Thursday, March 28, 2013

Should You Drink Lots of Water?

     I once had a patient who almost died from drinking too much water.  In fact, there were multiple occasions when she was rushed to my office with life-threatening electrolyte disturbances, because she would not stop drinking water.  She said she couldn't.  Then she proceeded to tell me, in a didactic tone, that water is good for you, it flushes out your system.
     On one of those occasions I called the ambulance because she kept pulling out one of her gallon jugs and chugging down the water, even after I tried to impress upon her that her serum potassium was at a dangerous level, and her heart was likely to go into an arrhythmia, causing her to die.  She didn't flinch.
     The condition is called psychogenic polydipsia, and it's considered a mental disorder.  The patient's husband had, over the years, become distraught so often, that by the time he brought her to me (prior doctors had "fired" her, I was told) he appeared bored.  It's not that he didn't love her, but he couldn't be staring down the cliff of her death any longer in their marriage.  He read a book in the exam room while I spoke with and examined his wife.  He told me he accepted that one day she would die, no matter what he or I or anyone did.
     Many patients tell me they "drink lots of water," "about a gallon a day," as though I'll be pleased and impressed.  I'm not sure where it came from, this idea that drinking more water than you need to quench thirst is good.  Some people carry the belief, like an article of faith, that "flushing the kidneys" will keep these exquisite filters in top form--and they cite frequent urination as proof.
     But our kidneys are organs that adapt well to dehydration, and can't be "exercised" by being made to process large quantities of fluid.  Perhaps there is a genetic component, derived from environmental triggers, but the Bushmen of the Kalahari live on almost no water.  What they do get is obtained from eating ground squashes and other wild foods foraged from their desert habitat.  They don't "need" more water, and neither do you.  There is no evidence that drinking lots of water is good for your health.
     It takes seven days to die of dehydration if you're normal weight and are deprived of all fluids.  Without any water, the kidneys respond to a surge of antidiuretic hormone by producing highly concentrated urine, and by slowing their activity.  The series of messages that pass, via polypeptide hormones, across the pituitary, hypothalamus, right atrium, adrenals and kidneys are not well understood, but these structures "know" more about how much fluid our bodies need than we do.  Therefore, a feeling of thirst is the most reliable measure of whether you should get a drink of water (or any fluid) or not.
     As people age, their thirst sensors become less efficient (or the brain response to thirst isn't as vigorous), so very old folks over 85) are more likely to become dehydrated from inadequate fluids.  I tell people over 65 to drink what they need to quench their thirst, and then slightly more.
     Athletes who do a lot of anaerobic exercise may need extra water to carry away lactic acid and other byproducts of muscle metabolism.  But no one needs a gallon of water a day.  Getting too much water isn't a big problem, if it is drunk a little at a time throughout the day.  But drinking a large amount in a short time, for a person who isn't dehydrated, sends the kidneys into emergency overdrive as they scramble to excrete unnecessary fluids before the blood becomes so dilute it leads to toxicity in the heart or brain.
     Psychogenic polydipsia is a rare problem and usually requires hospitalization, fluid restriction, and frequent weight checks to determine whether the patient is obtaining water on the side.  You are unlikely ever to come across someone with this disorder.  But it's instructive, for those who don't think water can kill you.
    A normal total fluid intake, in a twenty-four hour period, is about 64 ounces.  When people drink more than this, and they say it's because they're thirsty, there are three main causes:
    First, they're on medications that cause thirst (e.g.,  many antidepressants, IBS and overactive bladder medicines, and sleeping pills).   Second, they eat too much of everything, especially foods containing salt.  Third, they drink too many caffeinated beverages.  Caffeine has a diuretic effect, stressing the kidneys, and can cause dehydration.  The natural response to this is to drink more--but satisfying thirst with more caffeinated beverages makes the situation worse. 
     In general, you should not drink more than you need to quench thirst, and you should avoid silly advice to drink lots of water as an aid in weight loss, or to rid your body of toxins.
     For people over 50,  drinking too much water can cause congestive heart failure.  All that water has to be pumped, by an increasingly tired heart, to the kidneys, and it's simply too much.   Congestive heart failure is the biggest reason for hospitalizations in this country--due, in many cases, to a basic misunderstanding about the body's response to too much fluid..
     Lots of people die of heart failure, a preventable condition.  Too much water--too much fluid of any kind--is the cause of death, in most cases.  These patients have hearts that get worn out pumping excess fluid to the kidneys.  The signs of heart failure are easy to read:  shortness of breath, coughing, swelling of the legs, and an increase in weight.  (One quart weighs about two pounds.)  The first line of treatment:  fluid restriction.
     No, it's not good to drink lots of water.   

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