What Really Causes Kidney Stones
(And Why Vitamin C Does Not)
(OMNS Feb 11, 2013) A recent widely-publicized study claimed that vitamin C supplements increased the risk of developing kidney stones by nearly a factor of two. The study stated that the stones were most likely formed from calcium oxalate, which can be formed in the presence of vitamin C (ascorbate), but it did not analyze the kidney stones of participants. Instead, it relied on a different study of kidney stones where ascorbate was not tested. This type of poorly organized study does not help the medical profession or the public, but instead causes confusion.
The study followed 23,355 Swedish men for a decade. They were divided into two groups, one that did not take any supplements (22,448), and another that took supplements of vitamin C (907). The average diet for each group was tabulated, but not in much detail. Then the participants who got kidney stones in each group were tabulated, and the group that took vitamin C appeared to have a greater risk of kidney stones. The extra risk of kidney stones from ascorbate presented in the study is very low, 147 per 100,000 person-years, or only 0.15% per year.
Key points the media missed:
- The number of kidney stones in the study participants who took ascorbate was very low (31 stones in over a decade), so the odds for statistical error in the study are fairly high.
- The study was observational. It simply tabulated the intake of vitamin C and the number of kidney stones to try to find an association between them.
- This method does not imply a causative factor because it was not a randomized controlled study, that is, vitamin C was not given to a group selected at random.
- This type of observational study is fraught with limitations that make its conclusion unreliable.
- It contradicts previous studies that have clearly shown that high dose ascorbate does not cause kidney stones.[2-6]
- The study authors’ conclusion that ascorbate caused the low rate of stones is likely due to a correlation between the choice of taking a vitamin C supplement with some other aspect of the participants’ diet.
- The study could not determine the nature of this type of correlation, because it lacked a detailed study of each patient’s diet and a chemical analysis of each stone to provide a hint about the probable cause.
So we have a poorly designed study that did not determine what kind of stone was formed, or what caused the stones that were formed. These are serious flaws. Drawing conclusions from such a study can hardly be a good example of “evidence based medicine.”
Different Types of Kidney Stones (Renal Calculi)
There is a considerable variety of kidney stones. Here are five well-known ones:
Calcium phosphate stones are common and easily dissolve in urine acidified by vitamin C.
Calcium oxalate stones are also common but they do not dissolve in acid urine. We will discuss this type further below.
Magnesium ammonium phosphate (struvite) stones are much less common, often appearing after an infection. They dissolve in urine acidified by vitamin C.
Uric acid stones result from a problem metabolizing purines (the chemical base of adenine, xanthine, theobromine [in chocolate] and uric acid). They may form in a condition such as gout.
Cystine stones result from an hereditary inability to reabsorb cystine. Most children’s stones are this type, and these are rare.
The Oxalate Oxymoron
The oxalate/vitamin C issue appears contradictory. Oxalate is in oxalate stones and oxalate stones are common. Ascorbate (the active ion in vitamin C) may slightly increase the body’s production of oxalate. Yet, in practice, vitamin C does not increase oxalate stone formation. Emanuel Cheraskin, MD, DMD, Professor of Oral Medicine at the University of Alabama, explains why: “Vitamin C in the urine tends to bind calcium and decrease its free form. This means less chance of calcium’s separating out as calcium oxalate (stones).” Also, the diuretic effect of vitamin C reduces urine concentration of oxalate. Fast moving rivers deposit little silt. If on a consultation, a doctor advises that you are especially prone to forming oxalate stones, read the suggestions below before abandoning the benefits of vitamin C. Once again: vitamin C increases oxalate but inhibits the union of calcium and oxalate.
Oxalate is generated by many foods in the diet, including spinach (100-200 mg oxalate per ounce of spinach), rhubarb, and beets.[8-10] Tea and coffee are thought to be the largest source of oxalate in the diet of many people, up to 150-300 mg/day.[8,11] This is considerably more than would likely be generated by an ascorbate dose of 1000 mg/day.[5,12]
The study we are discussing didn’t tabulate the participants’ intake of oxalate, but on average they had relatively high intakes (several cups) of tea and coffee. It is possible that those who had kidney stones had them before the study started, or got them during the study, due to a particularly high intake of oxalate. For example, the participants that took vitamin C may have been trying to stay healthy, but the subset of those who got kidney stones might also have been trying to stay healthy by drinking a lot of tea or coffee, or eating green leafy vegetables such as spinach. Or they may have been older people who got dehydrated, which is also very common among men who are active outside during the summer. Among the most important factors in kidney stones is dehydration, especially among the elderly.
- Ascorbate in low or high doses generally does not cause significant increase in urinary oxalate.[2-6]
- Ascorbate tends to prevent formation of calcium oxalate kidney stones.[3,4]
- Risk factors for kidney stones include a history of hypertension, obesity, chronic dehydration, poor diet, and a low dietary intake of magnesium.