BRITISH BROADCASTING CORPORATION
RADIO SCIENCE UNIT
CASE NOTES Programme no. 6 - Water
TX DATE: TUESDAY 18TH MAY 2010 2100-2130
PRESENTER: MARK PORTER
CONTRIBUTORS: BRIDGET BENELAM
PRODUCER: ERIKA WRIGHT
NOT CHECKED AS BROADCAST
ACTUALITY AT UNIVERSITY OF BEDFORDSHIRE
We need you to pop on the bed and take your socks and shoes off. Okay.
So this is going to do what?
The machine itself sends different frequencies of electricity through your body. Very
low voltage - no danger. No danger but what it will do is uses that to measure how
much water's in the body.
How old are you? Are we allowed to say?
Yeah, I'm 48.
Oh it doesn't go up that high.
The indignities I suffer for the sake of this programme - in this case being wired up to
find out what proportion of my body is water, because, as I will be finding out, it's by
far the biggest single ingredient in our make up. And maintaining it at just the right
level is crucial to good health.
But how much water do we actually need to maintain optimum hydration? Is
dehydration as much of a threat to our well being as people seem to think? And does
it have to be water?
Nutrition Scientist Bridget Benelam has just published a comprehensive review of the
evidence linking hydration to health in response to confused messages in the media.
One week it seemed that there'd been a scare story about somebody drinking too
much water and getting very ill as a result and the next week there was a story that
suggested that you needed to drink litres and litres of water on top of all your other
fluids and the next week it would be something different. So it really seemed an area
that was very confused. And obviously water is absolutely vital for us to survive and
is almost the most important nutrient there is when we look at nutrition as a whole.
And although we can survive for probably a good couple of months without any food
we can only survive a few days without water. So good hydration is absolutely vital
for us to survive.
So how much do we actually need?
Well our recommendations of six to eight glasses and that's about 1.2 litres of fluid
per day are a good place to start but actually everybody needs a different amount of
water and even one individual will need different amounts of fluid on different days,
depending on what they're doing and depending on the environment and how active
they are. So although we have that recommendation as a starting point, actually
everybody needs to take their own individual needs into account. So if you're very
active one day you will need more than the previous day, if it's hotter than you'll need
more, if you're going through any particular illnesses, anything like that, then you'll
also need to take on more fluids.
Does it vary between the sexes?
It does vary, yes. The main variation is down to your body composition and how
much fat do you have to compared to how much muscle. Muscle is about 70-80%
water, whereas fat is only about 20% - so the higher your body fat the lower your
proportion of water as a whole.
These are special electrodes, long thin ones, and then we'll attach the machine to these
So they look like just - well they're sort of little sticky backed electrodes, I've got two
on my right hand and two on the top of my right foot.
Here comes the machine.
Can you shuffle over that ...
... put the machine on, that's fine. And very quickly in fact it will tell you how much
body water you've got.
How much water a person's body contains will vary depending on their make up, but
it's generally at least 50%. The water is spread between two compartments -
intracellular fluid inside the trillions of cells that make up every tissue, and
extracellular fluid which is everything else including blood. Dr Nick Sculthorpe is
senior lecturer in exercise physiology at the University of Bedfordshire.
Alright so the machine's finished its analysis, it's told you you've got 25.4% body fat
but for your age you're looking at an average of between 22 and 28, so that's okay. So
your total body water is 50.5%, so just over half of your body mass is made up from
And that equates to sort of 43 ...
Forty three litres.
Well I think a lot of people would be surprised that looking at a human body that it
can possibly be that much water. Because when we think of water most people think
of course of the blood but actually that's only a small percentage of the fluid, most of
the fluid is inside ourselves ...
Must of it's inside the cells or in between the cells. So for you your total body water
was 42.9 litres. Of that 49 litres maybe five and a half litres will be blood, so
although most people think of blood that's really a relatively small amount of the
water that's in the body. The majority of it is inside each cell because the cells need
water inside them because they have lots of reactions going on.
So they're effectively little capsules of ...
...and made up of little cells full of water.
And the interstitial fluid - this is what people might think of as tissue fluid effectively.
Tissue fluid, so this - if you think of cells and you tend to think of cells like bricks and
they're not quite that jammed packed together but if you think of bricks and think of
the mortar in between bricks the interstitial fluid will be that mortar that you would
see in between different bricks, it doesn't glue them together but it allows for transport
in and out of those cells.
And that's in every tissue?
That's in every tissue, yeah, exactly so muscle tissue, bone, brain, everything.
An average sized man's body will be made up of around 50 litres of water, a typical
woman closer to 30 litres - but whatever an individual's ideal level, it has to be kept
constant for optimum health. A loss of just 1 - 2% can lead to symptoms of
dehydration such as fatigue, headaches and impaired mental and physical
Some loss is inevitable - even at rest in a cool environment we all lose around 900
mils a day via our bowels, in our sweat or on our breath - but the body goes to great
lengths to conserve fluid. Take the gut - we all produce around 10 litres of digestive
juices a day, that's saliva, bile etc., but the bowel reabsorbs 99% of them. A level of
conservation that pales into insignificance compared to what the kidneys are capable
of - a function that evolved from a time when our surroundings were as salty as our
insides. Detlef Böckenhauer is a consultant paediatric nephrologist at Great Ormond
Crucial parts of our body developed when we were still living in the ocean, so there's
a high concentration of salts around and our nerve cells and our muscles they really
only can function if there's a steady concentration of these certain salts around which
when we were living in the sea was a given, that was just our environment. But
eventually life changed and organisms moved out of the sea and on to land and that
really was only possible by the kidneys maintaining this internal milieu of salts, sort
of conserving the ocean within us. So bathing our muscles, our nerve cells, our brain
in this constant environment of salts so that our nerve cells can fire, our muscles can
move, that we can think a thought, that we can jump up and scream of joy or
something like that. That's really only possible because the kidneys maintain this
constant internal milieu.
And the kidneys themselves obviously can't influence what we're taking in, so they
must be controlling what we're putting out - how do they do that?
Yes, so that's - the first thing the kidney does is it just filters the blood, so we have
tiny little filters in our kidney, about a million in each kidney which we call glomeruli
and these filters are there to hold back all the big stuff that we don't want to lose in the
urine, like the blood cells, the red cells, the white cells, the platelets but also big
proteins and so on. And the small stuff - the water and the solutes - fall through these
filters and that forms what we call the primary urine. And we make a lot of that, we
make about 150 litres of that primary urine per day and it contains about one and a
half kilos of salt, table salt. So obviously that's not what's coming out in the end, in
the urine, it would be terrible if we would be weeing out 150 litres per day. So the
vast majority of that needs to be reabsorbed by the kidney back into the bloodstream.
So following these little filters are small tubular structures, which we call tubules, and
they have lots of transporters and channels in there which are designed to reabsorb the
water and the salt back into the bloodstream and to adjust that exactly to what's going
in. So if you're drinking a lot we will be making more urine, if we're eating lots of
salt we will be excreting more salt in the urine and the other way round.
And does it make any difference what time of day we're talking about? I mean most
people will be familiar with the fact that we're obviously drinking more and probably
passing more urine during the day but at night we're asleep, we're not drinking.
Yes, so it's adjusted to what's going in, on a minute by minute real time basis, an
orchestra of transporters and channels in there, each of them having exactly the
defined function and they all have to work together in order to get this composition
just right. So if you're having a big meal with lots of salt and protein, well your
kidney will immediately in the hours afterwards try to get rid of all the waste products
that come from the proteins and the salt and if you've been drinking with it then
obviously also the fluid that came in with it.
Our fluid conservation systems are finely honed and capable of rapid adjustment to
meet changing situations. So is significant dehydration - the 1 - 2 % loss widely
regarded as necessary before we notice any symptoms - as big a threat as most people
Just how long would you need to go without drinking in a climate like ours before you
approached this sort of level of dehydration? Bridget Benelam
Well when people have tested this and done this by not allowing subjects to consume
any fluids and to only consume very dry foods they've then measured the percentage
loss of bodyweight due to dehydration and after about 12 hours, so sort of during the
day, they found about a 1% loss of bodyweight, over 24 hours about a 2% loss of
bodyweight and 3% after one and a half days, so 36 hours, 37 hours. They did note
that the subjects were thirsty and really wanted to drink and this probably wouldn't
have happened accidentally. So I think the idea that we're all walking around at very
high risk of getting dehydrated is not really founded in fact.
Which suggests the risk of dehydration are overblown, given that it would take most
of us around 12 hours to get to a stage when our well being and performance are
likely to suffer - and to do so you would have to ignore the pangs of thirst. But what
drives that thirst? Nick Sculthorpe.
If you've lost a lot of water and you need to drink that becomes what we call
hypertonic, so you've lost water but you've still got lots of salt and so on, so it's very
So it becomes saltier?
It becomes salty, your plasma, if you like, becomes saltier. And what happens in
certain parts of the brain is you've got this salty plasma, if you like, and you've got
cells that have got water in and it pulls the water out of those cells to try and even out
the concentration. And as those cells become more dehydrated, as they become more
dehydrated that sets off a thirst reflex that says I need to go and find some drink
because I've got insufficient water. If you get quite dehydrated then you produce
again certain hormones that will cause a thirst reflex of itself.
So once the thirst reflex is triggered I go to the tap, pour myself a large glass of water,
there's no way that the water I'm drinking can get into my body quick enough to
equalise out that salty concentration, I mean presumably that takes an hour or so?
That takes a little bit of time but you've got - again you've got another set of sensors -
the body's full of sensors - but you've got another set of sensors, some are in the
mouth and some are in the gut, and they also play a role on both sides of the thirst
reflex, so if you are thirsty you've probably got a dry mouth and again there's nerves
that go from the mouth to that same area of the brain that says I've got a dry mouth, I
need something to drink and you've got sensors in the stomach that will tell that area
of the brain - I've just had a drink, I know we haven't sorted out the saltiness of the
blood yet but that's coming, there's water in the system, it's on its way. And that will
start to switch off the thirst reflex, even though you haven't quite normalised the
And that's important presumably because if we had to depend on settling down,
switching off the thirst reflex ...
It would be too slow.
... you'd have to wait an hour by which time I might have drunk 20 pints which would
be too much.
Yeah, I'm still thirsty, I'm still thirsty and that in itself is dangerous at the other end.
So you need a - there's a long term mechanism to tell you you're thirsty but you need a
fast switch to say I've taken on some fluid ...
Let's wait and see what happens.
So generally the first reflex would err on the side of it's conservative?
It tends to be conservative, you tend not to get particularly thirsty until you're already
But the thirst reflex, like many others, can blunt with age making it a less reliable
pointer for the elderly. Bridget Benelam.
We do have evidence that the thirst response decreases in older adults, so over the age
of 60 years it appears that the sensation of thirst is not as accurate as it is when we're
younger. And this combined with other physiological issues as people get older, say
reduced kidney function, greater use of drugs, being less mobile, this can impact on
people's hydration status and actually when it comes to hospital admissions it seems
that quite a large proportion of older people are actually admitted to hospital fairly
dehydrated. And I think this is a really important issue and trials in old people's
homes have actually found that increasing water consumption, so helping people to
drink, making sure they've got water available to them, really improved health, it
reduces falls, it reduces urinary infections and constipation. And being dehydrated
has detrimental effects, you can measure decreases in cognitive function, decreases in
physical function that will result in symptoms like headaches, feeling lethargic,
feeling tired and not performing our best.
And performance is key in sports and athletics - particularly if they involve exertion
in conditions that accelerate dehydration through sweating. John Brewer is Professor
of Sport at the University of Bedfordshire.
Mark this is the University of Bedfordshire's environmental chamber.
Right, well let's go in.
Ooh it's like getting off the plane, when you fly on the plane and get off somewhere
hot and exotic like Thailand. It's hot in here and humid.
Very hot and humid, it's been set to imitate the conditions that you would find in a
very hot and humid climate. For example: Delhi, where the Commonwealth Games
will be in October will have a very similar climate to this. Somewhere in the region
of the mid 30 degrees Celsius and 60-70% humidity, clearly placing a real challenge
of people who will be exercising.
So you can - I see there's an exercise bike in the corner, which I'll have a go on in a
minute I suspect. But you can put athletes through their paces in here and that helps
them in what way?
It helps them to acclimatise because sweating is a real friend of people who are
exercising in these types of conditions. You need to sweat in order to stay cool. So
when you exercise as well as producing energy the body also produces heat and if we
don't lose that heat the body will have reached boiling point within about eight or nine
miles of running. And the way we avoid that is through sweating. And it's that loss
of latent heat from the sweat evaporating off the skin into the environment that
causes, or one of the main reasons why the body will stay cool. The problem we get
is that if we stop sweating or if we can't sweat efficiently then the whole ability of the
body to stay cool is impaired, the body temperature, the core temperature, goes up and
up and performance is very quickly impaired.
And is that something that athletes can train - can they improve their sweating
Yeah they can, they can - by acclimatising to hot and humid environments they can
improve that sweat ability, they will lose more sweat and stay cool in a much easier
and more efficient way.
Of course the downside of that, the reason why we're here, is that if you sweat more
you may keep yourself cool efficiently but you're losing fluid.
Absolutely, it's a real chicken and egg situation I suppose, the more you sweat clearly
the more of the valuable fluid stores that the body has you will lose and use up. So
the balance, or the crucial balance is to ensure that the fluid that you take in tries to
match the fluid that you will be losing when you are sweating and that's a real
challenge for people in hot and humid environments. Or indeed, to be quite honest, in
the more temperate environments and cooler environments that we have in this
Looking at the exercise bike here if I have a go - I mean I just imagine - I'm sweating
here, it's uncomfortable just being in here let alone cycling. But presumably you get
somebody working on here, I mean what sort of loss are we talking about?
Well if we take people who are working really hard, particularly endurance athletes,
then they've been shown to lose two, three, four litres of fluid per hour, so really high
fluid loss rates and unless that fluid is replaced then increasingly the body becomes
more and more dehydrated and that's when performance starts to suffer. The science
would suggest that if you lose more than about 2% of your bodyweight from sweating
then your performance is impaired. Now if you think - say somebody who's around
70-75 kilograms 2% of bodyweight is one and a half to two kilos at most. Now given
a sweat rate of two, three, four litres an hour you can clearly lose that 2% within the
first half hour, 45 minutes of exercise in an environment like this when you're
sweating very rapidly. So that challenge of sweating, weight loss, performance
impairment is one that really hits home very quickly.
I can see it working - exercising, cycling, even just going for a gentle cycle ride in
conditions like this is going to make you lose a lot of fluid but for Joe Bloggs going
for a jog in the UK at this time of year in spring is fluid loss a series consideration?
I think being completely honest it probably isn't, as long as you're not going for more
than 20 or 30 minutes and most people will have perfectly adequate fluid stores to last
for 20 or 30 minutes and as long as they rehydrate sensibly after a run then they
should be fine. Clearly even if they run for 20 or 30 minutes or cycle for 20 or 30
minutes and then don't rehydrate you can have a cumulative effect of dehydration, if
they went back into the office into an air conditioned environment and carried on
sweating then they will eventually lose sweat and become inefficient and their work
day or work life performance will suffer. But by and large for your average person in
a cool environment 20-30 minutes is okay. Once we get beyond that, if you're playing
football for the Saturday afternoon pub team or playing a game of netball or whatever
it might be, even in the climate that we have in this country you will still sweat, your
performance will be impaired and even if your performance isn't impaired during the
event itself it's still important that you rehydrate properly afterwards, so that you can
go back to your normal daily life in a more efficient manner.
Well I think I've got a bit of adapting to go, I think we should head for some fresh air,
I'm going to get off this bike.
You can probably just see perhaps if you go to an exercise class, for example, that
some people will sweat buckets and some people will hardly sweat at all while doing
the same activity.
Does it make any difference what we replace that sweat with, does it have to be
It doesn't necessarily have to be water and indeed if you're losing a lot of fluid and a
lot of sodium in your sweat then water may not actually be the best thing to rehydrate
with, so at the sort of elite level of athletics or sport people will be using drinks that
contain sodium and sugars in order to absorb the water more quickly and also to
replace some of the sodium that you lose in sweat.
How do we know if we're having enough fluids?
One way, although it may not be so palatable for people, is to look down the toilet, to
look at your urine colour and to check if it's about pale straw colour then you're about
right in terms of your hydration but if it's much darker than this this means that your
urine is getting more concentrated and you may not be getting enough fluids.
The link being that obviously if you're on the dry side your kidneys are concentrating
your urine to try and conserve fluid but if it's a pale straw colour it means they're not,
so everything's working fine?
Exactly and equally if your urine is completely clear then you may actually be getting
a bit more than you need and you can probably hold back on drinking so much.
Well John we're back outside in a much cooler lab I'm pleased to say, I'm sweating a
lot, let's give some practical advice now. I'm a casual runner, I run for half an hour,
what should I be doing before, during and after?
The first thing that you should do and it's slightly indelicate is check the colour of
your urine, so literally the target should be for it to be a light straw colour and as long
as that's the case then you are already properly hydrated.
So I'm starting in the right place.
Starting in the right place. The next thing is - it's a bit like consider watering your
garden on the morning before the garden show - if you think about hydrating with
only an hour or so to go before exercise you've probably left it too late. So wind the
tape back a couple of days and make sure you stay properly hydrated. And then a
very simple technique that people can also do is to weigh themselves before their
exercise - whether it's training or competition - and then weigh themselves afterwards
and not mislead themselves into thinking that that weight loss is a loss of body fat
because I'm afraid it isn't, the majority of that weight loss has come from a loss of
fluid. And for every kilogram of weight that you've lost that's the equivalent of about
one litre of fluid. And by doing that you can get a feel for your own personal rate of
fluid loss because different people will sweat at different rates. So if you're somebody
who knows that you lose say one, one and a half kilos on average after a game of
football or a game of netball or a game of hockey it's about one to one and half litres
of fluid that you need to consume afterwards to replace the fluid that you've lost.
In terms of timing does it matter when I do it, I mean obviously people have the odd
drink during a match but difficult to take on a lot of fluid?
Yeah it is, again it's down to personal preference but generally speaking people would
say sort of an hour or so beforehand is when you should stop taking in too much fluid.
The biggest problem that you will have if you take in too much fluid too close to an
event or even during an event if you're not used to it is stitch - that sort of cramping
feeling that you get in the stomach and I'm sure you, like me, have experienced it in
Had plenty of that.
Yeah and it really is very debilitating, very difficult to stop, it seems as if it's the
diaphragm muscle that's cramping. So it's really a case of getting an idea of what
suits you best and I think the main recommendation is small amounts of fluid
frequently and don't suddenly gulp down loads of fluid, particularly if it's cold fluid,
in one large volume because that is more likely to cause nausea and stomach cramps
and stitch than taking small amounts of fluid frequently.
But if you know you lose one and a half kilos, so we're saying that's one and a half
litres of water and you've only had half a litre during the match, you've got another
litre to make up afterwards and presumably get that in as quickly as possible?
Within reason, you don't need to be stupid but you need to just make sure that you
drink that fairly quickly and of course in many recreational sports there is a
temptation to head straight to the bar in the club house after a game. I think the
important thing is to make sure that the first drink that you have is a non alcoholic
drink to replace the fluid and then obviously the odd social pint or two with the rest of
the team to celebrate or commiserate. But if you try to rehydrate exclusively with
alcohol then unfortunately you're not really following the right scientific strategy.
So what should we be doing to ensure we are optimally hydrated? And does it matter
what we drink? Do tea, coffee, juices and soft drinks count? And what about
alcohol? Bridget Benelam.
Anything really above normal strength beer will actually dehydrate you so wines,
spirits, all of those kinds of drinks will actually lead to a net fluid loss. Medium and
lower strength beers will cause you to lose a bit of extra fluid in your urine but
actually because you drink them in large volumes then you'll get a net fluid gain.
So this perception that when you wake up the next morning after you've had slightly
too many beers that it's due to dehydration, that's not the case, although it might be
with what you're saying stronger drinks like wine and spirit?
Yes anything above the strength of sort of normal weak strength wines will actually
cause a net loss in fluid, so will actively dehydrate you. So if you're only drinking
wine and you're not consuming any other fluid, such as water or other drinks, then
you'll be progressively dehydrating yourself by drinking wine or spirits.
And can you quantify that net loss in terms of specific drinks?
Yes sure, so if we're looking at a large glass of sort of standard strength wine, we're
looking at over 100 millilitres of fluid lost overall. And if we look at a double
measure of spirit that goes up to sort of 150 or so millilitres lost.
So that's quite significant, if you drink a lot of single shot spirits during the night with
no mixers and no other fluids you're going to end up losing quite a lot of fluid?
Yes absolutely and your body has to lose a small amount of fluid anyway, there's no
way we can completely stop losing fluid, we're constantly losing a little bit, so
consuming those kinds of drinks will actually add on top of that and gradually
dehydrate you if you're not consuming extra fluid.
Alcoholic drinks are the other problem with people slaking their thirst, particularly
Tom Sanders is Professor of Nutrition and Dietetics at Kings College Hospital
The problem is people are drinking too much what I call sugar sweetened beverages
now and in the States you see this very obviously where they're consuming 600, 700
mils a day of sugar sweetened beverages and that's probably one of the main reasons
for the increases in calorie intake in the USA.
So as a nutritionist I mean your concern would be that they're slaking their thirst with
other drinks, I mean even some that might be outwardly healthy like orange juice and
smoothies can carry problems too?
Oh yeah I mean I think something like a juice, a glass of juice a day is fine, but
slaking your thirst with that is extra calories, just as many calories in a fizzy soft drink
as there is in fruit juice and some of the smoothies are even higher. So the best way to
hydrate is water, tea is okay but it's a diuretic, so it makes you want to go to the loo
and coffee even more so. Water is neutral.
In terms of tea and coffee and their caffeine content it is true that caffeine can be a
mild diuretic but actually in the sort of moderate amounts that most people consume
tea and coffee, say up to about five cups of instant coffee a day, it appears that the
level of caffeine that causes the additional urine production is not actually reached.
So in terms of our day to day consumption of tea and coffee, say two or three or four
cups a day, it's probably not going to make any additional difference to urine
production so it counts towards our fluid intake, just as a glass of water would. And
indeed even more than that there would be some additional loss of urine but that's
probably more than compensated for by the fluid in the tea or coffee itself. So unless
you're drinking very strong coffees, for example, say espresso coffee which doesn't
provide you with much fluid, then tea and coffee can count towards your fluid intake,
just in the way that water can.
So, to recap. As long as you are not sweating heavily, thirst is a reasonable indicator
of the need to drink and if you respond accordingly, you are unlikely ever to get
And, contrary to common perception, there is no evidence that drinking more will
improve how you feel by flushing out toxins. It just means that you will need to go to
the loo more often.
You don't have to drink water, but it is calorie and tooth dissolving acid free, and the
healthiest way to rehydrate.
I guess the danger is that people are unnecessarily alarmed by the idea that they're
going to become dehydrated. I think it's getting that balance message across that
obviously we need water, it's very important for our health but it's not necessary to be
constantly drinking litres and litres of fluids on top of our tea and coffee and to feel
like no fluids apart from water will count towards our dehydration.