In hotter weather, the risk of dehydration is greater. But is water always the first thing you should be reaching for?
Water makes up more than half of our body weight. To sustain this amount of water in our bodies, we're advised to drink six to eight glasses of fluid per day. Water is the healthiest drink we can reach for – no sugar, no calories – but is it always the best way to rehydrate on hot days?
The answer is: it depends. It seems water is sufficient for most people most of the time, but the best way to rehydrate depends on who and where you are, and what you're doing.
"The needs of a physically active person with an outdoor job on a hot day may be different from the person who lives in an air-conditioned house, who drives an air-conditioned car to an air-conditioned office," says Ron Maughan, professor at St Andrews University's School of Medicine.
The obvious answer is that the active person will need to drink more fluid than the sedentary one – but there's more to it than that.
When we sweat, we lose water and salt, so we need to replace both. If we consume too much of either, the body will take steps to even things out using osmosis, which is the process of passing water through cell membranes.
"Replacing lost fluid just with plain water means the body has too much water and not enough salt, so to even things out, it will get rid of water by producing urine," Maughan says.
For this reason, milk can actually be more effective than drinking water. Milk naturally contains salt and lactose, a sugar, which we need in small amounts to help stimulate water absorption in the gut, Maughan says. Coconut water is also effective, as it contains salt, potassium and carbohydrates.
Milk also contains electrolytes and macronutrients, which are absorbed into the body. This slows down the time it takes water bound up onto these molecules to travel across the stomach and small intestine, allowing the body to better absorb and retain liquids.
Since milk contains enough sugar for this process to occur, but milk is very different to sugary drinks. In fact, sugary drinks can dehydrate us in the short term, Maughan says, because of their high concentration of solutes, which are substances that can be dissolved to make a solution.
Cell membranes, which water travels through to get around the body, only let through water and very small molecules. Water moves from the side of the cell with the lower concentration of solutes to the side with the higher concentration, keeping things in equilibrium.
Sports drinks contain carbs, which is essentially sugar – not necessary if you're sitting at a desk all day as you don't need the extra energy – Sophie Killer
This means the first thing that happens is that water passes into the intestines to digest it – taking it away from other parts of the body which might need it.
That said, evidence suggests sports drinks containing electrolytes – which include sodium, potassium, magnesium and calcium – can be better at hydrating us than water.
"Sports drinks designed to target these physiological mechanisms slow the transition of liquid across the small intestine where it's reabsorbed into body," says Owen Jeffries, a lecturer in sport and exercise physiology at Newcastle University.
Athletes lose high amounts of sweat over prolonged periods of time, so it's important for them to replace the electrolytes they lose. But there's no need for the rest of us to drink sports drinks to stay hydrated. A balanced diet, alongside the normal drinks we consume, will contain sufficient fluids for a person to get everything they need, says Sophie Killer, a performance nutrition consultant for elite and professional sports players.
"Sports drinks contain carbs, which [are] essentially sugar – not necessary if you're sitting at a desk all day as you don't need the extra energy," she says.
The same goes for salt, which many people consume too much of already.
"Sodium plays an important role in hundreds of biochemical reactions in the body, and it's the electrolyte we lose in the highest quantity in our sweat when we exercise," Killer says.
The average person, with the average exercise regime, doesn't lose so much water that they need sports drinks, says David Nieman, a professor of biology at Appalachian State University and director of the Human Performance Laboratory at North Carolina Research Campus.
But hydration is important among the general population, especially since studies show that one in five of us are dehydrated within a given week.
Consuming any carbohydrates, in fact, will help slow down the absorption of water into the body and help us retain what we drink
Having a glass of water and a piece of fruit before going on a run will do the trick, says Neiman, who has found that eating fruit during exercise aided recovery.
"Half a banana contains sugars and 24 types of polyphenols. This will keep water in the body, and contains nutrients to support exercise," he says.
This advice also applies to children, says Gabriella Montenegro, nutrition researcher at the Center for Studies in Sensory, Impairment, Aging and Metabolism in Guatemala. In her research, she found that children who regularly consume fruit and vegetables are better hydrated. She recommends children and elderly people in particular, who are at higher risk of dehydration, eat more fruit and vegetables.
Consuming any carbohydrates, in fact, will help slow down the absorption of water into the body and help us retain what we drink, Killer says.
"The simplest thing to do is to have water with a meal because it will allow the body to slowly absorb fluid and retain it, allowing it to cross the membranes and get to where it needs to be to hydrate you, rather than going straight to the bladder, causing an increase in urine production," she says.
Another way to stay on top of our hydration is to drink tea and coffee. Some people worry caffeinated drinks dehydrate us, but this is only true when we drink large doses of caffeine and not enough water.
Caffeinated drinks do make the body produce more urine, but they also contain water, which will usually more than compensate for the fluid caffeine makes you lose, says Maughan. In fact, he says, tea and coffee are a good way to hydrate because we're likely to drink more of something we enjoy. (Read more on this in BBC Future's article: Do coffee and tea really dehydrate us?)
Sometimes even high doses of caffeine don't result in dehydration – if the drinker is well accustomed to it. Regular caffeine-drinkers are less susceptible to the diuretic effect of caffeine. In a 2014 study involving 50 male coffee-drinkers, Killer found that drinking four cups of coffee a day for three days provided the same levels of hydration as the equivalent amount of water.
"Coffee contributes to daily fluid requirements, and in regular coffee-drinkers, the kidneys adapt to retain fluid from coffee," she says. "There's no reason moderate amounts of coffee or tea would dehydrate those who are used to having regular caffeine."
If the body is telling you you're thirsty, there needs to have been relatively significant changes there to alert our perceptual awareness – Owen Jeffries
Staying hydrated depends not just on what we drink, but how we drink it. The body goes between being mildly dehydrated and over-hydrated as part of daily life, and will only register dehydration once it gets to a certain level, Jeffries says. At that point, it might be already past the optimum moment to take in fluid.
"If the body is telling you you're thirsty, there needs to have been relatively significant changes there to alert our perceptual awareness," he says.
To combat this, he advises drinking water throughout the day to maintain our hydration levels. Drinking a lot of liquid in one go can cause more water to pass through the body and come out as urine without hydrating us first.
"Drinking a litre of water quickly overwhelms the bladder, it's not truly hydrating you. Your urine may well be clear but that's not a representation of hydration status," Killer says.
It might sound like athletes get the better deal, while the rest of us are stuck with plain water. But water is underrated, experts argue.
"Water is often the forgotten nutrient," says Killer. "It's free, it's healthy, and it doesn't damage our teeth."
Montenegro says hydration hasn't been studied so much in the last few years, but there's probably more to it than we realise.
"Water stopped being important – but it's a very important nutrient," she says. "I'm sure there's many more metabolic things associated with hydration that haven't been studied yet."