How much water should I drink?
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Do you know how much water you drink? Do you know how much water you should drink? If you're anything like me, you'll have very little idea. This is the Food Chain from the BBC World Service. I'm Ruth Alexander and this week we're finding out how to hydrate. Water is a nutrient and many times it's forgotten. We'll be discovering how much we need. So the variation is absolutely enormous.

Which drinks are the most thirst quenching? What happens if we don't drink enough or too much? I had literally just drunk so much water that I had washed out of myself all the salts and nutrients that you need to function. And we'll hear your tips for getting it right.

As you can probably hear, I'm in the office and so far today I've had one cup of coffee, one and three quarter cups of tea and I'm on my third glass of water. I don't normally track how much I'm drinking but today I'm going to. And something I've noticed in the office, I think quite a few people do monitor how much they're drinking judging by the large water bottles on people's desks with markings on them.

Now here is Gideon, my colleague. Gideon, every time I sit next to you, I notice you have that water bottle with markings on the side. I do. It's a litre-sized water bottle. I fill it up each morning when I come into work and I try and make myself drink it before the end of the day. Not always successfully, it depends on how busy I am. And quite often I get to the end of the day, I look at the water bottle, I realise there's still water in it and I'll finish it before I go home. I'll make myself get through my litre. Thirsty or not? Exactly.

And by the very end of the day, you're going to bed, how much are you trying to drink in a whole day? I kind of aim at about two litres. I don't know if that's the amount I should be drinking, if that's too much, too little, I'm not sure. OK, well, we're going to find out. Good, let me know. Will do. So, Victoria over here has got a nice turquoise one with some orange stripes. Hello, Victoria. Are you sort of very specific about how much you're drinking every day?

Yes, I am, to be honest, because I feel dehydrated, especially in office with our very dry air here. And at the end of the day, I have horrible headaches. And I've realized if I drink enough water, I don't have it. So I at least try to drink two of these big bottle waters. What are you aiming for? Usually two liters, two and a half ideally. But not often I can do this.

Ah, everyone's pointing at Henry. Hello, Henry. And look at this big water bottle that you have by your desk. Yeah. Why? Well, it's really depressing. I actually bought it so I'd have to get up from my desk less to go to the kitchen, which is a bit of a tragic reason, yeah, but I think that's about a litre and a half. Because you work so hard? Because I work so hard, yeah, I think that's over a litre and I probably drink about four of those a day. Programme-making must be thirsty work.

Why is it that we need water? I've been talking about this with Nydia Rodriguez-Sanchez, an expert in hydration at the University of Stirling in Scotland. Water is a nutrient and many times it's forgotten. We focus on protein and we focus on the vitamins and the carbs and having fibre, but many times we don't think about water as such an important substance or such an important nutrient in our lives, in our diets. And water...

Why is hydration so important? Well, we know that we are made of protein and different molecules in our body, but water, in general, we say that it's a form of 70% water. And, well, that's in our cells, in our organs, our blood, and many of the reactions and different pathways that we have in our body will be in a water media. So, yeah, it's very, very important. And changing constantly.

I always like to think that, well, our fluid balance is like our bank balance pretty much because, well, we are losing water all the time. And since, for example, when we are having this conversation, well, or just by the fact of being inhaling, exhaling, well, when we are exhaling, there is water that we are losing. So we are losing water in a continuous manner. However, when we are drinking water is in episodes. So it's all day we're kind of

moving up and down the amount of water that is in our body. What happens to our bodies if we don't take in enough water?

Well, we cannot live very long without the water that we require. So, well, there are different factors that will be indicating when we need water. Probably the main one is the plasma, which is the water component in our blood. Our blood is formed around 90% by water.

So when we have more concentrated blood, that will trigger different hormone responses and that will trigger our brain and that will bring the third sensation.

And also many times we start getting dehydrated and we don't realize we are dehydrated. So it's very common that we feel tired or we feel like with a bit of a headache or even we think we are hungry. We go and get some food and actually what is happening is that we are thirsty, that we are dehydrated. We're starting to show some signs or some symptoms of dehydration. Nydia Rodriguez-Sanchez.

So how much should we be drinking? The number I've always heard is two litres a day. For example, the European Food Safety Authority recommends two litres for women and two and a half litres for men. But on closer inspection, I see that that's taking into account the water in food.

I've been speaking to Professor John Speakman of the University of Aberdeen in Scotland and the Shenzhen Institute of Advanced Technology in China, who's one of several scientists to have been looking at this. I asked him how the two-litre figure was arrived at.

We did, when we did the paper, look a little bit into where it came from, but we couldn't find any sort of very strong evidence for where the number came from. So I'm not sure how it was arrived at, but it seems to be a number that has taken grip on a very large number of countries. So if you look at...

Government recommendations around the world, they're pretty much all the same. They pretty much all say everybody's got to be drinking two litres of water, which is eight eight-ounce glasses in the US is how they frame it. And pretty much every country has come to the same opinion. But our work tends to suggest that actually for the average person, that's a little bit too high.

So our estimate is that males on average age between about 20 and 60 are probably requiring about 1.8 litres per day. And females in the same age group are using about 1.5 to 1.6 litres per day. As you go above 60, that gets progressively even lower.

So once you're up to about 85 years old, then you're really only needing about a litre per day. The study, which involved the work of numerous scientists, tracked more than 5,000 people in 23 countries. We are in a water balance, so we regulate pretty carefully between what comes in and what goes out.

And that total balance is called the water turnover. As Nydia said, we lose water when we breathe, sweat and go to the toilet. We take in water with food and drink. And something I didn't know, we even make our own water too. Let's say you eat a meal that consists of carbohydrates, proteins and fat...

When you burn those carbohydrates, you produce two things. You produce carbon dioxide, which you breathe out in your breath, but you also produce water. And that water contributes to your water intake. So how much water is coming from what we drink? So on average, it's estimated that of the water turnover, you probably need to drink between 40% and 45% of that value. The remainder...

comes as water in the food and the water that we generate when we metabolise the food. The study John was involved in tracked the rate at which water was being replaced in individuals. So it's not easy to measure it, of course, because ideally what you want to do is capture all the intakes and all the losses. But there is a shortcut that we can use.

and that is to use an isotope. So what we do is we get people to drink some water where the hydrogen has been replaced by a heavy isotope of hydrogen called deuterium. It's completely safe, doesn't have any negative effects on you. And when you drink that, it floods into your body, into the water pool, as we call it, in your body. And then water coming in...

And water going out slowly washes that isotope out of the body. So there's a decline over time. So obviously, if you have a very big water turnover, it comes up more quickly. And if you have a very slow water turnover, it comes up much more slowly. And what did you find in terms of variations? So the variation is absolutely enormous.

Some people are only needing about a litre per day and I think the maximum value was about seven and a half litres per day. And so there are big differences between individuals. What sort of factors makes a difference? The biggest thing that influences how much you need is actually how big you are. So the bigger you are, the bigger your water turnover is.

And so if you look at, for example, males going from having a lean body mass of about 40 kilos, they have a water turnover of about three and a half liters per day.

Whereas an individual that is 80 kilos of lean tissue has a turnover of about 5 litres per day. What about where you are in the world? Another factor that influences it is temperature. So if you go to a warm place...

You sweat more, you're losing more water, you need to replace that water. And so it will be the case if you live in somewhere that's hot and humid, your water requirements are going to be considerably higher than somebody who lives in a cold and dry place. And that's why it's just crazy to say, you know, everybody's got to be drinking two litres of water a day. That just doesn't make any sense. Professor John Speakman, you're listening to The Food Chain from the BBC World Service.

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I'm Ruth Alexander. This week, how to hydrate. While most of us don't need as much water as we might think, athletes would do well to get ahead of their thirst, according to Nydia Rodriguez-Sanchez.

It's very common to find that athletes finish very dehydrated. And if we are training or competing in different environments, of course that will be different. That will be changing. So it's not the same if we have a marathon athlete, that if they are running the Arctic marathon, won't be the same if they are running in warm or dry environments. So we need to adapt that to the conditions, to the humidity, the temperature, etc. And if you are dehydrated...

do we know by how much it can affect your performance?

When we are dehydrated, the cognitive function can be impaired. Or, for example, the decision making that also can be affected. A marathon runner or an endurance runner, probably the speed or the performance, if we are thinking about the time to complete a task, might be reduced by 3 or 4%. Now, if we think in elite athletes or if we think about Olympic Games, well, that might represent the fact of winning or not winning a medal.

Is it possible to drink too much water, though? In rare instances, it is, as Joanna Perry discovered when she ran the London Marathon in 2018. It was the fifth time, I think, and I decided to do it with my daughter and my son-in-law. The training was good. However, the conditions on the day were especially challenging. It was really, really hot. I think it's still the hottest marathon day they've ever had.

We set off. When you do the marathon, there's loads of people to chat to, especially when you're virtually walking. So there's always people dressed as a rhino or a train or a boat or something and me. Anyway, so we sort of chat along and everything was all fine. And the last thing I remember was the half marathon sign, the big thing that says halfway. And that's the last thing I remember.

Until I woke up, that was Sunday. So I think I woke up on the Wednesday in intensive care. What had happened? I did finish the race. They've got a little video of me going over the finish line. And because Richard, my partner, and some other friends were there, they waved and everything. And I just, in this video, you can see that I was

saying how wobbly I was. You know, you could see I was physically wobbly. We got home and I was really, really sick and obviously passed out. And then Richard did CPR. In that time, the air ambulance came and the paramedics came. And then I think for those few days, as far as I know, they told my family that

They didn't really know when I'd wake up and what sort of situation I would be in when I woke up. When you did wake up, what did the doctors tell you?

was wrong? I really don't remember. I only know what Richard's told me and that I had literally just drunk so much water that I had washed out of myself all the salts and nutrients that you need to function. Do you know how much you had drunk? Well, no, not really. Because I mean, I suppose because it was so

Everybody said, you know, drink water, drink water. So at every possible stop, you know, I had my bottle of water and carried one and had more water.

thinking that that was the right thing to do. I asked Nydia Rodriguez-Sanchez about what happens when someone drinks too much. That's what we call a hyponatremia. That means that we are diluting our blood. We were mentioning earlier that our blood is formed by plasma. But when we drink too much water, the plasma gets diluted and we start having lower levels of sodium. That the sodium is

It's an electrolyte that is really important. And if the plasma gets diluted, also the amount of sodium in the plasma will be diluted. And this can be related to brain edema. So that basically means that the brain gets swollen. And well, this can be lethal. This can be a very serious condition. So we also need to be very careful and avoid hyperhydration or to drink too much.

too much. If we think in a marathon, for example, the risk is when they drink too much water and if they are not ingesting also sodium or electrolytes, they need to be not just relying on water. Joanna says that's been her big takeaway. I really think other people should know this because we're all encouraged to drink these great big bottles. I mean, everyone I know has got these bottles with all these markers on them. I think that

Anybody contemplating doing a marathon, from my experience, they should make sure that all they need to do is to take an electrolyte before they start and maybe take another one along the way, along with their sweets and the sugary things that you need. Joanna Perry. Now, all drinks are not equal. Some are better at quenching thirst than others. I've got a decaf coffee here at my desk. Does that count when it comes to hydration? I asked Nydia Rodriguez-Sanchez.

Absolutely. So we investigated the impact of different drinks, seeing how much water was conserved in our body. So we tested coffee, decaf coffee. We also tested export drinks, cola drinks, skim milk or full fat milk.

And well, we observed that those that were containing electrolytes such as sodium, potassium and also a protein that was helping to keep the water for longer. So, for example, protein in the meal that stays longer in our stomach.

And that helps to like having a slower release of the water to our intestine. So that helps to keep us hydrated for a longer time. For example, it would be relevant if we think about surgeons or drivers that you want them to be well hydrated. You want them to be fully focused when they are in the theater or when they are driving. So for example, milk will be

really helpful to keep that hydration balance for longer. And where did decaffeinated coffee come in your list? Well, actually it was as hydrating as water. Really? Yeah. Many times we associate caffeine with the hydrating effect. However, well, we studied a dose of zero milligrams of caffeine, 50 milligrams, 200 and 400 milligrams. So we went up to ingesting the equivalent of

three or four cups of black coffee. And we actually observed that there was no negative impact on fluid balance. So yeah, so we can actually consider coffee as a hydrating source. How did tea score? Same as water. What about sugary drinks, sodas? Well, we observed that those with a greater content of sugar, well, it was retained for longer. So actually, in terms of hydration, it

kind of help. However, well, we need to keep in mind that probably the amount of sugar that they contain is not ideal. So we need to find that balance.

There's a bit of a fashion for waters with a little bit of extra electrolytes or intelligent water. I mean, is it necessary to go out and buy water with things in to help hydration? For the general population, this is not required. They can get their electrolytes, their sodium, their potassium through a varied process.

diet that is including different sources of fruits, vegetables, that those are the main vehicles of sodium, potassium, and just in a natural way. It's not needed to have that fancy drink. But in the case of athletes, probably in those cases, if they are training for longer than 90 minutes, if they are having big losses of sodium or potassium, well, in those cases, we can consider any of these products. Nydia Rodriguez-Sanchez

If, listening to all this, you're feeling motivated to drink a little more, are there any tricks that can help? We've asked people who follow the BBC World Service Facebook page. Hi, my name is Jamie and I live in the Philippines. My tip for drinking enough water is if you're using a big bottle right now, switch to a smaller one. That way, when you see it already empty...

you are in some way motivated to refill it right away. And before you know it, you're already meeting your goals and drinking as much water as you need and as you can. Thank you. My name is Kathy and I am in North Dakota, USA, which is very close to the Canadian border.

So I was one of the ones that toted a water bottle around with me wherever I went. I was never without it, but I was never really drinking from it either. If I did, I would just take small sips and it wasn't really about hydrating. It was more about sort of wetting my whistle. And so my idea was, what if I just drank from a cup? You know, every hour or so it got me out of my desk, use the restroom, fill up my mug.

I am drinking more as a result, and I'm getting to say Merry Christmas a lot more because the mug that I found in the cupboard at work happened to be a Merry Christmas mug. And I thought, why not have Christmas every day, right? Hello, my name is Hagai Abraham Mwapi. I live in Zambia, Cobabos Province. During the course of the day, I eat a lot of salted roasted groundnuts.

Now, in order to quench the thirst which follows, I drink at least one litre of water. By the end of the day, I discover that I have drank three or so litres of water. That is my tea for drinking more water. Thank you very much. I drink more when I eat salty snacks too, although I'm not sure it's a good thing. I asked Nydia what she thinks.

Well, that's a very interesting point because actually salt is sodium. Those salty snacks, they are providing electrolytes. So that will help to recover what was lost through sweating and overall to keep the right balance in our blood. But again, it's always about moderation.

How can you tell if you're dehydrated? Of course, you may feel thirsty. You can also pay attention to your urine. And we normally recommend that it is pale, like the colour of a lemonade, but not as dark as if it was apple juice. That means that we are dehydrated. The recommendation is to go to the toilet to urinate around five to seven times a day. So that's an easy way to know if we are well hydrated. What other top tips would you have for people

getting more water into your diet, drinking more.

Make the most of the fruits and the vegetables. For example, something that I love from Mexico and something that I miss when I'm not there, the different sources of fruits, the flavor of the watermelon, the mango, etc. But for example, broths. Now that we are moving into the winter, that's a wonderful way to get electrolytes, to get some protein in our diet and also to keep us well hydrated.

It's early evening. How much have you drunk today? I've drunk probably around 1.5 litres of fluid, I think. I actually went for a run earlier this morning. So I came home and I had a half a litre bottle and I would say probably two cups of coffee. And

And yeah, I still have a couple of hours to go. So hopefully that will help to keep me well hydrated. Where will you get to by the end of the day? I would hope around 2.5. Okay. I don't think I've done that well. We'll see. But it's not every hydration expert who measures what they drink.

Here's Professor Speakman. This seems to be the only government guideline people follow for some reason. You know, everything else the government tells you to do, people just go, oh, I'm not going to be doing that, like exercise and things like that.

But for some reason, the water recommendation's taken hold. And I see students all the time, you know, I mean, if people are giving lectures, you know, there's students in there with a bottle of water on the table, like they're not going to survive a one-hour lecture unless they have some form of hydration there to rescue them.

But I think generally there has been too much emphasis on what our water requirements are. It's not dangerous to drink two litres per day. You need to drink an awful lot of water before you get into a kind of danger zone that you're drinking too much.

But it is wasteful. So if you imagine that producing this water costs money, all that happens is if you consume more than what your needs are, then you just urinate it out because you need to maintain that balance for your physiology. So it's just a waste. If you only need 1.5 liters, then...

then a third of the water that you're drinking each day is just a waste of money for you and also a waste of resources for the environment. How much water have you drunk today? Do you know? I have no idea. I don't monitor it. I don't track how much water I drink. I never carry a bottle of water around with me.

And I drink when I'm thirsty. Nydia Rodriguez-Sanchez, on the other hand, is happy if people are choosing water over other drinks. I would love to see, for example, in the movies, in the TV, we never see people drinking water. So if we have a programme, yeah, we can see people having a cup of coffee, having a glass of wine. But very rarely we see people taking the time to hydrate. Yeah.

Yeah, having water, having milk, of course. Well, we never see that. So if there are any film stars out there listening, any Bollywood or Hollywood directors, please make water cool. That's your message. Absolutely.

By the end of the day, I estimate that I drank around two and a quarter litres, more than I expected, probably because I ate quite a few salty snacks that day. Too many, in fact. Of course, in some countries there are water restrictions and people have to be very careful about how much water they use. Is that your experience? We'd love to know. For another programme we're making, please do email thefoodchain at bbc.co.uk. From me and the rest of the team, Beatrice Pickup,

Thanks for listening and join us again next week.

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