Because crystals are at the heart of all kidney stones, when we talk about preventing stones we mean preventing crystals from forming and from growing. Fortunately, universal and quantitative laws govern how atoms and molecules form crystals so we can predict the risk of crystals using equations that apply everywhere, even in the kidneys and urine.

Supersaturate your own sugar solution

One way to understand these laws is to make crystals on your own. Find a heat resistant glass container, fill it with water, and stir in table sugar until no more will dissolve and there is extra sugar at the bottom. The water is ‘saturated’ with sugar, meaning it holds all it can. It is ‘full’ of sugar.

Heat the water; you do not have to make it very hot. All the sugar will dissolve because heating raises the solubilityof sugar, how much the water can hold when saturated. At room temperature the solution was saturated because the concentration of sugar in the water had reached its solubility. At a warmer temperature the solubility is higher, the solution has ‘room’ for more sugar.

Let the solution cool. It will remain clear. It is supersaturated with sugar. You know this because all the sugar that had been heaped at the bottom is now dissolved. Your kidneys do what you have done. They create a liquid filtrate from blood containing a variety of kidney stone forming materials like calcium, oxalate, phosphate, and uric acid out of blood, where their salts are below saturation. They leave some of what has been filtered to go out in the urine, and extract most of the water that was filtered so the stone forming salts become increasingly concentrated. Depending on the balance between the salts and water left to go out, the urine will be below or above solubility.

Get your solution to crystallize

Your sugar solution is clear at room temperature, but it is supersaturated and can collapse into crystals, letting go of all the ‘extra’ sugar until it reaches solubility. Tap the glass, shake it to make some bubbles, and you will see. Urine is a lot more stable than your solution. Many of the same molecules that glue crystals together to make stones slow their formation. So even if it is supersaturated, urine in a glass will not make crystals just because you tap on the side. But it can make crystals. We can estimate the supersaturation of urine in patients, and that value is one way of gauging the risk of new kidney stones.

If you work at it, you can dip thin wood sticks into your supersaturated sugar solution and make pretty rock sugar lollipops like in the lead picture of this post. Food dyes help. To do this you would want the solution to crystallize slowly, so it is best to put the sticks into the solution in the refrigerator, and let things happen without disturbances such as shaking. There is an art to this.

Why would kidneys create supersaturated urine?

You might at first wonder at evolution to create conditions for crystallization as if a random bedevilment for humans. It is in fact the dark side of water conservation. We evolved on an earth not so rich in fresh water as one might like, so thriftiness was a virtue that fostered survival. Not only that. In a world with little salt to eat, urine would have contained very much less calcium than it does not – a matter I shall take up much later on – so water extraction would not so much supersaturate urine as it does now.

What can patients do about supersaturation? 

Think a lot about water

It is to lower supersaturation that physicians hector patients about drinking water. If you consider that crystals are always waiting to form, and those already present are always waiting to grow, steady and copious water drinking makes a lot of sense. It is one of the few acts a patient can always undertake and it is always worthwhile. I will offer a lot more about this over time.

Help your doctor

Modern 24 hour urine analyses present your doctor – and you if you wish to look – with supersaturations for all of the main stone forming crystals except struvite. But to use these intelligently requires three things be known, and patients can help with all three.

What are the crystals in your stones?

It is silly to speak of supersaturation in general. In your home experiment it was supersaturation with respect to sucrose – table sugar. In your 24 hour urine samples, it is supersaturation with respect to each of the stone forming crystals individually. So all I have said about the crucial importance of stone analysis comes to focus here: lost analyses, lost stones, are lost opportunities to understand urine supersaturation with respect to the actual crystals you form. I have said and will say again, the stone analysis in a dresser drawer, or some dusty filing cabinet in a local hospital or in the office files of a prior physician, those analyses might just as well never have been made if your current physician cannot see them.

Are you really making new stones now?

Essentially, with all the complexity of urine chemistry, supersaturation and crystal formation are not so tightly linked as in your experiment. The stone matrix proteins and urine proteins alter the rates of crystal formation so much that one cannot simply set a ‘normal’ supersaturation value and work to lower your values to that supposed normal point. Instead physicians need to work with a more rough and ready rule: the supersaturations of an active stone former are too high with respect to the crystals in stones and need to be lowered. But are you active in making stones?

Active means new stones are forming, and that judgment requires your physician compare the stones you pass or have removed with the stones seen on your x rays and decide if stones are forming or old stones are causing new symptoms. You cannot do this. But you can keep careful track of when stones pass, and keep all records from emergency rooms where stone attacks have been treated. Your physician may practice at other hospitals from where your emergency visits occur and will not have access to the information. If x rays are done, get the images on a CD so your physician can view them and count the stones.

Are your 24 hour urines valid?

Everyone likes to show off and it is most convenient to collect 24 hour urines over a weekend. Put those two facts together and everyone can see the problem. My honest if overly wordy memoir about how I practice stone prevention presents a patient whose work as a nurse totally altered her supersaturations from what would happen on weekends. If ‘she’ is indeed a composite of many patients, blurred enough that no one could ever know who the real people were, the image is perfect. The physician who is trying to prevent your stones will use what is offered and if weekend urine collections are much less supersaturated than during the working week, your treatment will be inadequate because based on inadequate information. No one can correct this problem but you. And I, here, am telling you this so you are informed and can act in your own best interests.

Return to Walking Tour on Supersaturation


  1. maxbet

    You’ve probably already read the New York Times article about how bean leaf surfaces can entrap

    • Fredric Coe, MD

      Hi Maxbet, I did indeed. No doubt medical applications will be pending. I have no knowledge of any related to stones, at least not now. Regards, Fred Coe

  2. Mary Tilton

    Look forward to more info on your web site. Thanks.


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