Archive for February 22nd, 2008

Glug glug glug … why those eight glasses a day don’t HAVE to be water (or eight).

February 22, 2008

The super successful campaign to persuade people to drink vast amounts of bottled water really has two parts.

One part, which we have already talked about, is to persuade people that tap water is dirty and harmful, while bottled water is clean, pure and healthy – not to mention aspirational.

The other part is to persuade people that they have to glug down vast quantities of water – not just liquid, water specifically – every day to stay healthy.

This latter part, it turns out, is just as much of a crock as the first bit.

It is A MYTH. An Urban Legend, peddled in the media and on the internet, and repeated by word of mouth, so often that many people believe it is true.

The standard version of it, repeated ad nauseam, is “drink eight glasses of water a day”. So prevalent is this message that even rather good newspaper articles like this one, which squarely nail the giant bottled water sales-fest, trot it out without question.

Sometimes you are also told how big the glasses should be: “eight ounces”, which to us metric European types is just under 240 milliliters (ml).

Thus the advice is sometimes short-handed (especially in the US) as “8 x 8” or “8 by 8”.

BUT… it turns out there is a big, big, piece of bullshit floating in this glass of nostrum.

Wait for it…

Any fluid will do!

Yes, as trailed in the title, the fluid doesn’t need to be water.

Truly. The total amount of “fluid intake” being suggested here (1900 ml per day, so nearly 2 litres) is sort-of reasonable (though still anywhere from 10 to 50% above what scientists and doctors think of as the normal daily drinking requirement), but it can be ANY fluid. Everything that you drink counts.

Water. Coffee. Tea. Herbal tea. Beer (yes, beer). Wine (yes, wine).

But…. If you knew that, then you wouldn’t need to keep slurping down water. Not to mention buying it in handy bottles.

So why DO so many people carry on with the endless water-sipping?

Answer: Because they have bought the message.

But… why weren’t we told 8×8 wasn’t true? And where does 8×8 come from?

Here is where it gets interesting. No-one seems to know. The evidence that the “8 x 8” figure was totally unscientific has been around for ages. And to most scientists, the idea that “only clear water counts as fluid” is so transcendentally silly that they probably never thought anyone would be crazy enough to believe it.

But people clearly did. So perhaps some eminent scientist needs to review the scientific literature, and explain just exactly why all the legends peddled by what I call the “Hydrationistas” are nonsense?

Well, it has been done. You can find a comprehensive scholarly demolition of all the water myths peddled by the Hydrationistas in a review written by Professor Heinz Valtin for the American Journal of Physiology here.

Heinz what…?

Heinz Valtin is an Emeritus Professor of Medicine and Physiology, noted for his seminal research over 40 years on how fluid output from the kidney is controlled. He originally qualified as an MD (medical doctor) and is the author of three textbooks on kidney physiology. Valtin was Head of a well-regarded Physiology department in the US (at Dartmouth University) and trained many other notable kidney physiologists and nephrologists (kidney doctors) over the years.


Above: Heinz Valtin and furry friend

Valtin is probably best known in science for his work on the Battleboro rat. This is a rat strain which does not produce any vasopressin (anti-diuretic hormone) and thus cannot concentrate urine (vasopressin is the hormone, released in the brain when your body is a bit short of fluid, that signals to your kidney to reabsorb water extra-efficiently).

Brattleboro rats, lacking the signal hormone, cannot reabsorb water very efficiently and thus cannot produce concentrated (v. dark) urine. So they pee out lots (and lots) of dilute wee. They compensate for this by drinking bucket-loads – it can be the equivalent of 70% of their body wt a day – and the famous picture above shows a Brattleboro rat with a beaker containing its daily fluid intake. Brattleboro rats, which are a natural “vasopressin knockout animal”, in the jargon, have lots of uses in research. However, what they show us very clearly – just by looking at the picture – is that your kidney urine output, and water intake, adjust to match one another.

This is a slight over-simplification, since you take in water other ways than drinking (in what you eat, about 1000 ml /day), make some water via cellular respiration (about 300 ml / day) and lose water other ways than via the kidneys (in poo, about 100 ml / day, and via sweating and via evaporation from your mucous membranes and lungs, around 800 ml / day). But the basic principle is good – if you drink more, you will pee more. There are people with the same kind of problem as the Brattleboro rat (lack of urinary concentrating ability). They have a rare disease called (slightly confusingly) ”Diabetes_insipidus”, or more specifically “Central Diabetes Insipidus” if their problem arises from lack of vasopressin secretion from the (central) nervous system.

Er… what was all that stuff for?

The purpose of this lengthy preamble is twofold: first, to acquaint you with some basic principles of fluid balance and fluid and electrolyte physiology (including that good old rule: In = Out); and second, to make clear why what Heinz Valtin doesn’t know about body fluid balance isn’t worth knowing.

So what happened when Valtin went looking for the source of the “eight glasses of water a day” line, and for any evidence that it was based on any science, or that drinking this much was beneficial?

The answer is simple.

He couldn’t find any evidence. Not one bit.

The source of the 8×8 advice is a real mystery. The best guess Valtin has was that about 60 years ago, round about the end of WWII, a US Govt report said something like “the total amount of all fluids you need to drink a day is about the equivalent of eight glasses of water” – although this was not based on any particular scientific study.

Over time, and with the aid of the Water-Nuts and of literally billions of pounds/dollars in marketing spend, this has been transformed into “Eight glasses of water – you must drink this! – and other liquids don’t count!”

The other “Hydrationista Myths”

In his review Valtin also nails many of the other silly, but widely repeated, lines the Hydrationistas employ:

“Caffeinated drinks don’t count as part of your fluid intake, because they dehydrate you”

Not true. Of course they count – they are mostly water, so they are fluid. The slight diuretic effect of the caffeine in the drink does not “offset” all the water that is in it. We will come back to this one in a minute.

Note that I’m not talking here about small drinks with loads of caffeine, like a triple Expresso or a can of Red Bull. I’m talking about the kind of coffee, or soft drink, that most people drink. My standard cup of coffee is about 275 ml, which would take a fair bit of peeing out.

“If your urine looks dark, you are dehydrated”

Not true. It would be more accurately to say that “pale yellow” or “almost clear” wee (which the Hydrationistas tell you you should look for) means you have water –loaded yourself (i.e. you have drunk loads extra) and thus are peeing out unusually extra dilute stuff.

“When you start to feel thirsty, that means you are already dehydrated”

Not true. Your body detects tiny changes in “body water” very well. To put it more precisely, the body detects how concentrated your body fluids are, that is, their “osmolarity”, and a change of 2% is detected easily. The body then tells you to correct this small change by drinking more and peeing less out. The wonders of evolution. Anyway, the system is both incredibly sensitive, and fast. So you get thirsty because it is time for you to have a drink, but NOT because you are dehydrated.

[Dehydrated is what you will get if you start feeling thirsty and then drink nothing for the next several hrs. Dehydration is usually taken to mean that your body osmolarity has risen by 5% or more. So you get thirsty, and drink, without ever being close to being dehydrated.]

– and finally, one Valtin doesn’t discuss in precisely this form, but which has recently turned up in the literature for “Brain Gym” (comprehensively trashed by Ben Goldacre here).

“Liquids (other than water) are processed in the body as food, and do not serve the body’s water needs.”

Again not true, and total scientific nonsense. Common sense should tell you that a bit of dissolved sugar does not stop water being water. So the other fluids count. And so does water in what you eat, roughly a litre a day of it, see below.

Being charitable, this statement COULD have its origins in the fact that there is a bit of evidence, which Valtin discusses, that taking more fluids along with your meals might promote satiety (feeling full). Therefore one could hypothesize that consuming foods with high water content might make you feel fuller (all else being equal, which it rarely would be, so that all other satiety cues were the same).

Anyway, this might mean that it is a good idea to drink something with your meal (which most people do anyway, of course) since it could help moderate how much you eat.

I can just about see how, if you were a bit confused, you might interpret this as “that water in what you ate was processed as food” and hence didn’t count as water. You would be totally wrong, though. Of course, your confusion suits the Hydrationistas perfectly.


Back to your Daily FLUID (not water) intake

Your total water requirement is “filled” by the fluids you take in over the courses of a day. Some of that water is in your food, depending in amount on what food you eat – a general value that appears widely in physiology textbooks, as we already noted, is that you get about a litre of water this way. It depends on exactly what food – it wouldn’t take a genius to work out that grapes, or watermelon, or soft fruit generally, is mostly water. But all food contains water. The milk you put on your cereal (if you do) contains water. And so on, and so on.

If humans needed clear fluid above and beyond “other water intake”, then breast-fed new-born babies would all be dying of dehydration.

By now it should be abundantly clear that, if you take a quantitative look, a large part of the daily “drinking requirement” will be met by drinks OTHER than water. Hence all those family stories about “Grandpa Albert never drank water, only tea”. Valtin gives an example of a day’s intake from himself:

Representative daily fluid intake of H Valtin recorded on 29th Aug 01

Breakfast coffee with milk 650

Orange juice 175

Lunch cranberry juice 240

Dinner cocktail 125

Water 250

Total fluid intake 1,440 ml

And here, for comparison, is mine, from last Sunday:

Representative daily fluid intake by the author recorded on Feb 17th 08

Breakfast Coffee with milk 550

Lunch Diluted apple juice 325

Coffee 275

Dinner White wine 300

Water 350

Total fluid intake 1,800 ml

These amounts match well to the widely-recognised daily fluid requirement of a standard human, that being typically summarized in physiology textbooks as “the 70 kg man”. Valtin summarizes various measurements of daily fluid intake, all of which tell you about the same thing – your daily drinking requirement, assuming you eat an average sort of diet, is probably 1.2-1.7 litres, give or take. Inter-person variability will mean a range of values.

Another thing you can measure easily is the amount you wee out in 24 hrs. This will be balanced, very approximately, by your intake. Again, Valtin summarizes the studies that suggest that average daily urine output is around 1.7 litres (1700 ml). The similarity of this to the fluid intake is clear.

The above is slightly simplified since, remember, there are there are other ways you gain and lose water. But the clear take-home message is that your body’s fluid balance system turns over nice and happily if you drink about 1.5 litres (1500 ml) or liquids (all of them), and wee out something slightly more. And if the mean fluid intake value is about 1500 ml, then anything between 1200 and 1800 might be right for you. Humans, after all, are not all the same.

But to feel you have to sluice down nearly two litres of water, as well as whatever else you drink a day – give me a break.

But we KNOW all that extra glugging is good for us! You can’t prove it isn’t!

Of course, some of the Hydrationistas insist they have an answer to this. They cannot really dispute the scientific body of evidence, as so masterfully summarised by Valtin. Instead, they simply claim it is all irrelevant, as there is no trial to show that drinking loads of extra water ISN’T good for you.

Dr John Briffa exemplifies this approach. A couple of months ago he discussed this issue on his blog, prompted by an article in the Christmas 2007 issue of the British Medical Journal that debunked several “health myths” including “drink eight glasses of water a day”.

“Absence of evidence”, intoned Dr B in one of his favourite lines, “is not evidence of absence”.

And other Hydrationistas take similar lines. That standard intake (around 2-2.5 litres by all routes, including food, or 1.5 litres of fluid) is simply that needed to avoid dehydration, they say. But it is healthier to drink lots more.

Note that they give no evidence for why this should be true – because there isn’t any. As Valtin clearly sets out, if you are a normal human individual, and you take more water in, your homeostatic system will rapidly adjust and pee more out. That is what your body has evolved to do. The extra flushing will mean more trips to the toilet, and considerable wallet-lightening if you are a bottled water enthusiast. But it won’t make you healthier.



And… absence of what kind of evidence?

Ah, say Dr Briffa and the other Hydrationistas, how do you know that? There is no rigorous clinical trial that says so!

Now this is an interesting general point, because it relates to a wider question about “Alt Health modalities”.

Is it really necessary to run complex, and expensive, large modern trials to demonstrate something is useless, when there is 50+ years worth of scientific and medical evidence, extensively tested, re-tested, critiqued and reviewed, showing that the theories on which the idea is based have zero validity?

In the water context, there is not a scintilla of scientific or medical evidence giving any grounds for the idea that super-slurping water will make you “better than well”

On the contrary, to repeat it for the umpteenth time in this piece, there is shed-loads of evidence that around 1.5 litres a day of LIQUID, plus what is in your food, is what your body has evolved to be happy with. And furthermore, that your body will “adjust” your thirst to get you to drink just that.

In statistical terms, what we would say is that the “prior probability” of the Hydrationistas’ view – “drinking lots of extra water, over and above what your body actually requires, is good for you” – being true is extremely low.

So under these circumstances, why spend millions (and it would be millions) running a trial to try and show that extra water drinking has health benefits?

Or put another way, on whom should any “burden of proof” lie for the idea that drinking an extra litre of water a day is “healthy”?

On the scientific and medical mainstream?

I don’t think so. The burden of proof clearly lies with the Hydrationistas. It is not the job of mainstream medicine and science to have to keep proving what is already brain-achingly obvious, and supported by all the science, simply because the Alt-ies find it convenient to say “No fair! No evidence!”

And it’s not as if there’s no money in water for anyone who could prove super-slurping made you “better than well”. Bottled water is a global mega-industry, with a market worth something close to a staggering Two Billion pounds a year in the UK alone. Plenty of potential funding there for some enterprising Hydrationista to run a proper study to demonstrate the benefits of super-consumption of bottled water.

Indeed, when he wrote his review five years ago, Heinz Valtin said exactly this.

“In view of the strong suggestive evidence cited [that people don’t need 8 glasses of water a day]… I would argue further that… the burden of proof that everyone needs 8 × 8 should fall on those who persist in advocating the high fluid intake without… any scientific support.”


And a bit later:


“Having found no evidence in support of 8 × 8 has placed me in the awkward position of having to prove a negative. I hope, therefore, that anyone who knows of contrary scientific evidence will bring it to my attention.”


Five years later, Valtin, and the rest of us, are still waiting. From which you can draw your own conclusions.



Drinking water – or bathing in it – can be deadly (not) – part 2 The men in grey suits… are actually on the case

Drinking water – or bathing in it – can be deadly (not)