This is 6 Minute English from BBC Learning English.

I'm Phil.

And I'm Beth.

Nowadays I often see people carrying water bottles with them to make sure they drink enough.

How much water do you drink a day, Phil?

I don't know.

Maybe about a litre.

OK.

And do you know how much water you should drink a day?

I think it's probably about two litres.

Ah, well, the number many people have heard is two litres a day.

Of course, everyone needs to drink some water.

Over half the human body is made up of it.

But exactly how much water do we need to stay healthy?

That's what we'll be discussing in this programme, along with some useful new vocabulary as well.

And speaking of vocabulary, remember you can download all the new words and phrases from this programme plus worksheet exercises to help you learn them on our website, bbclearningenglish.com.

Great.

But first I have a question for you, Phil.

I mentioned that over half the human body consists of water, but there's an even higher percentage of water in our blood.

But how much?

Is our blood a.

Around 80% water, b.

Around 90% water, or c. 100% water?

I think it's around 80% water.

OK, I will reveal the correct answer later in the programme.

Our blood needs water so it can carry nutrients to the body's cells and organs.

The amount recommended is often given as 2 litres a day.

But why?

Here to discuss this with BBC World Service programme The Food Chain is biologist Professor John Speakman.

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 2 litres of water.

Professor Speakman doesn't know how the number of 2 litres a day was arrived at, how it was decided or calculated.

Nevertheless, the idea of drinking 2 litres a day has taken grip on many governments around the world.

When you say an idea has taken grip on something, you mean it's taken control of it.

Yes, Professor Speakman says that pretty much all governments are giving pretty much the same recommendation.

He uses the phrase pretty much to mean almost.

For example, pretty much all governments advise drinking 2 litres a day means almost all governments advise it.

What Professor Speakman doesn't have, however, is any scientific evidence for this advice.

The number of exactly 2 litres isn't based on scientific fact.

It's more of a ballpark figure.

A number which is a guess, but which you still believe is approximately correct.

Actually, the amount of water in our bodies is changing all the time.

Like your bank balance, which goes down when you spend money, your body loses water all the time when you breathe, sweat or go to the toilet.

Exactly how much you need to drink depends on how much water your body needs to replace, and that mainly depends on your size.

But what happens when we drink less than we should?

Here's hydration expert Dr Nidia Rodriguez-Sanchez describing the effects of dehydration to BBC World Service's The Food Chain.

And also many times we start getting dehydrated and we don't realise 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 and we go and get some food and actually what is happening is that we are thirsty, that we are dehydrated.

We are starting to show some signs or some symptoms of dehydration.

Dr Sanchez describes the effect of dehydration, the condition of not having enough water in your body so that you feel ill or weak.

She mentions feeling tired, having a headache or a dry mouth as symptoms of dehydration.

Symptoms are signs or feelings in the body showing the presence of some illness or condition.

So whether you drink a little more or a little less, it seems that around two litres of water a day is a good way to keep your blood and body healthy.

Speaking of which, Phil, it's time to reveal the answer to my question.

What percentage of our blood is water?

You said 80% and the answer is 90%.

Our blood consists of around 90% water.

OK, let's recap the vocabulary we've used, starting with the phrase to arrive at a number, meaning to decide it by doing calculations.

If an idea takes grip on something, it takes control of it.

The phrase pretty much means almost.

For example, pretty much everyone likes chocolate, which means almost everyone likes chocolate.

A ballpark figure is a phrase meaning a number which is an acceptably accurate approximation.

Dehydration is the condition of not having enough water in your body so that you feel ill or weak.

And finally, a symptom is a sign or feeling in the body showing the presence of an illness or condition.

Once again, our six minutes are up.

Why not join me now and head over to our website bbclearningenglish.com, download the worksheet for this programme and test yourself to see how much you remember.

See you there soon.

Bye. bbclearningenglish.com