Kelly Turner
It is said again and again that vitamin C can cause kidney stones. Many people therefore no longer dare to take vitamin C supplements. We explain what really causes kidney stones and what you can do to prevent kidney stones.
Vitamin C and the risk of kidney stones
A small part of vitamin C is metabolized into oxalic acid or oxalate, which is then excreted in the urine (oxalate is the salt of oxalic acid). So when you take vitamin C, the amount of oxalate in your urine increases. However, the more oxalate in the urine, the higher the risk of kidney stones. Because many kidney stones consist of calcium oxalate, the compound of oxalate and calcium. So it is said that vitamin C can cause kidney stones or increase the risk of kidney stones.
This assumption was confirmed e.g. in 2013 by a Swedish study (JAMA Internal Medicine) which claimed to have found a link between vitamin C intake and the formation of kidney stones.
Study: Vitamin C is said to double the risk of kidney stones
More than 23,000 men (45-79 years old) took part in the study and were scientifically monitored over a period of 11 years. At the end of the study, it was found that those men who were supplemented with vitamin C had twice the risk of developing kidney stones. At first, that sounds like a lot. In reality, the numbers look like this:
- Of 22,448 men who did not take any supplements, 405 men developed kidney stones. That’s 1.8 percent.
- Of 907 men who took vitamin C supplements, 31 men developed kidney stones. That’s 3.42 percent.
The exact dose of vitamin C taken by the men is particularly interesting at this point. The study states that men who took fewer than 7 tablets per week had a 66 percent higher risk and only those who took more than 7 tablets had twice the risk.
It depends on the vitamin C dose
The researchers give the estimated vitamin C dose per tablet as 1000 mg, which means that only those men who took more than 1000 mg of vitamin C daily had a twice as high risk of kidney stones – a dose that was usually only increased at times risk of infection or illness, but rarely permanently. Normally you take 200 to a maximum of 1000 mg of vitamin C, in case of illness sometimes 3000 to 4000 mg per day for a few days.
Unfortunately, one consequence of this study and the corresponding reporting in the media (“Vitamin C causes kidney stones”) was that many people stopped taking vitamin C immediately.
Low risk of kidney stones even at high doses
But if we assume that 1.8 percent of men would have gotten kidney stones anyway, i.e. 16 men (like in the group of men who never took supplements), then the supposedly increased risk of kidney stones from vitamin C would have dropped to just 15 shown by 907 men, so that not even a very high vitamin C intake is a particular problem in terms of kidney stones.
Other risk factors are much more important!
In addition, this is a purely observational study that only establishes correlations (simultaneous factors that could also be present at the same time by chance or for other reasons), but cannot prove any causal relationships.
For example, no investigation was carried out into whether these men might not have regularly drunk too little water. But that is exactly one of the most common causes of kidney stones (along with a genetic predisposition). Because the less you drink, the higher the risk that salts (e.g. oxalate) will crystallize in the urine and can no longer be kept in solution.
Also, the pH of the urine of these men was not examined. However, permanently acidic urine (e.g. due to an unhealthy diet) indicates a higher risk of kidney stones.
Vitamin C has more advantages than possible disadvantages
Likewise, the Swedish study does not provide any other details on the men’s state of health after the end of the study. For example, it could be that the vitamin C group now had a healthier cardiovascular system than the other group, a stronger immune system, more balanced gut flora, better tooth and bone health, etc., etc.
So it could have been that taking vitamin C had so many far-reaching benefits that one could have accepted a really minimally increased risk of kidney stones, although the latter could also be reduced by the factors mentioned (drink more and eat healthily and/or take alkaline citrates to avoid permanently acidic urine pH).
Even with a high dose, there is no increased risk of kidney stones
It is also interesting that an earlier (1996) and much larger study on this subject came up with a completely different result. At that time, the participant group consisted of more than 45,000 men between the ages of 40 and 75 who were under the scientific observation of researchers from the Harvard School of Public Health (Harvard University Medical School) over a period of 6 years.
The conclusion of this study read: Our results show no connection between the daily intake of vitamin C and the risk of kidney stones – not even when the vitamin C is taken in high doses, with high doses of 1500 mg and more per day were meant.
Three years later (1999) a study was published that was carried out over 14 years with more than 85,000 women: The result was that women who took more than 1500 mg of vitamin C per day did not have a higher risk of kidney stones than women who consumed less than 250 mg. It is therefore of no use in terms of kidney stones to restrict vitamin C intake in any way, the researchers conclude.
Vitamin B6 reduces the risk of kidney stones
Another interesting finding from this 1999 study was that taking 40 mg of vitamin B6 per day reduced the risk of kidney stones by 34 percent (compared to taking just 3 mg of B6 per day). The intake of vitamin B6 could therefore be integrated very well into the prevention or therapy of kidney stones.
Study 2016: Risk only increased in men
In March 2016, researchers analyzed data from more than 156,000 women and more than 40,000 men to find a link between vitamin C and possible kidney stone formation (5). Such a connection could not be established for women, even if more than 1000 mg of vitamin C per day were taken.
A 20 percent increased risk was observed in men when they took more than 1000 mg of vitamin C per day as a dietary supplement. No increased risk of kidney stones was observed at lower doses, especially when vitamin C was taken in the form of food.
In this regard, it is important to remember that it may be the synthetic ascorbic acid that may be responsible for the small increase in kidney stone risk seen in some studies and not the natural vitamin C.
Why vitamin C could protect against kidney stones
The alleged risk of kidney stones from vitamin C is therefore very low if it exists at all. In addition to a slight increase in oxalate excretion, vitamin C naturally also has other effects on the organism – and these could actually counteract the formation of kidney stones.
First of all, an interesting report from 1946 (11). It comes from the Canadian doctor William James McCormick (1880-1968), who devoted many years to vitamin research, especially therapy with high doses of vitamin C:
“In many cases, I have observed that cloudy urine is generally associated with low levels of vitamin C. As soon as correct amounts of vitamin C are given again, the crystalline “precipitates” disappear immediately and the urine becomes clear again. The patient is given a single dose of 500 to 2000 mg and with this dose, the urine is cleared within a few hours. Then you go to a maintenance dose of 100 to 300 mg per day, which is enough to keep the urine free of deposits. It, therefore, appears that vitamin C deficiency is the decisive factor in kidney stone formation.”
The quote from Professor Dr. Emanuel Cheraskin of the University of Alabama in the 1983 book The Vitamin C Connection:
“Because even though vitamin C increases the formation of oxalate, it also inhibits the connection of oxalate with calcium, so that it does not lead to increased kidney stone formation. Vitamin C also has a diuretic effect – and the faster the urine can be excreted, the o less likely it is for crystals to form.”
(An effect that is of course particularly noticeable if you ensure regular fluid intake, i.e. drink enough water throughout the day).
Vitamin C does not increase oxalate excretion
In addition, the increased oxalate excretion after vitamin C intake (from which one always concludes an increased risk of kidney stones) is firstly dose-dependent and secondly cannot even be observed in every person:
Levine et al. wrote in 1999 that increased oxalate excretion after taking vitamin C can only be observed in people who suffer from what is known as hyperoxaluria, i.e. pathologically increased oxalate formation, and only if these people consume more than 1000 mg of the vitamin take C Therefore, according to the researchers at the time, it is better to take less than 1000 mg of vitamin C per day in the case of hyperoxaluria. Whether you suffer from hyperoxaluria can be quickly determined by a doctor with a 24-hour urine test.
In March 2003, researchers wrote in the journal Kidney International that after oral intake of 1,000 to 2,000 mg of vitamin C, urine oxalate levels increased significantly more in people predisposed to calcium oxalate kidney stones than in people who were not. However, people with predisposition already had higher values in advance. With them, the oxalic acid excretion increased from 31 to 50 mg after taking 1000 mg of vitamin C, with healthy people it rose from 25 to 39 mg.
It is interesting here that the oxalic acid value in the predisposed patients did not rise much higher if they received double the amount of vitamin C, i.e. 2000 mg (from 34 to 48 mg) instead of 1000 mg.
A study from 2005 also showed that the oxalate content in the urine of 60 percent of the participants hardly changed even with the daily intake of 2000 mg of vitamin C. It, therefore, increased significantly in only 40 percent of the study participants.
The normal levels of oxalic acid in the urine
Oxalic acid excretion, which is considered normal, is stated to be up to 32 mg within 24 hours in women and up to 43 mg within 24 hours in men.
A value of 45 mg within 24 hours should not be exceeded, otherwise, this could be an indication of the presence of kidney stones. Sometimes the value is also given in mmol. In this case, the value should not exceed 0.5 mmol per 24 hours.
Only a small part of oxalic acid comes from vitamin C
In the discussion about the increase in oxalic acid levels through the intake of vitamin C, it is usually not mentioned that oxalic acid is a normal component of many everyday foods and luxury foods that may increase the oxalic acid level much more than vitamin C.
Foods rich in oxalic acid include B. spinach, beetroot, rhubarb, and tea (green, black). For example, 100 to 200 mg of oxalate are produced per 30 g of spinach, which is more than an intake of 1000 mg of vitamin C.
If you drink green tea (from 2-4 g of tea), the oxalate level in the urine increases from an average of 0.24 mmol to 0.32 mmol. Nevertheless, according to a study published in Nutrients in 2019, people who drink green tea daily do not have an increased risk of calcium oxalate kidney stones – again a sign that a single substance (in this case oxalate) is far from alone for the development of diseases may be responsible.
How to reduce the risk of kidney stones in hyperoxaluria
It is very important in this discussion that an increased excretion of oxalic acid via the urine does not mean that one now also gets kidney stones, which is unfortunately always concluded. Many factors contribute to the development of a kidney stone, of which a chronically elevated oxalic acid level is just one.
Even people with primary hyperoxaluria, in which the liver constantly produces too much oxalic acid due to an enzyme defect, can reduce their risk of kidney stones, e.g. B. drinking 2 to 3 liters of water daily, taking base citrates (sodium or potassium citrates), trying to take vitamin B6 as described above, take care of a good magnesium supply and take probiotics.
With regard to probiotics, it is known that there are probiotic bacteria that have oxalic acid-degrading properties, e.g. B. Enterococcus faecalis, and presumably also lactobacteria (lactic acid bacteria). The rehabilitation of the body’s own microbiome (intestinal flora, vaginal flora, oral flora, etc.) with suitable probiotics is also part of the holistic therapy concept.
The intake of base citrates (0.1 to 0.15 g per kilogram of body weight) is recommended as they can prevent the formation of calcium oxalates and thus the formation of kidney stones.
The risk of kidney stones may only increase if there is a magnesium deficiency
What is usually not considered in the connection between vitamin C and the risk of kidney stones is the magnesium status of the respective person.
As early as 1985, researchers in the International Journal of Vitamin and Nutrition Research wrote after a corresponding study with guinea pigs that a magnesium deficiency – regardless of whether vitamin C is taken in high or low doses – can lead to increased calcium storage in the kidneys.
From an even older publication (1964) it is known that 420 mg of magnesium oxide per day could be very beneficial for patients who regularly develop kidney stones, as it prevents the formation of calcium oxalate stones. However, if magnesium oxide, which is considered to be poorly bioavailable, already has such a good preventive effect, then this can also be achieved with other magnesium compounds (possibly even in lower doses), according to the corresponding report.
More recent publications (2005 in Magnesium Research) showed that the administration of magnesium alone cannot prevent the formation of new kidney stones containing calcium oxalate in all patients, but that the additional administration of other measures, e.g. B. to citrates is an important therapy component, of course, especially for those patients who have a magnesium deficiency.
Magnesium is therefore important for kidney stones in any case – whether you take vitamin C or not. You should discuss the exact dose of magnesium that is right for you (usually 300 to 400 mg) with your doctor or naturopath, as this should be adjusted to your diet, moreover, the more calcium-rich the diet, the more magnesium is required to achieve the ideal ratio of 1:1 to 2:1 (calcium: magnesium).
