By Steve Hickey, PhD and Hilary Roberts, PhD.(OMNS) It is strange how some medical authors seem desperate to show that vitamin C causes harm. One recurrent scare story is that vitamin C might cause kidney stones. However, although such warnings pop up regularly, these reports do not demonstrate an increase in the number or size of stones; instead, they rely on vague indicators of improbable risk.
The authors of such uncritical papers have probably not read the literature, for this is an old story. Decades ago, the idea that vitamin C causes kidney stones formed part of the medical attack on Linus Pauling. While it was initially a reasonable hypothesis, unexpected kidney stones are not found in people taking large amounts of vitamin C. (1,2)
There is no evidence that vitamin C causes kidney stones. Indeed, in some cases, high doses may be curative. (3) A recent, large-scale, prospective study followed 85,557 women for 14 years and found no evidence that vitamin C causes kidney stones. (4) There was no difference in the occurrence of stones between people taking less than 250 milligrams per day and those taking 1.5 grams or more. This study was a follow up of an earlier study on 45,251 men. This earlier study indicated that doses of vitamin C above 1.5 grams reduce the risk of kidney stones. (5) The authors of these large studies stated that restriction of higher doses of vitamin C because of the possibility of kidney stones is unwarranted.
People with recurrent stone formation may have an unusual biochemistry, leading to increased production of oxalate from vitamin C. (6) Oxalate and urate can accumulate in kidney stones. In practice, there is an increased excretion of both oxalate and urate with gram level doses of vitamin C (ascorbate). Various authors over the years have used this increase to predict that vitamin C will cause kidney stones; however, these predictions have never been confirmed.
Around three quarters of all kidney stones are composed of calcium oxalate; unlike some other stone types, these can form in acidic urine. Although vitamin C does increase the production of oxalate in the body, there is no evidence that it increases stone formation. It could even have the reverse effect, for several reasons. Firstly, vitamin C tends to bind calcium, which could decrease its availability for formation of calcium oxalate. Secondly, vitamin C has a diuretic action: it increases urine flow, providing an environment that is less suitable for formation of kidney stones. Finally, stone formation appears to occur around a nucleus of infection. High concentrations of vitamin C are bactericidal and might prevent stone formation by removing the bacteria around which stones form.
Vitamin C could also prevent other types of kidney stones. Less common forms of stone include uric acid stones (8%), that form in gout, and cystine stones (1%), which can occasionally be formed in children with a hereditary condition; these stones are not side effects of vitamin C. Other stones include those made from calcium phosphate (5%), which dissolve in a vitamin C solution. Acid urine, produced by ascorbate, will also dissolve the struvite stones (magnesium ammonium phosphate) that often occur in infected urine.
Recently, Linda Massey and colleagues from Washington State University have claimed that vitamin C increases the risk of kidney stones. (7) Their paper illustrates how the claims of risk have little basis in fact. Massey claims that vitamin C supplementation can increase the amount of oxalate. Vitamin C can increase oxalate absorption and, if degraded in the body, ascorbate can be converted into oxalate. However, while oxalate is a constituent of some types of kidney stone, an increase in its concentration does not mean that more or larger kidney stones will be formed. The formation of kidney stones is influenced by many factors and, as we have seen, vitamin C might be predicted to inhibit several aspects of stone generation. Massey suggests that this increase in oxalate may increase the risk of stones. This is a weak suggestion, which is contradicted by substantial evidence, quoted above. Continue reading >>
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