How concerned should we be about oxalic acid and kidney stones? A technical explanation.
One of the "scare" weapons used by the critics on high daily doses of ascorbic acid is the oxalic acid-kidney stone hypothesis. Meakins[36] states that the chief factors in the formation of renal calculi are perversions of metabolic processes, infection and stasis in the urinary tract. There are two schools of thought on stone formation: 1) That there is a central nucleus of colloids on which the crystalloids are precipitated; 2) That the crystalloids are deposited from the urine in which they are present in concentrated solution, in which salt and hydrogen ion concentrations are important factors. In all cases stasis and a concentrated urine appear to be the chief physiological factors. The only way that oxalic acid can be produced from ascorbic acid is through splitting of the lactone ring. This happens above pH5. The reaction of urine when 10 grams of vitamin C is taken daily is usually pH6. Oxalic acid precipitates out of solution only from a neutral or alkaline solution-pH7 to pH10. Kelli and Zilva[37] reported that "Nutrition experiments showed that dehydroascorbic acid is protected in vivo from rapid transformation to the antiscorbutically impotent diketogulonic acid from which oxalic acid is derived." Values reported in the literature for normal 24 hour urinary oxalate excretions for humans range from 14 mg to 56 mg. Lamden et al.[38] found in a group of volunteers that the ingestion of 9 grams ascorbic acid daily resulted in oxalate spills as high as 68 mg for 24 hours and in the controls without extra vitamin C the high was 64 mg for a 24 hour period.
These critics have overlooked the individual with diabetes mellitus. The amount of oxalic acid found in the diabetic patient approximates that found in the urine of a normal person taking 10 grams vitamin C each day. With the diabetic we find a paradox. Give this individual 10 grams ascorbic acid daily, by mouth, and the urinary oxalate excretion remains relatively unchanged. Diabetics are known for their diuresis. The individual who takes 10 or more grams of vitamin C each day will find that this organic compound is an excellent diuretic. No urinary stasis; no urine concentration.
The ascorbic acid kidney stone story is a myth. Methylene blue will dissolve calcium oxalate stones giving 65 mg orally 2 to 3 times a day. (Dr. M. J. Vernon Smith: Med. World News, Dec. 4, 1970)