A report from EPIC-Oxford did not find a higher risk of kidney stones among vegetarians (which included vegans).
Because many vegetarians and vegans eat a high-oxalate diet, I have been interested to see if a prospective study might find a higher risk of kidney stones.
A recently released report from EPIC-Oxford measured the risk of being hospitalized for a kidney stone over the course of five years for people in various diet groups (1). Vegetarians (including vegans) had a 31% lower risk (.69, .48–0.98) as compared to high meat-eaters.
The other diet groups were:
Moderate meat-eaters, 50–99 g of meat per day – 0.80 (.57–1.11)
Low meat-eaters, < 50 g of meat per day – 0.52 (.35–0.80)
Fish-eaters – .73 (.48–1.11)
Oxalate intake was not measured.
I have updated Oxalate at VeganHealth.org with this info.
The authors also found a correlation between zinc intake and kidney stones. However, this finding barely reached statistical significance and the rate of kidney stones were very low in this population (.6% over five years). Kidney stones were not a common enough side effect in the recent Cochrane Database Analysis of clinical trials on zinc to be mentioned in their report.
Considering all of this, I am not worried that a modest zinc supplement will lead to a kidney stone.
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1. Turney BW, Appleby PN, Reynard JM, Noble JG, Key TJ, Allen NE. Diet and risk of kidney stones in the Oxford cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC). Eur J Epidemiol. 2014 Apr 22. [Epub ahead of print] | link
Since my post of November 22, Calcium Chart Updated – Calcium Absorption Modified, I have had some correspondence with an oxalate researcher at the University of Wyoming, Dr. Michael Liebman. He told me that the numbers I was using from the USDA were not accurate and that new methodologies are providing more accurate numbers.
So I replaced the USDA numbers with those from a table put out by Harvard School of Public Health, Oxalate Content of Foods.xls. These updates resulted in no changes regarding which greens are good sources of absorbable calcium.
And with that housekeeping out of the way, the moment I have been pursuing since mid-October is finally upon us – my article on oxalate is finished! I do not suspect that it is, by any means, the final word on oxalate and the vegan diet, but it’s a good start.
Why do I care so much about a silly little molecule like oxalate? Well, at the time I started the article, oxalate was, by far, the most popular topic people were writing me about.
In the past, I had been aware of the issue with calcium absorption from high-oxalate greens, of course, and that oxalate plays a role in kidney stones. But what I was not aware of was that there is an entire community that considers oxalate to be the source of their health problems.
This is not an entirely unusual phenomenon – there are many foods, or components of foods, that communities have sprung up around in order to help each other avoid as the underlying mechanism for their health problems. But oxalate is kind of a big deal because it is hard to eat a low-oxalate vegan diet, and in my search for things that might be causing some vegans failure to thrive, after looking into oxalate, I think there might be something to this one.
I read about 70 papers relating to oxalate and the VeganHealth.org article, Oxalate, summarizes that research in more detail than most of my readers probably care to know. I have put the main points in the Summary, which I have reproduced below.
If you read the summary, you will see that I suggest that vegans, and especially vegans who might have oxalate issues, eat high-calcium foods or take calcium supplements with meals. There is some evidence that this can hamper iron absorption. So if you are someone prone to iron deficiency, this would be something to monitor and I will probably have more to say about iron absorption on low-oxalate vegan diets in future posts.
I also hope to put together some low-oxalate vegan menu plans in the near future.
I’d like to thank Mimi Clark of Mimi Clark’s Vegan Cooking Classes for her help with my oxalate research. She may soon be adding low- to moderate-oxalate recipes to her site and says vegans can write her for ideas for eating low-oxalate as a vegan.
And here is the Summary from the VeganHealth.org article, Oxalate:
Oxalate is a molecule many in the vegan community are familiar with for preventing the absorption of calcium. This article does not address that subject – info can be found in the article Calcium and Vitamin D.
Oxalate is also known for the part it plays in calcium-oxalate kidney stones, which is the most common form of kidney stone.
In many cases, getting a kidney stone is a one time thing and does not occur again. Increasing fluid intake can cut the incidence of getting another stone in half. Cutting down on the amount of oxalate in the diet is another strategy for reducing stone recurrence. Some calcium-oxalate stone formers are prescribed potassium-citrate tablets which are also effective at reducing stones.
Oxalate is generally not found in animal products while many plant foods are moderate or high, and some are extremely high (such as spinach, beets, beet greens, sweet potatoes, peanuts, rhubarb, and swiss chard). Despite this, a study from the Harvard School of Public Health found that people following a plant-based eating pattern had a lower occurrence of kidney stones (1).
There is no research on kidney stone frequency in vegans, though anecdotally I know of some who have gotten stones. Of course, I also know of meat-eaters who have gotten kidney stones. But is the average vegan at a higher or lower risk? Vegan diets are higher in some elements that increase the risk of stones, lower in some, and higher in some things that prevent stones, so it is hard to say.
But the story regarding oxalate does not end with kidney stones.
There is currently an entire community built around the idea that absorbing too much oxalate, known as enteric hyperoxaluria, either causes or exacerbates many diseases such as fibromyalgia, interstitial cystitis, vulvodynia, depression, arthritis, autism, and a variety of digestive disorders (which, in turn, exacerbate hyperoxaluria by allowing even more oxalate to be absorbed).
According to LowOxalate.info, leaky gut syndrome, in which molecules are absorbed from the digestive tract at a higher than normal rate, can cause hyperoxaluria. They also say that you cannot rely on getting a kidney stone as a warning sign before oxalate accumulates in other tissues.
There is not much research (on humans) regarding hyperoxaluria and diseases other than kidney stones and vulvodynia, so it is hard to say much about them with any certainty. However, many people have reported improved health on a low-oxalate diet and given the high amount of oxalate in some plant foods, it might be a good idea for vegans to be aware of this issue and not eat unusually high amounts of these foods.
Here are some other tips for minimizing problems from oxalate:
Boil high-oxalate leafy greens and discard the water.
Meet the RDA for calcium. Eat high-calcium foods or take calcium with meals; calcium citrate if you have a history of calcium-oxalate stones.
Drink plenty of fluid.
Do not include large amounts of high-oxalate vegetables in your green smoothies.
Do not take large amounts of vitamin C.
If you have a history of calcium-oxalate kidney stones or suspect you have hyperoxaluria, there are a few more things you can do such as limit oxalate as much as possible, add citrate to your diet (through orange or lemon juice, or calcium citrate), minimize added fructose and sodium, or try a probiotic supplement.
Please see the Contents for quick links to more details about all of these topics as well as tables of the oxalate content of foods and other helpful resources.
I highly recommend anyone with digestive disorders to check out that section of Oxalate.
1. Taylor EN, Fung TT, Curhan GC. DASH-style diet associates with reduced risk for kidney stones. J Am Soc Nephrol. 2009 Oct;20(10):2253-9. | link
For the last many weeks, I have been in an undisclosed location working on an article on oxalates. I think it has been my 3rd biggest project to date, the other two being the epic adventure Vitamin B12: A Love Story followed by Soy: What’s the Harm?
But before I release the article on oxalates, I am writing to let you know that upon getting a bit more information about the oxalate content of foods and re-analyzing the data, I have expanded and moved the table Calcium & Oxalate Content of Foods to a new page and also slightly modified the absorption category for some greens:
– Studies have shown that calcium in fortified soymilk, bok choy, kale, and mustard greens is absorbed well.
– Based on oxalate levels, the calcium in turnip greens, watercress, and broccoli should also be absorbed well.
– Based on oxalate levels, the calcium in collards should be absorbed moderately well.
– Studies have shown that the calcium in spinach and rhubarb is not absorbed well.
– Based on oxalate levels, the calcium in beet greens and swiss chard should not be absorbed well.
I know a lot of people have oxalate stories, but please do not send me any links to oxalate info before I publish my piece! Once it comes out, I’ll be happy to receive any info you think I have missed. Thanks!