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Oxalates and kidney stones - what is the connection?

Updated: Jul 21, 2021


Oxalates (also known as oxalic acid components) are an inseparable part of the body of most plants. They serve provide an important protective and detoxification role in the plant. Oxalates, as a potential problematic ingredient in nutrition, have been identified some time ago after it has been found that most people who suffer from kidney stone formation, at least once in their lifetime, usually get a type of stone called oxalate stone, made of calcium oxalate.

For this reason, an extensive investigation has started in the scientific community trying to find out if oxalates in the diet are causing the formation of kidney stones in people. Since 1 in 10 people will experience kidney stone in their lifetime and out of those who do, nearly 50% will have a recurrence in 5 years; it made sense to try to identify what is it that is responsible for this.

Radical dietary adjustments aimed at removal of oxalates from the diet have been suggested by doctors by here comes the tricky part, “nearly 100% of plants have oxalic acid inside of them”, some more than other of course, but a complete oxalate elimination would mean that people need to remove all fruits, vegetables, legumes, grains, seeds and nuts and on the other hand we know that consumption of these foods actually REDUCES occurrence of kidney stones in people.

We also know that not all people who consume a high amount of oxalates in their diet end up with oxalate stones; in fact, most do not. Statistically speaking, the more oxalates someone has in their urine, the greater the chance of ending up with an oxalate kidney stone. Let’s start by understanding the difference between “oxalate consumption” and “oxalate excretion in urine is”.


When oxalates are consumed, the majority are not absorbed. They get attached to fibre and other undigested bits and get excreted in stool after passing through the small and large intestine. However, in order for something to appear in the urine, it first has to be absorbed through our intestines into the blood. The blood then takes it into the liver through something called “first hepatic pass” and then the rest is either distributed to the body in cells, removed in urine via kidneys or reabsorbed back to intestines and removed in the stool. In order for oxalates to become problematic, they have to go through this entire process and end up in kidneys and urine in large amounts.

You can already tell that it is more complicated than saying “oxalates are all bad” or “oxalates are all good”.

The first question that has to be answered he is “Why do some people absorb more oxalates through their intestines than others?” What is it (or isn’t) in the bodies of some that isn’t in the bodies of others?


The first part of the answer is a bacteria species living inside the human gut. A small strain called Oxalobacter Formigenes (OF). OF belongs under a bacterial colony called Proteobacteria, one of the six major bacterial colonies in the human gut microbiome. It has been found that this unique strand of bacteria is able to do what our gut cannot, break down the oxalate molecular structure and prevent them from being absorbed inside the gut. In fact, it has been found in one study that people who had their gut well colonized with Oxalobacter Formigenes had up to 70% reduced risk of kidney stone development. Similar results have been replicated across other, small studies.

Nevertheless, here comes the painful truth. We have NO helpful capsules or probiotics that will add more OF into people’s guts. Unlike typical Lactobacillus or Bifidobacterium probiotics that can be very useful, studies that tried to supplement people with OF had very poor results in kidney stone formation, and in most cases, these supplements were useless. Additionally, OF is extremely sensitive to antibiotic therapy and may effectively become extinct after only few rounds of antibiotics, with repopulation being very very difficult. Another explanation is the type of diet we have in the west, called the Western Diet.

One study from Alabama found that children from the USA have significantly lower gut colonization with OF compared to children in tribal Africa and South America. (see chart below, the yellow spot marks the OF colonization of American children). The researchers in this study concluded:

We provide evidence of higher rates in a larger population of both children and adults in remote Amerindian and African populations than in the USA, including those in this study as well as in the literature. A possible explanation for this difference is that developments associated with Westernization, such as changes in diet or antibiotic use, may contribute to the loss of this commensal organism.”

A final "nail to the coffin" is the fact that people treated for kidney stones are usually put on at least one (often more) rounds of antibiotics during which their further protection against future oxalate stone formation could effectively be destroyed.

This begs another question;

Is it then possible, that in our society, through our excessive use of antibiotics while eating western type of low-fibre, high-salt, high-protein diet, we have effectively been stripping our guts from the ability to digest oxalates in a non-harmful way by the killing of OF, predisposing ourselves and our children to kidney stone formation? Yes, this is very much possible, and personally, I believe this to be the main contributor. However, we still require more research to be sure.


It appears that it is actually when our diet contains some oxalates that our gut microbiome diversity changes, and Oxalobacter Formigenes multiplies, even grows back after having been killed of. This is a natural form of microbiome adaptation, a wonderful effect seen in human bodies whenever we change our diets & environment. This is exciting news and while we only have tiny amount of research to show for this

In addition, as you can see from the chart below when our diets contain more oxalates, the absorption gradually decreases.

Source: Holmes, R. P., Goodman, H. O. and Assimos, D. G. (2001) ‘Contribution of dietary oxalate to urinary oxalate excretion’, Kidney International, 59(1), pp. 270–276.

This chart represents a study that was done in 2001. For 6 days people were fed an oxalate-free diet and so their oxalate urine content dropped to 10mg (this is how much our liver makes every single day regardless of dietary oxalates). This is the starting point of the curve. The first group only received an extra 10mg from food. (Notice the dramatic spike in oxalate in the first point on the line) But once people were fed a more oxalate-rich diet up to 250mg/day, notice that the curve nearly flattens and at some point the urinary excre. While we would expect 25-fold increase when going from 10mg to 250mg what we see in reality is only around double the excretion in urine. This was a fascinating finding that showed that when we consistently consume oxalates, there is an initial spike but after some time, the absorption stops on a certain point. We could speculate that this is where Oxalobacter kicks in or when oxalate transporters in the intestinal walls get oversaturated. But so far we can just speculate why this happens Regardless, positive finding that shows that more is not always worse. At least not in majority of people who have healthy kidneys and are not predisposed to kidney stone formation.


Another fascinating finding was that when calcium intake in these people was reduced to 30% of the original intake, their oxalate content in the urine increased, showing that somehow calcium helps bind oxalic acid in the gut and supports its removal in stool rather than in urine where it could contribute to kidney stones.

However it does not mean we need to load our bodies with calcium, the opposite. Calcium, the same as vitamin D follows something called a “U-Curve” where too little is bad but too much is bad as well. The ideal rate of calcium intake is somewhere between 800-1000mg/day, and anything extra may not only be unnecessary but may actually be detrimental in the long run.

Another study on 3500 participants found that those with the highest levels of magnesium, potassium and citrate in the blood had lowest levels of kidney stones despite having higher levels of oxalates

Like I said before, the relationship between oxalates and kidney stones is more complicated than “oxalates are bad” or “oxalates are good” and I believe the reader is starting to appreciate the complexity of this topic 😊


According to data from two foundational studies, Nurses Health Study I and Nurses Health Study II (together 238,130 participants) it was found that those regularly supplementing ascorbic acid (synthetic vitamin C) were predisposed to more kidney stone formation. This, however only applied to men, not women and the higher the supplemental dose, the higher the risk (peak risk at >1000mg/day)

On the other hand, as previously mentioned, we know that people who have more citrate in their blood are protected. Citrate (citric acid) comes naturally from nearly all fruits and vegetables but mostly from citrus fruits like lemons, oranges and grapefruits. Coincidentally these fruits also have the highest levels of natural vitamin C.

This means that while dietary vitamin C from food wrapped in the original packaging of fibre, citric acid and other vitamins is very beneficial, once we start to megadose isolated synthetic vitamin C, we could be causing ourselves more harm. Personally, I advise my clients to only get vitamin C from food as that is the safest and most natural way to get it. There is time and place for vitamin C supplementation, but it is definitely not daily or long-term.


It appears that people with chronic kidney disease (CKD) do indeed need to control their oxalate intake more thoroughly than others. People diagnosed with CKD need to refer back to the advice of their nephrologist and their dietitian trained in CKD management as working with or giving advice to people with CKD is beyond the scope of my practice.


So what can we learn from all this?

We need to start by realizing that that oxalates ARE indeed problematic for some people. We cannot deny that fact. People with a familial or personal history of kidney stones would do well to control their diet for the highest sources of oxalates (see the tab below). However, even for these people, it is not necessary to remove oxalates from the diet completely. (For those with Chronic Kidney Disease, please refer to the advice of your specialist CKD dietitian ONLY, as this advice may not be useful in CKD management)


  • Always aim for large variety of different plant foods and dose them in common amounts. Where possible try not to megadose the foods highest in oxalates.

  • Do not consume oxalate-rich foods as a single-meal portion (e.g. spinach juice or rhubarb juice) but serve them alongside a larger variety of other plants

  • Don’t juice these foods but eat them in their whole form.

  • Avoid dried and powdered forms of these foods (e.g. beet powder, spinach powder)

  • If you really enjoy a particular meal that could be oxalate-rich, eat it alongside other high-fibre meal


1. First of all, we need to protect our guts from devastation by unnecessary antibiotics, smoking, alcohol, excessive junk food and sleep deprivation. All of these can damage or even kill off a species called Oxalobacter Formigenes that has been found to reduce the absorption of oxalates and protect from kidney stones. Unless oxalates are problematic, do not remove them from the diet, as doing so may further decimate this precious colony of bacteria. There is no need to megadose oxalates either.

2. Secondly, we need to ensure that we consume enough calcium-rich foods in the diet but not an excessive amount. The best way to obtain this is to regularly consume vegetables, greens, nuts, seeds and legumes alongside our main meals. Fermented soy products are also excellent sources of calcium. While dairy is probably the highest source, too much dairy can tip the fragile balance and give us excess calcium, which can also be detrimental. The ideal way to achieve this is not to drink milk as a liquid but to serve small amounts of cheese alongside the main meals occasionally. This is, however, not necessary, and calcium target can be reached through a 100% plant-based diet as long as it is properly planned.

3. Thirdly we need to ensure our diets are sufficiently rich in fibre, vitamin C, magnesium, citric acid and potassium. How do we achieve this? Again as above, get a wide variety of fruits and vegetables. Get some of your protein from plants, don’t be afraid of fermented soy products. Eat whole grains like oats, buckwheat, quinoa and millet regularly. Avoid junk foods, packaged foods and all sorts of takeaways that are loaded with phosphates and salt, both of which have been associated with kidney stone formation.

4. And finally, while this hasn’t been discussed, nor is there an abundance of research, personally, I believe vitamin D is an essential nutrient here. While I wasn’t able to find any good studies that would link kidney stones to vitamin D deficiency, we know that vitamin D is essential in calcium utilization and calcium metabolism, and it is possible that healthy calcium balance may be disrupted in a deficient state. As such, I would recommend that the reader gets his/her vitamin D under control and get it to optimal levels. I went into this topic in my very first blog post and also in the free e-book Brain Resurrection that you can find in the section “Free Gifts” on

TOP 30 OXALATE FOOD SOURCES (per average portion)


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Curhan, G. C. and Taylor, E. N. (2008) ‘24-H Uric Acid Excretion and the Risk of Kidney Stones’, Kidney International. Elsevier Masson SAS, 73(4), pp. 489–496.

Holmes, R. P., Goodman, H. O. and Assimos, D. G. (2001) ‘Contribution of dietary oxalate to urinary oxalate excretion’, Kidney International, 59(1), pp. 270–276.

Kaufman, D. W. et al. (2008) ‘Oxalobacter Formigenes may reduce the risk of calcium oxalate kidney stones’, Journal of the American Society of Nephrology, 19(6), pp. 1197–1203.

Mitchell, T. et al. (2019) ‘Dietary oxalate and kidney stone formation’, American Journal of Physiology - Renal Physiology, 316(3), pp.409–413.

Ferraro, P.M. et al. (2016). ‘Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones’, HHS Public Access, 67 (3), pp.400-407.

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