Would anyone bet against water to prevent stones?

Here is the first of new articles that highlight stone treatments in a simple format. These new articles were provoked by what patients asked for, and by Freakonomics.

Because patients asked for clear answers about treatments, I lifted ‘water’ out of my long article on treatment of calcium stone formers, and simplified the style so the main points stand out. I also suggest betting, something we all understand.

Freakonomics offers a three part podcast about the woeful state of medicine. Doctors, they say, rely on received wisdom, poor clinical trial design, and bad data. At the end I ask you to vote if we kidney stone physicians suffer from these three defects, and by how much. Voting sharpens the mind, or should.

The beautiful ‘Hunters in the Snow’ by Pieter Bruegel the Elder, illustrates water in Chicago these days. Painted oil on panel in 1565, 17 x 162 cm, it hangs in the Kunsthistorisches Museum, Vienna. This image is in the public domain.

Water and the Causes of Stones – Some facts

Supersaturation causes crystals to form and grow – that is physics. Crystals make stones – no crystals no stones. Crystals and stones require supersaturation.

Kidneys supersaturate urine by extracting water in excess of calcium, oxalate, and phosphate, the main stone components. Because promptly eliminated in urine, water in ‘excess’ of needs prevents kidneys from from supersaturating urine as much as they would have otherwise. As a consequence, supersaturation must – not may but must – fall unless the amounts of calcium, oxalate, or phosphate lost in urine rise. Since they do not rise from water alone, extra water must lower supersaturation.

The brain gauges water needs by measuring blood sodium concentration and balances desire and urine loss accordingly. Water in ‘excess of needs’ means more than you would drink if left alone, or in other words more than the brain would dictate. Such excess ensures urine dilution as kidneys promptly remove it.

People who make stones supersaturate their urine excessively even if the actual measured values are no higher or even lower than those commonly found among normal people. The proof is that they make stones. The message is that whatever their supersaturation we need to lower it.

As a corollary, supersaturation itself need not cause stones. Some factors must protect healthy people from their own urine supersaturation. Only research can disclose these protective factors and what defects they may have that foster stones.

How Much Water?

Gallons would do, but few will drink that much. We need the sweet spot – enough but not so much no one will drink it.

If we observe people over time and compare urine volume to the risk of becoming a stone former, that risk becomes very low at around 2.25 to 2.5 liters a day.

In this one panel from a larger set of risk factors in the original article, red bars depict two female cohorts, blue bars men. The vertical axis plots relative risk of becoming a stone former against 7 ranges of 24 hour urine volume.

Risk falls rapidly as volume rises, but the bars above the line at 1 – meaning relative risk is really not low enough – remain until about 2.25 liters.

As a complement to this graph, data from Jack Lemann show that urine supersaturation falls steeply with urine volume between 1 and 2 litersBy about 2 to 2.5 liters a day the curve flattens out, so you get less and less benefit from more volume. Because they have more calcium and oxalate in their urine – more stuff to dissolve, stone formers supersaturate urine more – to the right – than do normals at all urine volumes

From both studies we can say about 2.25 liters of urine volume a day should reduce stones. Because we lose water sweating and breathing, we must drink at least 1/2 liter more, or 2.75 liters of fluid a day to get 2.25 liters of urine. Common sense says we must drink more with heat and exercise, and everyone must figure out how much more from experience.

Does Water Reduce Stones?

Given the results above, just under 3 liters of water should reduce stones, but does it? Has anyone tried it?

Here is the one trialin first time calcium oxalate stone formers. Group 2 patients were told to stay hydrated. Group 1 patients were urged to hydrate lavishly and met frequently with nurses who coached them on. Urine volumes at five years were 2.62 liters/day in the Group 1 and 1.01 liters/day in the Group 2 patients.

By five years, 12 hydrated and 27 low flow patients had formed a new stone. Time to first new stone was 38 vs. 25 months, so water delayed the few failures in Group 1.

What about supersaturation? Did water lower it?

Of course it did: CaOx SS 9.9 in the low flow group and 2.6 in the high flow group. For CaP SS, values were 1.58 vs. 0.48. All p values <0.001.

Lets Think About Water

Suppose the National Institutes of Health (NIH) funded trialists to perform another trial.

How Could They Redo the Trial?

Given what we already know, they cannot redo this exact trial. One liter of urine flow a day for five years would condemn first time stone formers to over twice as many new stones as they would have with 2.6 liters of urine a day.

But they could ask if we really need all 2.6 liters of urine flow.

For example they could compare 2.6 liters/day – the volume that worked in this trial – to perhaps 2.25 liters/day. That lower figure comes from the graphs I showed you. At 2.25 liters prospective risk of becoming a stone former is low. In experiments, that urine volume reduces urine supersaturation to its plateau.

Why Should They Redo the Trial?

Less than 2.6 liters of urine, 2.25 or even 2 liters, might do and be less onerous. If a lower volume gave the same results as the higher volume, we could tell our patients they need not drink so much to prevent stones.

How Much Is a New Water Trial Worth?

Let’s put some money on what I just said. Is it worth a million dollars to know that 2.25 or even 2 liters/day of urine will prevent a second stone as well as 2.6 liters/day? Is it worth $100,000 to know that fact?

As you think about this question, remember research money is tax dollars, the amount for kidney stone research is very limited, and what would used for another water trial cannot be used for something else.

Given this, should another trial consume research funds? Would a ‘yes’ answer delight you, make you feel your money had been well spent?

Is Water All We Need?

Maybe water is enough for a first time stone former. But even with drugs prevention is less effective as pretreatment stones mount up.

Here are all the trials we have that used medications and hydration. It plots the percent new stones – relapse percent on a log scale from 10% to 50% – against the numbers of stones formed before treatment – log scale from 1 to 20 pretreatment stones. Plotted that way, relapse rate rises with the number of stones.

Given this pattern, maybe you would want to avoid the second stone as best you can by doing more than water at the beginning.

At the beginning, I not only increase fluids but also advise all the other diet changes that have a good hope of reducing stones because they are safe and because they point patients to eat what experts have fashioned as the ideal US diet. In my clinic – not a trial – patients with one stone given multiple treatments are labelled ‘multi’ on the graph. They have the same outcome – 11% – as those from the trial treated with high fluids – labeled ‘water’. But many were in treatment much longer than 5 years, so they had more time to make new stones.

What About Freakonomics?

Do We Depend on Received Wisdom about Water?

Only that I do more than increase fluids for first time stone formers.

Do We Depend on Poor Clinical Trials about Water?

A trialist always wants another trial, and no doubt statisticians will say, ‘Do more’.

But another trial must be about how much, not whether. This is because the first one was strong and there is good a priori science lurking just behind the curtains – water lowers supersaturations, supersaturation drives crystal formation, stones require crystals.

Do We Depend on Bad Data About Water?

Bad means data are untrue – either not proper measurements, or proper measurements misused to bolster statements to which they do not apply. Between the links and figures I am pretty sure that neither is the case for water as a stone prevention.

Let’s Bet

A new trial compares 2.25 liters to 2.6 liters of urine flow/day to prevent more stones in first time stone formers. Will the lower volume be no less effective than the higher volume. Bets are $1,000. How do you bet?

Let’s Vote 

How many NIH kidney stone research dollars would you pay to know if 2.25 liters is as effective as 2.6 liters of urine flow/day to prevent more stones in first time stone formers? Name your price.

Is water for kidney stone prevention based on good data and good trials? Yes, or No.

Are you completely clear about what we know about water as a kidney stone treatment? Yes, or No

Leave Your Bets and Votes in a Comment


  1. Ron Lach

    I don’t think a study to determine if drinking 1/2 of a glass less of water is a good use of research money. If one has been through the pain of kidney stones there should be no hesitation on drinking slightly more water. You have provided a reasonable amount to drink from the previous study that showed it helped to prevent kidney stones. The key is to distribute it throughout the day and night. As a side note I was told by doctor John Aspin that my 24 hour urine output of 8 liters was too extreme and that I should reduce it to about 6 liters. That is why I think patients should just drink water and not try to get by with the minimum. I still drink a lot of water and fortunately have not had any stones since 1996. I have had 3 procedures for stone removal starting in 1974, then1982, then in 1996.

  2. Steve Thompson

    Some frustration here… having had a number of stones in my life (and passing one now), and each doctor telling me to “drink more water,” but never able to base that on any real data for me as an individual. There are generalities, but what test could be created/developed to determine what is right for me, my body, my stone production, my issues. I have followed doctor’s advice and do nearly 3L water per day. And stones continue to generate. I follow stone free diet as prescribed by Urologist, and still stones. So… I’m all for additional research… but is there research going on that will evaluate my body… my needs? Where is the research on individual factors? Where is the specific detailed testing for an individual for water consumption? Where is the detailed testing being developed for other stone factors in my body? Am I missing something?

  3. Don R.

    Thank you for your article. It dramatically impressed upon me the need to drink all of the pitcher of water on my desk each day. You’ve explained it in words a non-medical professional can understand. I don’t think that there was a single statistic or study referenced – just your direction on what I should do. Beyond that, your style was lovely as well.

  4. Al R.

    Hi Dr. Coe,
    I’m not much into betting, but I can vote!
    Yes, with respect to the first order effects, that is.

    Come on everyone, the polls are open!
    P.S. I plan to post my rationale and comments later, but I don’t want to influence the voting now.

    • Al R.

      Interestingly, Hippocrates (460-370 BC) reportedly already recognized the importance of diluting urine to prevent stones. I do see value in general health recommendations based on studies to determine what level of fluid will best help a typical person to avoid their first stone. Shout the answer from the mountaintops. But that is water over the dam for all of us already in treatment. Data on averages are little help to us because, as you know, our supersaturations aren’t average or even consistent as a class.
      So a very interesting question is how to most effectively manage the quantity and timing of water intake to minimize risky peaks in supersaturations through day and night in order for a particular patient to prevent stone and plaque formation and growth?
      Just my 2 cents. Best regards, Al

  5. Pam Myers

    No I was not really clear answer. I had to read and reread the information before I could figure out what you were saying. I’m not very familiar with some of these termsbut as I get more into my research I’m sure it will get easier.

  6. Laurie Gower

    While water is certainly important for supersaturation, I believe (simply based on my own body, and no scientific evidence) that not all people make use of water the same. You can tell me to drink more water until I’m drowning in it, and I would need to urinate every hour, but I might still have dry mouth for example due to the way my body processes water. So my kidneys might also retain/dispose of the wrong amount of water as well. I would think that a person who has dealt with the painful stone would likely follow the doctors advise and drink more water; but does it always solve the problem? Just my naive input.

    • Fredric Coe, MD

      Hi Laurie, It certainly does not solve all problems mainly because people rarely can maintain even hydration so supersaturations will rise at intervals. It is best thought of as necessary but not sufficient. So you are right to say it does not suffice. As for water intake and renal water loss things are a bit clearer. Kidneys will clear excess water in response to variation in AVP levels which respond to slight changes in serum sodium – I know who you are and that you are a very skilled scientist! Unlike sodium balance that can be irregular over a day or so, water balance tends to be exact and prompt. Maybe I should write an article on this subject? Warm regards, Fred

  7. Lisa M Viviano

    Dr. Coe,
    Another great article. Question: I have increased my water intake to 2.5 L/day and have reduced my oxalate intake. I hadn’t seen how high phosphate could be a factor in stone formation before. I am hypoparathyroid secondary to MEN1 and my endocrinologist to increase serum calcium levels (to appropriate levels) and keep my phosphate levels normal. Of late, my phosphate has spiked. Any comments or suggestions? THANK YOU!

    • Fredric Coe, MD

      Hi Lisa, hypoparathyroidism is indeed a real stone risk as PTH helps conserve calcium so as blood calcium rises urine calcium will rise a lot. Serum phosphate will be high because of loss of the phosphaturic action of PTH so that is a problem as well, not for stones but for the blood. In general the goal is a level of blood calcium sufficient to prevent symptoms from low blood calcium, with attention to 24 hour urine data to prevent undue stone risk. Serum phosphate will be sensitive to phosphate intake, so bloods need to be fasting if one wants some stability. Regards, Fred Coe

  8. Michelle

    Thank you for another informative article. However, I have noticed that if I drink anywhere close to that amount(unless it is hot and humid out or exercising a ton), that I feel sick. I have msk. I actually felt better by having less water. In my case, it seems to be overhydration causing an imbalance of electrolytes causing me to feel sick mainly in the stomach but sometimes with other symptoms as well. Do you have any thoughts on how to feel fine drinking the amount of water you listed? I ask to be prepared for any future stone I might have to pass. Thank you for your time.

  9. Lisa accardi

    I would not risk it. I will gladly drink just under 3 liters of water every day of my life to not have a kidney stone. I would also track it for you for free . I think the data is clear. Drink more water. I think the data has proven the point.

  10. Michele

    I do believe you’re right. Such a struggle each day to meet the requirement which for me is at least 80 oz or more. I love water, but by the end of the day after dinner, it’s harder and then getting up twice during the night isn’t great either but anything to prevent these stones! Thank you for all of your well written and informative articles. I read them all.

  11. Phyllis Axon

    I originally had 7 stones, one passed test removed and/blasted. Have worked with Jill, HRvard list and mostt of the time drink a bit over 2 liters of h2o a day . I Have , so far , not had any more stones . I certainly support research n trials as I have learned how many people suffer regularly . I gave birth to 9.5 and an 8.5 girls and sone passing right up there. Due for KUB so I’ll know how I am still doing . I am learning much from your articles and yet still struggle with recipe creations some days . I am 116lbs and since stone Diey seem to have activated arthritis , totally left sided. Not sure if there is any connection with diet , vitamins or lack of taking them . More research ha ha . My full hip replacement was a shock as well! Any thoughts, things I should read ? Study to participate in lol? Thank you for your sharing with all of us . Much to learn

    • Fredric Coe, MD

      Hi Phyllis, thanks for the comment. The stone diet is really that recommended for all US people, and I sure hope it does not activate joint symptoms. Regards, Fred Coe

  12. Jeff Finesilver

    It would be of use to place in ( ) ounces after the liter values decreasing the need to consult conversation tables. I suspect that for many it’s easier to recognize that 2.75 liters (93 oz.) translates to a bit less than 8 12oz. glasses per day.


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