There have been comments from two people on the blog recently who have developed symptomatic kidney stones. Very symptomatic in one case.
I did a quick Google for kidneys stones and found that they can occur in up to 10% of the population, peak incidence between 30 and 50 years of age. A "significant" portion are asymptomatic.
So why should two people on a high fat, lowish protein and low carbohydrate diet develop symptomatic kidney stones?
That depends on what you think is happening and what actually causes kidney stones. There is quite a lot of information on PubMed about the physiology involved. One of the core findings is that magnesium is lost in to the urine under conditions of hyper insulinaemia and/or hyperglycaemia, most especially under hyperglycaemia.
Some of the core observations were made by Djurhuus, predominantly looking at type one diabetics. While he accepts that elevated insulin causes Mg loss in the urine, hyperglycaemia appears to be the main drive. This gets to the point where you can correlate magnesium deficiency with HbA1c in type one diabetics. As an elevated HbA1c suggest relative insulin deficiency in this group, then hyperglycaemia appears to be the problem.
It's open to speculation whether Mg deficiency is a specific cause of metabolic syndrome or a result of the hyperglycaemia associated with it, but there is undoubtedly a clear association between the two.
Once you have mangled your magnesium status you appear to be wide open to calcium based stones.
In fact metabolic syndrome might be enough to trigger calcium stone formation on its own, especially if you are not thinking about magnesium status...
But the message I get is that Mg, Ca and PO4 are lost through the kidneys under glucose/insulin dysregulation. These strike me as the reason for the massive requirement of both calcium and magnesium in diets which promote hyperglycaemia. Calcium and magnesium are elements. You don't "break them down", they're there to stay unless you put them down the loo. If they are so essential (which they are) I doubt your body would do this if it was working correctly.
So we have hyperglycaemia and/or hyperinsulinaemia as the most likely cause of urinary calcium, magnesium and phosphate loss.
Once these ions are in to the urine subsequent stone formation depends on urine concentration and pH. In alkaline urine you get magnesium based struvite, in acid urine you get assorted calcium derived stones.
Ultimately urinary stones appear to be a common feature of metabolic syndrome. They may well be present in much more than 10% of this population. What happens when you have metabolic syndrome and suddenly start living within the carbohydrate limits imposed on you by that syndrome? When you suddenly become normoglycaemic and norm-insulinaemic?
I doubt any of us starting out on low carbohydrate diets gets an MRI done to check if we have renal stones before we begin, just on the off chance. A sizeable number of the population drawn to low carbohydrate eating might well carry asymptomatic renal stones. The stones then begin to dissolve once people stop peeing their bones down the loo. How many will convert a large asymptomatic renal pelvic stone to a smaller stone which can enter the ureter to begin its agonising journey to the bladder?
Some, it seems!
I have vague memories of Kwasniewski and Lutz both warning about this feature of stone dissolution, and a similar scenario with gall stones dissolving and entering the bile duct too.
Of course all of this may be total BS and the case might be that saturated fat causes renal stones. You could always just ask any cardiologist.
The flip side to all of this is that the management for osteoporosis might just be normoglycaemia...
BTW Djurhuus did an intervention study supplementing Mg in type 1 diabeteics. It REDUCES insulin stimulated glucose uptake! It's hard to see what is happening here. Usually type 1 diabetics are exquisitely insulin sensitive until some joker pumps then full of insulin then says "there's the bread, eat it to stay alive". Then it's not so clear what might happen to insulin sensitivity in the medium to long term. Anyway, Djurhuus didn't seem to find Mg to be a panacea of any sort. Dropped the LDL particle count thought FWIW!