interventions to prevent recurrent kidney stones

the Am College of Physicians just released a clinical practice guideline on interventions to prevent recurrent kidney stones (see kidney stone secondary prevention AIM 2014 in dropbox, or doi:10.7326/M13-2908​). background:

--13% of men and 7% of women get kidney stones, and 35-50% have recurrence within 5 years without treatment
--80% are calcium oxalate or calcium phosphate or both
--dietary efforts include increasing water intake, reducing dietary oxalate, reducing dietary animal protein and other purines, and maintaining normal calcium intake

results of this systematic review:
--1 good quality and 28 fair-quality trials found insufficient evidence that assessing stone composition, or blood/urine chemistries reduces recurrences
--80 fair-quality trials of dietary interventions have found that: 
        --increased fluid intake, reduced soft drink intake (esp soda acidified by phosphoric acid, eg colas), and a high-calcium, low-protein, low-sodium diet reduce stone recurrences, though these studies were typically of low-quality and often with mixed results. 
        --one trial also found that low sodium intake (50 mmol/d) helped in patients with calcium oxalate stones. 
        --no trial specifically assessed low oxalate diet, though there was a trial finding benefit of high calcium (1200 mg/d) vs low calcium diets (400 mg/d) -- which has been attributed to dietary calcium binding oxalate in the gut and decreasing oxalate absorption.
--pharmacologic therapy:
        --thiazides -- moderate-quality evidence from 6 fair-quality trials of 24.9% vs 48.5% incidence of recurrent stones. no difference in type of thiazide.  though 8% vs 1% withdrew for adverse reactions. ​ (in terms of dose -- they note that these studies were done with higher dose thiazides. none with lower doses which have fewer adverse effects -- it is evident that even low doses of thiazides increase serum calcium levels, though i could find no good data on dose-dependent calcium excretion. and i  have prescribed lower dose thiazides with apparent good effect)
        ​--citrates (which interfere with stone formation) -- moderate-quality evidence in 5 trials of calcium stones with lower recurrence (11.1% vs 52.3%). though 15% vs 2% withdrew for adverse reactions.
        --allopurinol -- moderate-quality evidence in 4 trials in patients with calcium oxalate stones of decreased recurrence (33.3% vs 55.4%).  no increase in adverse effects found.

so, their recommendations:
--increase fluid intake spread throughout the day to achieve at least 2 L of urine/d (weak recommendation, low-quality evidence). they suggest avoiding colas (acidified by phosphoric acid) but not drinks acidified by citric acid (eg fruit-flavored sodas)
--use drug monotherapy with thiazides, citrate or allopurinol in patients where increased fluid intake fails to reduce recurrent stones (weak recommendation, moderate-quality evidence)

so, a couple of comments:

1. although i do give patients lists of high oxalate foods to avoid when they have calcium oxalate stones (and i do check stone chemistry analysis), my guess is that a more acceptable diet is the high calcium one, which i also recommend.
2. the issue of allopurinol. unclear what the mechanism is. 2 RCTs have found that patients who had hyperuricosuria put on allopurinol had fewer calcium oxalate stones, but an observational study found no difference in uric acid excretion in people with or without stones (observational study. controlled for some risk factors. but who knows?). ?uric acid as nidus for stone formation (unclear). as most of you know by now, i am very concerned about food additives, and one of the obvious targets is high-fructose corn syrup, which is associated with dramatic increases in fructose consumption (currently about 25% of calories in the US, mostly from sodas, with increase from about 15 gm/d when fructose was consumed naturally from fruits to 73 gm/day now), has a different metabolism from glucose (which is converted into glycogen in the liver, vs fructose, which  is converted to fructose-1-phosphate and depletes the liver of phosphates, increases uric acid levels, increases small dense and more atherogenic LDL particles, and may increase insulin resistance). i have had some patients completely stop soda intake and have found pretty dramatic decreases in uric acid levels. so, this is now one of my dietary recommendations for those with kidney stones.