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Thursday, May 2, 2013

Why do people develop kidney stones? What the risk factors for forming a kidney stone?

I had previously emphasized that the treatment of a kidney stone does not end with its passage or removal. Let's talk about why is that the case? Kidney stones could be of different kinds (the predominant types are made of calcium and oxalate). Why you form a stone, as well as what type of stone you form, depends on a number of lifestyle, dietary, and genetic factors. It could also depend on other disease conditions that have a bearing on the way stones form in the first place. Some of these factors might be treatable or modifiable, and hence prevention of future stone formation is possible if you can tailor treatment to those risk factors.

Before we talk about why kidney stones form, lets talk briefly about how stone formation occurs.
Stone formation is dependent on the concentration of critical substances like calcium, oxalate, phosphate, uric acid, cystine, citrate. It also depends on the level of acidity/alkalinity of urine (measured as the "pH"). Extremes of pH encourage stone formation. The concentration of the prior mentioned substances depends on the concentration of urine and on the absolute amount of the element being excreted in to the urine. Some of the above mentioned substances encourage stone formation (like calcium, oxalate, phosphate) and are considered "lithogenic". Other substances like citrate and magnesium prevent calcium from binding together by keeping it in a soluble form, and by influencing the acidity of the urine. These substances are called "anti-lithogenic". These two categories are like the Ying and Yang of urine, when it comes to stone formation! They need to be in balance. If the concentration of the anti-lithogenic factors goes down, or that of the lithogenic elements reaches a critical minimum (called "supersaturation"), the elements can bind to each other (for instance, to form calcium oxalate) and form crystals. These crystals then serve as a nidus for further stone formation and accretion. Over time, the crystals grow (assuming the same risk factors continue) and lead to discrete masses of stones in the urinary tract. I admit that this is a somewhat simplistic explanation of the process, but should suffice in getting my remaining message across.


1) In the Diet

Diet that is low in fluids, potassium, or phytates puts you at risk (I will cover the specifics of a good diet to prevent stones later). Conversely, diet that is high in oxalate, animal protein, sugar, salt, and vitamin C (which is converted by the body to oxalate) has been known to put you at a risk of stone formation. I would like to elaborate on the role of dietary calcium. I have had patients who told me that they "stopped all milk intake" after getting to know that they have kidney stones, fearful of its calcium content. However, dietary calcium intake actually protects you against kidney stones.  If calcium has such a strong role in kidney stone formation, why would you benefit from eating more of it? I must admit, this sounded pretty counter-intuitive to me the first time as well! The reason is that dietary calcium binds to oxalate present in food in the gut, thus preventing oxalate absorption in to the blood (and subsequent excretion in the urine). This reduction in the urine oxalate more than offsets any increase in urine calcium that might occur (if at all) due to increased dietary calcium. This phenomenon however does not occur should you take calcium supplements/pills where calcium is the lone element. In this case, the excess calcium that is absorbed from the gut is excreted in the urine, and hence, taking calcium supplements is a known risk factor for stone formation. 

2) In the Urine

A low daily volume of urine, or concentrated urine, would promote stone formation. People who drink insufficient amounts of water are hence at risk. So are people who reside in warm climes, and it is known that kidney stones occur more frequently in southeastern US, than in the northwest.  If your urine is high in calcium (either because of too much calcium excretion or because of high calcium concentration in a concentrated urine), oxalate, or uric acid, you will be at risk. Pound for pound, urine oxalate is the strongest promoter of stone formation, being 15 times more potent that urine calcium! Citrate fights stone formation, so low levels could be detrimental. Finally, extremes in pH will promote stone formation (calcium oxalate and uric acid stones in urine that is too acidic, and calcium phosphate stones in alkaline urine). 

3) Genetic and racial factors

Some people can be genetically predisposed to excrete high amounts of calcium/oxalate/cystine, or low amounts of citrate in the urine. These conditions will thus be associated with stone formation. Examples include Barter's syndrome, Hypermagnesemic Hypercalciuria syndrome, Dent's disease, Primary Hyperoxaluria, Cystinuria, Distal Renal Tubular Acidosis, etc. Congenital anatomic abnormalities of the kidney like horseshoe kidney, and medullary sponge kidney also come with increased risks. Caucasians are more likely to form stones than other races. 

4) Other medical conditions

Diabetes, obesity, and gout increase your risk of forming certain kinds of kidney stones. A condition where the parathyroid glands (primary hyperparathyroidism) produce excess parathyroid hormone could lead to excess calcium excretion in the urine, predisposing you to stone formation.  Patients who have diseases characterized by malabsorption from the gut, or patients who have had gastric bypass surgery (bariatric or weight loss surgery) absorb higher amounts of oxalate from the gut that is then excreted in to the urine, leading to stone formation. 

To summarize, not everyone forms kidney stones for the same reasons. However, knowledge of the factors involved is tremendously useful in treatment and further prevention. We will talk about that in my next post!


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