Much research seems to indicate that vitamin C is not a problem with regard to oxalate.
I have read the references you linked.
Essentially four studies are being referred to :-
Study 1, the Harvard Prospective Health Professional Follow-up study, referenced in 1 and referred to in 5, which did not show a link between vit C intake and kidney stone formation (no details given for number of participants or whether male, female or mixed)
Study 2, a prospective study of 85,557 women, referenced in 2 and discussed in 5, which found no link between vit C intake and kidney stone formation
Study 3, a study of 23,000 men which did find a link between vit C intake and risk of kidney stone formation, discussed in 3, 4 and 6
Study 4, by the same author as in 2, but this time in 45,619 men. This is discussed in 3. You give no direct reference.
Here it is. In contrast to women, this study did find a link between vit C intake and kidney stone formation.
Reference 4 discusses the limitations of study 3, essentially because of its prospective nature. Of course this applies to all the studies. All were prospective, all look only at association, none go to causation. All are limited.
Straight away these results suggest a gender divide. If the first study used a mixed population (which seems likely from the way it is described) this could negate any effect.
More importantly, there are other problems with these studies and the conclusions drawn from them, including that the presence or absence of kidney stones is a very crude measure.
The study I linked in an earlier post tried to refine things a bit by selecting a number of participants from several prospective trials and asking them to provide 24 h urine samples. A number of parameters, including urinary oxalate, were measured. This latter is a more sensitive measure than simply determining whether or not someone has kidney stones.
Here is a quote from the discussion of that paper.
Previously, a randomized cross-over trial demonstrated that 1000 mg of supplemental vitamin C consumed twice daily increased urinary oxalate excretion by 22% (
11). Our data suggest that significant increases in urinary oxalate excretion also occur after consuming much lower doses of vitamin C and that vitamin C intake is an important determinant of urinary oxalate excretion in free-living individuals. Higher vitamin C intake also is associated with an increased risk of kidney stone formation. A prospective study in HPFS found that the multivariate relative risk of kidney stone formation for men consuming 1000 mg or greater of vitamin C per day was 41% higher than those consuming less than the recommended dietary allowance of 90 mg/d (
26).
There are several notable things here.
1)The study showed that significant increases in oxalate excretion were observed after relatively modest doses of vitamin C.
2) The
randomised cross-over trial referred to, while small, is of the highest possible quality - far more valuable than prospective studies. It involved a randomised cross-over design with participants on a controlled metabolic diet. It studied only a single dose of ascorbic acid, viz 1000 mg X 2 daily and found this significantly increased oxalate excretion.
3) The study referred to at the end of the quote is study 4 above.
Finally, and MOST IMPORTANTLY - OXALATE DOES NOT EQUAL KIDNEY STONES.
- Calcium oxalate kidney stones are well known, but the model of urologists and nephrologists, which holds that oxalates only enter the rest of the body after kidney damage has occurred, is outdated and incorrect; oxalate can be stored in and cause trouble in many parts of the body independently of kidney stones
- Oxalate also binds to and disrupts the function of other minerals, notably magnesium, iron, zinc, copper and manganese.
- Fine crystals of oxalate salts can be deposited in many sites in the body and this is thought to be an important part of many pain syndromes such as vulvodynia, fibromyalgia and interstitial cystitis.
- Other known deposit sites for fine crystals include bone, the eye, thyroid, atherosclerotic plaques, testis, breast, lymph nodes and sites of old injuries
- The free oxalate anion binds to and compromises the function of many important enzymes, disrupting energy metabolism, methylation and sulphur metabolism, including glutathione processing and recycling, to name just a few.
As the Determinants of 24 Hour Urinary Oxalate Excretion paper made clear, most oxalate in the body does not come from the diet but is produced endogenously. Appreciation of the factors which can trigger excessive oxalate production, along with the widespread metabolic and other derangements which can flow from it, is relatively recent.
Ascorbic acid consumption is only one contributor to endogenous oxalate production but it may well be significant in a population already at risk from other triggers - eg chronic oxidative stress.
