water + body = ?????

January 28, 2007 7:59 AM

Beyond the idea that "Water is essential to carry out processes in the human body," I do not understand the more complicated health relationship between water and our bodies.

If you DON'T drink it, you lose weight? If you DO drink it you lose weight? Why do people retain water? If you retain it, why is that bad? Why do people take diuretics? How do they decide how much water someone needs to drink? I can understand drinking it if it "makes you feel better overall," but what if it doesn't?

And I'm not even going to start with the whole bottle vs. tap issue. Or fluoridation.

I can see it helping prevent cancer (sort of), but I can't find any studies indicating this. When I search pubmed, I get articles about the "mucus slurper" in sheep, but that's neither here nor there. In fact, mostly I just find articles about how drinking contaminated water is bad, and how there is a benzene cloud (not to mention estrogen) potentially creeping into my water supply.

posted by unknowncommand to health & fitness (25 comments total)

From this link:

Why We Need to Drink Water
Our bodies are estimated to be about 60 to 70 water. Blood is mostly water, and our muscles, lungs, and brain all contain a lot of water. We need to drink water because water is needed to regulate body temperature and to provide the means for nutrients to travel to all our organs. Water also transports oxygen to our cells, removes waste, and protects our joints and organs.

More info, written for kids.


and one more, that goes to many of your questions.
posted by Brandon Blatcher at 8:14 AM on January 28, 2007

I understand the info in the first two links, but I don't think that they help very much with my questions. And your last link is written by Aquasana, and says that drinking water prevents ADD. That's the problem when I googled for this.
posted by unknowncommand at 8:19 AM on January 28, 2007

I'm not a physiologist or a doc but I'll point you to what's relevant about water and the human body. First, you should know that the body is not a closed system. The food that you eat contains minerals that affect the ionic balance of the fluids in your body (i.e. intracellular fluid, blood, etc.). To make sure these fluids contain the right concentrations of the different ions (Ca, K, Na, H, etc), your kidney processes your blood to excrete or reabsorb fluid/ions.

Water rentention can affect blood pressure and by taking a diuretic, you can tweak the physiological feedback system that regulates blood pressure.

See: Wikipedia entry on Kidney Physiology. If you have access to a good library, Bern & Levy's Principles of Physiology has info on how the body controls all these different variables.
posted by scalespace at 8:42 AM on January 28, 2007

Snopes on drinking 8 glasses of water a day and other nonsense.

Summary: of course you need to drink some water, but you get some from your food too. Drink when you feel thirsty.
posted by gauchodaspampas at 8:46 AM on January 28, 2007 [2 favorites has favorites]

I have some brief answers to most of your questions. You might be interested in a college-level human biology text to get into the chemistry.
1) If you don't drink water, you will become dehydrated. Weigh a bottle of water sometime. This is why dehydrated people can claim to have "lost weight". (Spoiler: a bottle of water which around 1/6th your daily reccomendation weighs around a pound.) Enter steam rooms as a diet solution.
2) If you drink enough water it prevents overeating in your diet. The feeling of hunger and thirst are very similar.
3) People retain water because in general developed nation's diets have way WAY too much salt. Retaining water isn't really a health threat as it normally occurs in most people. It does cause puffy midriff. Which leads me to #4
4) Only desperate people take diuretics. They are not useful for anything besides screwing up the electrolyte balance in your body. You will probably lose about 5lbs if you take them, but simply see #1 if this begins to look tempting.

Sorry I don't know the last answer for sure. Personally I tried for the 8-10 reccomended until I really knew what dehydrated felt like, and then cut back a little because I'm a lot smaller than the average population. I drink like 4-8 glasses a day. Some days I eat all roughage and don't really feel thirsty.
posted by shownomercy at 8:51 AM on January 28, 2007 [1 favorite has favorites]

That snopes article is great. It's such an accepted fact that it never occurred to me to look there.
posted by unknowncommand at 8:56 AM on January 28, 2007

Drink when you feel thirsty.

Isn't that too late in some cases?
posted by Brandon Blatcher at 9:02 AM on January 28, 2007

This will not answer all of your questions and I don't have the attention span to type and type, but I hope this will at least give you some info for further research and that others will post additonal info. This is simplified/not quite reality - but go with the main ideas.

One big reason that you need adequate water (yet not too much!) is - revisit the term osmosis from intro biology. Your cells throughout your body have water (mixed with other substances, to make it easy, break those substances down to tiny granules). Now you drink in water, and some of this travels through your blood supply, mixed with other substances (ions, blood cells, proteins) - to make it easy, imagine a certain amount of tiny granules.
Don't remember the exact amount (it isn't this number, but you will get the main idea) - but let's pretend material in blood has a concentration of 97% water, and interstitial fluid and cells in your body 97% water. Water will move through cells from an area of high concentration to an area of low concentration.

Imagine drinking lots of water it immediately goes into your blood. Now you may have 98% in your blood - which will start to move into other cells. If there is way too much water - those cells could break open.

Now imagine the reverse. Don't drink enough water. Eventually, the concentration of water falls (let's say 97%), water will move out of cells. Either extreme is not healthy, and both cases can and have resulted in death.


One reason someone can retain water (although this is pathological, other underlying reasons too) - amount of those little granules in the water will create a pressure. In a healthy person, fluid and proteins (only a small amount) leave the capillaries - some of little granules come back through lymphatic vessels, but that same water material goes back into the capillaries (if you can find a picture of a capillary, with pressure at either end and direction of fluid this would be helpful). Some older people with blood pressure problems and other vascuature probs - fluid will leave the capillaries, can't come back into the capillary. At first this is a small amount but over time - you can actually see an accumulation of fluid (think swollen ankles, etc)

Deciding how much water to drink - if you are healthy, I am assuming the special cells in your hypothalamus work well. There are special cells here which do shrivel if you are dehydrated/or increase in size if you have to much - modified release of chemical signal released, which modifies your desire to drink water. Personally I would use that unless you have had problems (years ago, people were told to continuously drink water running a marathon and had it provided every few steps - people died from drinking too much/back to osmosis)

This is a start - anyway, I would suggest an intro bio or anatomy or physiology book right now over pubmed. There is far more to this story, but I'll let the other posters take over.

Have fun.
posted by Wolfster at 9:06 AM on January 28, 2007

Thanks guys, I have a degree in biology, not that I remember all of it :) I understand the physiology, though I'll definitely refresh my memory the next time I'm in a library. A main part of my question is how drinking and not drinking water both cause you to lose weight. If you feel perfectly fine drinking a minimal amount of water, is the damage just undetectable? And I still don't get that whole "not drinking water" can be confused with "hungry" thing. Sounds fishy.

Also, why would healthy people take diuretics? Why would you drink a diuretic like cranberry juice to prevent a UTI? Or maybe that's just the acidity; I've heard it both ways, though cranberry is definitly a diuretic.
posted by unknowncommand at 9:15 AM on January 28, 2007

Why would you drink a diuretic like cranberry juice to prevent a UTI?

Probably because you've heard an urban legend that drinking cranberry juice will prevent or cure a UTI.
posted by rxrfrx at 9:27 AM on January 28, 2007

Drinking a healthy amount of water on a regular basis will make your body regularly hydrated and decrease the amount of water retained in total, causing some weight loss. You're also going to be eating less food if you're drinking more water. Foods rich in sodium will cause you to retain more water, and make you thirsty at the same time, causing more water retention.

Healthy people may take diuretics as a quick-fix weight loss approach (like a wrestler weighing in), but it's not a long-term weight loss option. Many people don't drink that much water during the day or drink coffee, tea, soda, or other diuretics so they're getting hydrated for short periods. Eating food adds water to your system, so some amount of thirst is solved through eating and the urges are easily confused.
posted by mikeh at 9:28 AM on January 28, 2007

By the way, I see you're on the east coast. My experience is that the largest impact on physical need for water seems to be climate.

Try living in Phoenix or Tucson for a week. You'll find that 15 glasses of water per day isn't quite enough for you to avoid the headaches. I lived in Santa Fe for seven years, then moved to Boston, and was amazed at just how little water I suddenly felt like drinking.

The Snopes article really is dead-on-- there are four or five large factors which change the need for water, making it widely divergent for different people.
posted by koeselitz at 9:32 AM on January 28, 2007

Ah, so upon reflection, maybe my question is better posed, "How is dehydration/hydration quantified, in absence of obvious severe effects?" And also how was it quantified in the past to establish the "8 glass" standard. It's qualified anecdotally, I understand that, and that the physiology indicates that "dehydration is bad", I'm just wondering what counts as dehydration.

Consequently, how could studies even exist to determine that "dehydration" causes any longterm ill-effects.

I'm really digging that snopes article.
posted by unknowncommand at 9:33 AM on January 28, 2007

Well, medically dehydration is diagnosed through a number of external physical signs such as low skin turgor, low blood pressure and a dry mouth, and blood tests of blood cell count and electrolyte levels. If you are in the hospital for a condition that might affect kidney function, the nurses will pay attention to input and output. Obvious signs of water retention are bloating of the extremities, and low urine output.

But in general you really have to push things to get clinically dehydrated. I checked myself into a hospital after an out-patient surgery led to 18 hours of non-stop hiccups and dry heaves on top of another 12 hours of pre-surgery fasting. As long as you drink when thirsty, and avoid over-drinking your body should maintain reasonable hydration.

On the other hand, doctors might suggest increasing fluid intake for conditions such as UTIs and kidney stones based on the principle that more dilute urine will aid in flushing things out of the system.

And you can do behavioral longitudinal studies tracking behavior over long periods of time to identify correlations between some types of behavior and some types of disease. This is where the bulk of evidence regarding smoking risk comes from for example.
posted by KirkJobSluder at 9:57 AM on January 28, 2007

A main part of my question is how drinking and not drinking water both cause you to lose weight.

The basic answer is "it doesn't" The body stabilizes at some level of fluid balance. If you are dehydrated you'll weigh less, but the weight all comes back as soon as you start drinking liquids again, so calling this "weight loss" is a bit of a misnomer. You can accomplish this through use of diuretics and/or wrapping yourself in saran wrap which are techniques that some athletes in weight-sensitive sports use to "make weight." As far as the drinking water causing weight loss, mikeh has it pretty well covered.
posted by jessamyn at 10:13 AM on January 28, 2007

To add to mikeh's comments, most food has a lot of water in it. You may eat because you are thristy and do not distinguish between hunger and thirst. So if you want to lose weight, it can help to drink water. It also helps you feel a little fuller, so you don't eat as much.

If you are not getting enough fluid, you urine will be darker. See Wikipedia on dehydration.

Koeselitz is right about climate. I spent a week in a desert town in California, and got quite dehydrated with really bad headaches. You don't even notice you're perspring in dry climates.

In the developed world, we are incredibly blessed to have safe drinking water on tap.
posted by theora55 at 10:35 AM on January 28, 2007

Why would you drink a diuretic like cranberry juice to prevent a UTI?

Studies have shown that cranberry juice inhibits e.coli's ability to attach itself to the urinary tract.
posted by squeak at 10:44 AM on January 28, 2007

Why would you drink a diuretic like cranberry juice to prevent a UTI?

Probably because you've heard an urban legend that drinking cranberry juice will prevent or cure a UTI.

Cranberry juice acidifies your urine making it less hospitable to bacteria that cause UTI. Back in my uninsured days, I cured many a UTI by drinking exorbitant amounts of unsweetened cranberry juice.
posted by oneirodynia at 11:33 AM on January 28, 2007

This site, a UC Berkeley course on nutrition, has a podcast lecture on water on 4/11. I haven't listened to it-- her lectures were a bit sparse and slow for me-- but it might have goodness in there if you're interested in the topic.

Huh, on preview, that site doesn't seem to link the water lecture. Well, you can find it on iTunes at this link.
posted by ibmcginty at 11:59 AM on January 28, 2007

I spent a week in a desert town in California, and got quite dehydrated with really bad headaches.

Can also apply to dry winter weather in cold climates, especially when indoor air is heated but not humidified.
posted by gimonca at 1:30 PM on January 28, 2007

"Drink when you feel thirsty."

That's a little like saying, put oil in your car when it starts making funny noises. Thirst is a symptom of mild dehydration. Not a big deal perhaps, if you you are sedentary in a comfortable environment and don't mind dry lips and mild discomfort. But a better way to monitor your hydration is to use your dipstick. If your urine is darker than a light straw color you should be drinking a bit more. Keep a glass on hand and take small sips every now and again.

If you are trying to operate at peak physical performance, then waiting until you are thirsty can be a real bad idea. Performance drops markedly with even mild dehydration, and the body is limited in rate of absorption (about a liter an hour). So if you are perspiring heavily and/or in a dry environment such as the mountains or high desert, waiting until you are thirsty means you'll be performing at a sub-par level until you get the chance to rest and catch up.
posted by Manjusri at 1:34 PM on January 28, 2007

"If you DO drink it you lose weight?"

Water with meals is sometimes recommended to dieters, because it helps fill the stomach but doesn't contain any calories. Substituting water for beer or juice is an easy way to cut calories.
posted by mbrubeck at 2:40 PM on January 28, 2007

Thanks! All of this is really helpful.
posted by unknowncommand at 3:03 PM on January 28, 2007

I've been on a lot of diets and I've read a lot of diet books. Never have I seen one advise to drink water with meals, and never have I seen one advise to drink water to stave off hunger. The idea is to flush out your system and to aid your liver, kidneys, and bowels in processing what you've consumed and regulating your glucose levels properly. You can argue with that if you like - I'm not an expert - but I'm just giving you the philosophy as it usually appears. Usually the reader advised to drink water until his/her urine is clear, except for the first void of the morning.
posted by bingo at 5:23 PM on January 28, 2007

Remember the biology of osmosis, and how the only way to get rid of ions more concentrated that the maximum osmotic differences the body can maintain, is to dilute them. (Ie salt makes you thirsty, because you body is unable to rid itself of the ions because past a certain point, the osmotic pull at that concentration exceeds the difference the body can maintain across a membrane, and so the body needs a lot of extra water to dilute your urine so that it becomes possible to filter out the salt via a lower concentration with lower osmotic pull.)
posted by -harlequin- at 3:15 AM on January 29, 2007