground-to-air missile launchers.
It’s tempting to mock but they’re not wrong, of course. Hydration is a serious business. The brain, the kidneys, the heart, the muscles: all of them like a drink. In fact, every cell that makes up your body requires water to function.
And if you’re not getting enough, it shows. In 2023, a National Institutes of Health study in the US found that people who aren’t sufficiently hydrated age faster. They’re also more susceptible to chronic disease and are even at a greater risk of early death.
Not consuming enough liquid impacts your daily life, too. Research has shown that your mood and energy levels – potentially even your cognition – decline once your body mass reduces by at least 2 per cent due to dehydration. That’s a loss of 1–1.5 litres of body fluid, and until then your body won’t even tell you it’s thirsty.
Drinking more water can also support weight loss, with studies showing an increase in fat oxidation when people swap caloric drinks for H2O.
All this comes down to one question. How do you know you’re properly hydrated? The answer is complicated, says Dr Lewis James, who studies human nutrition at Loughborough University.
“The big thing with water is that the requirements can vary day-to-day and quite largely between different individuals,” he says. “The main thing that dictates this is the water loss that someone’s experiencing.”
In other words, the amount of water you need depends on the amount of water you pee and sweat out due to climate, activity or biological factors.
“If it’s somebody living in a very hot climate who does a lot of activity, then their requirements will be wildly different from a very sedentary person living in a cool climate,” James says.
To help you feel glass-half-full about your health, we’ve opened the taps on the latest hydration research for the top lessons on getting the right liquid intake.
1. How much water to drink per day
So, you’re drinking your eight glasses of water a day – good for you. But are they pint or shot glasses, or something in between?
Part of the confusion around hydration is caused by vague and outdated recommendations, says James. “The eight-glasses-a-day thing is a little bit old now.”
He points instead to more detailed guidelines produced by the European Food Safety Authority in 2010.
The guidelines offer tailored recommendations based on age and sex, plus specific guidance for anyone who is pregnant or lactating. For adult women, the recommendation is 2l per day. For men, it’s 2.5l. Increase that to 3l on very hot days or when you’re doing a lot of exercise.
Most experts recommend around 25ml per workout. If you’re doing a particularly gruesome high intensity interval training (HIIT) class, you might need as much as a litre to hand.
Water is generally the recommended liquid because it’s free of calories. But when you’re thinking about your daily intake, you can count other drinks too, says Prof Stuart Galloway, who studies physiology, exercise and nutrition at the University of Stirling in Scotland.
“Sometimes people view it as eight glasses of water on top of everything else they drink naturally,” he says. “Actually, you can get that from other drinks during the day as well. They’re part of the overall requirement.”
There are more surprises, too. “This total water intake requirement is typically based on getting about 80 per cent of your water from fluids that you drink, and about 20 per cent from foods that you eat,” Galloway says. “For an average man, about 500ml is coming from your food.”
However, Galloway’s research finds that the amount coming from food varies significantly from person to person.
He says some people get as much as 40 per cent of their daily hydration from the food they eat, assuming it’s water-rich produce like fruit and vegetables. Eating the cucumber in your cocktail? Yeah, that counts.
2. Do we need to add electrolytes?
If you’re anything like an Olympic athlete – and let’s assume that you are – you might lose 2.5g of salt with every litre of sweat you excrete.
“Many athletes lose one to two litres of sweat per hour of exercise,” says James. That’s 10g or more of salt if you’re exercising intensely for a prolonged period.
“At the extreme end, we’ve measured up to four litres of sweat being lost by really well-trained athletes.”
The problem with losing that much sodium is the imbalance it creates in your electrolytes. These chemicals don’t just help our hydration – they regulate our pH levels and play a role in muscle and nerve function.
When your electrolytes are out of kilter, you might experience muscle cramps or weakness, plus nausea and headaches.
So, it’s important to replace those electrolytes during prolonged exercise when even a salty Western diet won’t cut it.
That’s why those enthusiastic hydro homies add a spoonful of salt to their drinking water.
“The salt content of the drink can help water be absorbed more quickly in the intestine,” Galloway says. “That means the water gets into your circulation a bit faster and also can mean that you hold on to that volume of water a bit better.”
Instead of just being flushed out as urine, the water that stays in your system is used to properly hydrate your cells, maintain blood pressure, and support overall bodily functions.
It’s the same theory with sports drinks. The trouble is, most of them don’t contain the amount of salt you’d need to get a beneficial effect, James says.
“Common sports drinks contain a small quantity of electrolytes, in the range of around 20 millimoles per litre (mmol/l, a standard measure of concentration).
"Contrast that with oral rehydration solutions, which are used for diarrhoea and vomiting in children and come in little sachets – they have much higher sodium and chloride concentrations, normally in the range of 50–75mmol/l. And that kind of level is what really has a strong impact on hydration levels.”
Don’t stress about potassium, Galloway says. “We’ve done studies where you feed people high potassium drinks,” he says. “But that tends to cause more sodium excretion in the urine. No one has ever proven that potassium is necessary in a drink.” Especially if you’re not an Olympic athlete.
Even for someone who goes to the gym three times a week, electrolytes are not something you need to worry about.
“For the vast majority, we get more than enough electrolytes in our foods,” James says. “What a lot of the general population doesn’t do is drink enough water."
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3. Milk might be more effective than water
When we’re parched, we naturally reach for a glass of cold water. But is that always the best way to rehydrate? Not according to one surprising paper, which Galloway worked on in 2023.
“We did a study where we looked at about 12 different beverages and which ones you retained in your body for longer,” he says. “Milk was one that came out better than others, and better than water.”
After participants drank a litre of a given beverage, Galloway and his colleagues monitored their toilet trips.
“We looked at what happened with their urine output and how much of that water they were able to retain two and four hours later. Milk performs better than water on that basis.”
That’s not to say water is poor at hydrating you – of course it’s not. There’s a lot of water in milk, after all – but milk also has fat, sugars, salt and proteins.
“Casein protein in particular can delay the delivery of that water to the intestine, slowing the rate at which the stomach empties,” Galloway explains.
“And because it’s got quite good sodium and potassium content – those electrolytes again – it helps you to retain it a little better than drinks that don’t have those salts in it.”
The sugar in milk also plays a role in slower gastric emptying, but that doesn’t mean soft drinks are a good option for hydration.
Their sugar content is so high that your body has to use up water from its reservoirs in order to process those sugars when they hit the small intestine.
As for other drinks, a question that always crops up is coffee. Worn out human beings knock back an estimated 2.25 billion cups of coffee every day, but whether caffeinated drinks improve our hydration or hamper it is the source of some confusion and debate.
That’s because caffeine is a diuretic. Like alcohol, it’s a substance that helps the kidneys produce more urine, flushing salt and fluids from the body in the process. But researchers say that yawning commuters needn’t worry.
“For me, the diuretic effects of caffeine are so small that they’re not worth considering,” says James. “When most people have a coffee, they drink perhaps 200ml of fluid and the diuretic effect is very minor.
"Even if you only have an espresso, which is 40ml, the effect of that will still be net positive. And there are other health benefits to having coffee and even tea as well.”
Surprisingly, there’s a similar effect with alcohol. Most of us probably overestimate the extent to which it dehydrates us, Galloway says.
“You can probably drink the equivalent of about a litre to a litre and a half of beer and not get additional urine output compared to drinking the same volume of water.”
If you have multiple drinks, however, you will soon need the toilet, as anyone who’s been on a pub crawl will attest. And the diuretic effect is stronger in less diluted beverages such as wine or whisky, Galloway adds.
4. Spread out your water throughout the day
For some of the hydro homies on social media, drinking water is a ritual to be planned, optimised and meticulously executed.
And, sure, if you’re staring down the barrel of a tough workout or a big day at the office, then being well hydrated will make you feel energised.
For most of us, however, the apps and sodium tablets – any kind of official protocol – are a little overkill.
“In terms of maintaining hydration across the whole day, timing isn’t that important and the temperature of your water doesn’t seem to make a huge difference, unless you’re trying to maintain a cooler body temperature on a hot day,” says Galloway.
“But if you want to meet the fluid intake guidelines, you probably need to spread your drinks fairly evenly over the day.”
Some people make simple mistakes, he says. “What you typically find is people will get up in the morning and have a cup of tea or a coffee and then not drink very much until lunchtime. So they’re not really spreading out the water intake over the day fully.”
Starting the day with a 500ml glass of water is no bad thing. It’s a simple way for those of us who don’t drink enough to get our numbers up. “And we naturally dehydrate overnight because we don’t drink when we sleep.”
After that, a healthy drinking ‘protocol’ would simply involve having the same-sized glass of water to drink with each meal, James says.
“That would be a good way to space your fluid out over the day. Then other drinks, like coffees, teas and that sort of thing, can make up the rest of what you need."
5. How to tell if you're dehydrated
If you only drink something when you feel thirsty, chances are you spend a lot of your time dehydrated.
“It does seem that the thirst response lags behind where your hydration status is,” Galloway says.
The delayed thirst response isn’t a big issue for most people in their day-to-day lives, Galloway says, but athletes should probably pay more attention.
“It might be worth knowing about if you’re an athlete doing a prolonged event like an ultra-endurance race or Ironman triathlon.
"But even that is a bit controversial because you can probably lose 1–2 per cent of your body mass and not have much performance impairment. Some athletes can even lose 4 per cent of their body mass and still be fine."
Another common recommendation is to monitor the colour of your urine – the darker or more yellow it is, the more dehydrated you are. But like the thirst response, there’s a time lag.
“The urine that you excrete when you go to the toilet could have been in your bladder for several hours,” James says. “So it’s actually telling you what your hydration state was around three to four hours ago.”
Galloway says that if you’re peeing five times or less per day, that’s a sure sign you’re not drinking enough.
6. It's possible to drink too much water
At the other end of the spectrum, it’s possible – but unlikely – that you’ll drink too much liquid. While most of us would probably benefit from drinking more fluids, it is possible to drink more than we require.
“You probably see that when people go to the pub and you might drink three or four pints,” says Galloway. “You probably don’t need that much volume, but you’re quite happy to drink it. And what happens is, your body just processes that and gets rid of the excess.”
It’s not usually a problem – your body simply excretes any extra fluid it doesn’t need. However, there are cases where drinking too much water can become unhealthy and even dangerous.
“If you’re exercising and drinking a lot of water, for example, your body might release too much vasopressin, an antidiuretic hormone that makes you hold onto the water.
“Then you can end up diluting your electrolytes,” says Galloway. “You find yourself with a low sodium content and that can cause quite severe complications.”
People who take certain drugs may also experience excessive thirst and end up drinking more water than their bodies can handle, which can result in the same sodium imbalance.
James adds: “There’s a finite limit to how much urine your kidney can put into your bladder – about a litre per hour. In situations where people drink at rates faster than that, there could be an increased risk of water intoxication and ultimately death.”
In these cases, water moves from the blood into the cells in the body. “If extra water moves into your biceps, let’s say, that’s not a big problem.”
But if it moves into the brain, he says, it can cause the brain to swell – a condition called cerebral edema – which in extreme cases can be fatal.
James says there aren’t specific guidelines about how much is too much water, but keeping your water intake consistent throughout the day is the best way to prevent a flood.
About our experts
Dr Lewis James is a reader and researcher of human nutrition at Loughborough University, in the UK. He is published in the likes of Sports Medicine, Nutrients and Journal of Applied Physiology.
Prof Stuart Galloway is a physiology, exercise and nutrition professor at the University of Stirling, in Scotland. He's published in various scientific journals including Pediatric Diabetes, Sports Medicine and Nutrients.
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