Does drinking a lot of water reduce the chance of developing kidney disease?

For all these years, I have considered the good old recommendation of drinking "8 glasses of water daily" another one of those medical myths; an unsubstantiated idea that for some reason has come to be accepted as the gospel truth by all. Till date, not a single high quality scientific study has looked at whether this is indeed the appropriate amount of water to drink. Yes, dehydration is obviously harmful to the kidneys' function, and you should still drink according to the thirst/weather. Yes, drinking a good amount of water will prevent kidney stones. But, how do we know if the magic number is 6 glasses, or 8, or 10? Should the recommendation be a higher intake for my patients here in Bradenton, Florida, than for instance, someone living in Upstate New York? 

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Even nephrologists and other physicians have been guilty of doling out this advice to their patients for the longest time.
We also know that patients with certain disease states like congestive heart failure, advanced liver disease, advanced kidney disease, low sodium levels, can actually worsen if they take an inordinately higher amount of water. In fact, it could be downright dangerous, and water "intoxication" can lead to seizures, death, fluid build up in the lungs causing respiratory failure, etc.

My "kidney doctor" search for an evidence-based answer to the question, "Does more water translate in to a reduced kidney disease risk?", recently lead me to a recent study, the results of which were also presented at the Canadian Society of Nephrology's 2013 Annual Meeting. The researchers who performed this analysis used data from the National Health and Nutrition Examination Survey. To cut to the chase, the conclusion was that chronic kidney disease was 2.5 times less likely to develop in people who drank more than 4.3 liters of water a day than those who drank less than two litres a day. The purported mechanism is the effect of such "hyperhydration" to minimize the levels of a hormone in the blood called AVP (arginine vasopressin). Reducing AVP levels in already a known mechanism to possibly retard the progression of a genetic kidney disease called Polycystic Kidney Disease. So could low AVP levels be beneficial in keeping the kidneys healthy in the general population too? At this point of time, this is a question that still needs to be answered.

Notice that the study I mentioned above was a retrospective analysis of data. In other words, we are looking back once we have culled the data, and trying to look for associations between diseases and risk factors. Such analysis in medicine is often referred to as "hypothesis generating", and serves as a springboard for higher quality prospective studies that are better able to prove causation (and not just association). Sure enough, the investigators of this study have already launched a prospective study that will enroll 700 participants. 

I guess the appropriate water intake still technically remains unknown, but for the time being 6-8 glasses should work for most patients. Make sure to ask your doctor in case you have any of the above mentioned disease entities that could require you to take a lower amount. I am not sure I will start recommending daily water intake of over 4 liters to every patient that I see; at least not for the time being. However, I will wait with anticipation till the results of this prospective study come out. If the evidence is strong, we might have a cheap, non-prescription tool to fight kidney disease!

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