Intake of vitamins B6 and C and risk of kidney stones in women
ASN 30th Annual Meeting, San Antonio
J Am Soc Nephrol (Sep) 8:560A 1997
Pyridoxine (vitamin B6) is a co-factor for oxalate metabolism and is useful in some cohorts of patients with primary hyperoxaluria (PH). Whether this effect is relevant in patients with hyperoxaluria on other bases (dietary, idiopathic) is not known. Ascorbate (vitamin C) can be metabolized to oxalate and may thereby promote stones. This effect may be more important as an in vitro artifact, though a few case reports have shown increased urinary oxalate excretion on higher doses of vitamin C than those commonly employed (> 500 mg qd). The authors' previous study in men (J Urol 155:1847-1851; 1996) found no effects of these vitamins. The authors' prospective study here of 85,000 women measured both food intake and vitamin supplementation. This study also showed no effect of vitamin C to promote stone formation (comparing 1500 mg qd to < 250 mg qd). In contrast to the negative finding in men, high dose pyridoxine (> 40 mg qd) reduced the risk of stones compared to lower intake (< 3 mg qd) (relative risk 0.64; 95% CI 0.43-0.97).
Comment: Like the authors' other epidemiologic study reviewed here, these large prospective studies are valuable contributions as no other studies of their size and design are available. The relative importance of diet vs. supplementation cannot be investigated. This may be important as Curhan's studies on calcium intake show different effects if calcium is taken as food (protective) or as supplements (mildly lithogenic). Dietary confounders that vary in people taking vitamin supplements may also be significant and are not controlled for. In the absence of any convincing data that vitamin C prevents viral infections, and despite the lack of evidence that it is lithogenic, I suggest that my patients take only enough to prevent scurvy. Pyridoxine appears to be useful for some cases of PH. Its utility in preventing stones in other patients could be documented by urinary oxalate measurements before and after institution of therapy, and continued only in those demonstrating benefit. Since the etiology of many cases of oxaluria is not established it is certainly conceivable that metabolism can be facilitated by pharmacologic doses of pyridoxine. (David S. Goldfarb, M.D., NYU School of Medicine)
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ASN 30th Annual Meeting, San Antonio
Nephrolithiasis : Renal Stones: Treatment