Oxalates, Chocolate, & Kidney Stones

Photo credit: Geoseph Domenichiello

Today, we're constantly hearing and reading about the benefits of eating dark chocolate.  Dark chocolate contains an array of essential minerals (iron, magnesium, manganese, copper, zinc…) and is packed with high levels of polyphenols (antioxidants).  Some research has suggested (not proven) that these high levels of polyphenols contained in chocolate may lower inflammation, reduce the risk of cardiovascular disease, and perhaps even aid in cognition or memory.  Of course, research revolving around antioxidants doesn't come without controversy, and more research is required before we draw any conclusions.  Even so, many companies and individuals already market chocolate and cocoa products as a health food because of this suggestive research.  


Moderation.  An age old tale. 

Chocolate is also high in fat and calorie dense.  Dark chocolate generally isn't harmful to one’s health (unless you are allergic), but it also shouldn’t be over indulged in.  Dark chocolate, like all foods, can be consumed as part of a balanced diet if eaten in moderation (one study suggests 6-7 grams per day as moderate).  As well, dark chocolate, cocoa powder, and cocoa beans all contain moderate to high levels of oxalates, an antinutrient. You may have come across oxalates in reading about spinach or rhubarb.  Oxalates are of particular concern due to the role they play in the formation of some kidney stones. For more information on the causes of kidney stones, visit ohf.org.


An Intro to Oxalates

What are oxalates?

Oxalates are molecules produced by plants, concentrated mostly within the seeds (such as the cacao seed) and leaves.  They are also a metabolic byproduct produced within our liver, but have no known biological importance to us.


Soluble vs Insoluble

Oxalates can be soluble or insoluble.  If oxalate is soluble, then our body can absorb it, and it can end up in our blood and urine.  If it is insoluble, then it passes through our system via the faeces, and doesn’t pose much of a risk.  When measuring oxalates in food, both the soluble and insoluble levels are considered. For instance, a sample of raw cocoa powder can contain 624 mg of total oxalate per 100 g of raw cocoa powder.  Of that 624 mg, 565 mg is soluble oxalate, and 59 mg is insoluble oxalate.

It's the soluble form that may be of concern since it can be absorbed into our blood, and end up in our kidneys.  Once in our kidneys, high concentrations of soluble oxalate in our urin can lead to kidney stones. Levels of oxalate within our body are determined by the sum of ingested oxalate from our foods as well as the oxalate produced from our liver.  High levels of oxalate in the urine does not just come from over eating high oxalate foods.  There are multiple ways in which soluble oxalate can collect within one's urine. 

As well, high levels of ingested soluble oxalate can also bind to minerals (calcium, iron) before they have a chance to be absorbed by our body, reducing their bioavailability. Hodgson et al. (2008) observed that there was a correlation between the loss of bone strength and a high frequency of chocolate consumption by women.¹


Oxalates and chocolate  

Although oxalates are found in chocolate and cacao, oxalates are also found in other common foods such as rhubarb, parsley, beets, and even bran.  Obviously these foods are beneficial enough that an all out ban isn’t necessary, especially if you are not at risk for kidney stones. However, chocolate is considered a high oxalate food (Williams and Wilson, 1990; Massey et al., 1993; Noonan and Savage, 1999), and it's worth keeping this in mind considering the push to consume more dark chocolate these days.  

One suggestion for those at risk of kidney stones is to consume oxalate rich food in conjunction with calcium rich foods.  The calcium will hopefully bind to most of the soluble oxalate during digestion, making it insoluble before it has a chance to be absorbed into the bloodstream.  However, it's worth noting that milk products do interfere with the effects of polyphenols (antioxidants) found in chocolate.  Therefore, if you are consuming chocolate for its antioxidant capacity, and also consuming milk with it to limit the absorption of soluble oxalate from your chocolate, any benefits from the chocolate’s antioxidants may be reduced or limited.  


Oxalate and kidney stones

If soluble oxalate molecules are absorbed into our blood and accumulate in our kidneys, they can bind with calcium there to form calcium oxalate crystals (if these oxalate molecules bound with calcium before they were absorbed, they would become insoluble and pass out with our faeces).  These crystals are often eliminated with our urine. However, if the concentration of these oxalate crystals in our urine is very high, there is a greater chance of them aggregating and forming calcium oxalate stones, the most common type of kidney stone. The high concentration of oxalate in the urine is often a combination of factors, such as very high levels of ingested high oxalate food, low levels of fluid or water (which increase the chances of the crystals to aggregate), high levels of ingested salt, and biological abnormalities within our liver or gut.  Once the calcium oxalate stones form, they can grow, and then travel from the kidney down through the ureter and eventually into the bladder. If they grow too big (even 4-5mm) they can become extremely painful, and cause significant obstruction or damage to the ureter. Here is a short video on how kidney stones are formed.

Most healthy individuals not prone to kidney stones likely do not need to worry about adjusting their diet or cutting out oxalate rich foods.  The National Kidney Association in the United States suggests eating oxalate rich food with calcium rich food so that the oxalate becomes insoluble.  The calcium will bind with the oxalate before it's absorbed into the body, and passes through our digestive system instead. Kidney stone patients have been suggested to consume oxalate rich food at a level no more than 100 mg/day, combined with a calcium intake of 1000-1200 mg/day.²   To learn more about oxalates and diet in regards to kidney stones, visit Kidney.org for more information. According to the Oxalosis & Hyperoxaluria Foundation, less than 80 mg of oxalate per day (equivalent to 10g of cocoa powder or 30g of dark chocolate per day) is considered a low oxalate diet.  Chocolate and cocoa also find their way into other products such as cookies, snack foods, smoothies, and cakes.  


The Research On Oxalates In Chocolate

1. Oxalate levels in cocoa at varying stages of the process.³

This study on oxalate at varying steps of the chocolate making process was published this year by researchers Hà V. H. Nguyễn, Hùng M. Lê, and Geoffrey P. Savage.³  They observed the levels of oxalate in raw cocoa powder which they had manufactured for the purpose of this study.  What's unique about this research is that they created the raw cacao powder from cacao seeds at varying stages of the process of making chocolate and cocoa. They compared oxalate levels of cacao:

A) Harvested at 4 different maturity stages

B) Before and after it was fermented

C) Before and after it was dried


A) Cacao oxalate At varying Maturity stages

They compared the oxalate content of raw cocoa powder derived from cacao seeds which were harvested from pods at varying levels of maturity (immature, mature, half ripe, and fully ripe).  It's a known fact that oxalate levels in other plants we consume differ according to their maturity. For instance, the level of oxalate in spinach and kiwi decreases as they mature, while rhubarb oxalate levels increase with maturity.  They found that maturity of cacao had a small effect on the level of oxalates in the raw cocoa powder. The mean oxalate level was 632 mg / 100 g dry matter (89%) being soluble oxalate. Soluble oxalate levels were highest at maturity stages 3 and 4 (~92%), while insoluble oxalate levels (mostly as calcium oxalate crystals, CaOx) were highest at the two youngest stages, 1 and 2.  

table one oxalates.png

This CaOx crystal formation indicates not only a calcium regulation mechanism, but also a defence to protect the seeds from herbivorous pests.  As the pods ripen, the levels of these crystals drop, perhaps encouraging animals to disperse the seeds only after they are ready to be dispersed.   

B) Levels of oxalate pre/post fermentation

Overall, fermentation greatly reduced the levels of total oxalate.  Fermented cacao seeds contained about 280-235 mg fewer oxalates than unfermented cacao seeds.  As well, the levels of insoluble oxalate were greatly reduced during the first few days of fermentation from about 300 mg to 27-77 mg.  However, after this initial plunge, there was a slight steady increase between fermentation days 3 and 7. As the mass ferments, the acidity levels become more alkaline, which also encourages soluble oxalates to bind to minerals (hence creating more insoluble calcium oxalate). This could be the reason for the slight increase in insoluble oxalate as fermentation progressed.  It’s worth noting that in a study that observed the fermentation of kimchi, insoluble oxalate levels decreased as fermentation progressed, so this idea of increased pH and increased insoluble oxalate requires further studies. The soluble oxalates seemed to rise and then drop slightly between days 3 and 7.

During fermentation, yeasts and bacteria grow within the mass, which are important for the breakdown of proteins and carbohydrates within the cacao seed. Saccharomyces cerevisiae and Lactobacillus plantarum which occur during fermentation are also known to degrade oxalate, and therefore may be responsible for the reduction of oxalate levels during fermentation.  However, after the initial drop in total oxalate levels in the first few days, levels remained relatively the same between days 3 and 7.  Another organism doing some work is Bacillus spp., which occur in the fermentation mass about 60 hours in (2.5 days), and are known to produce oxalates. Therefore, the degradation of oxalates by Saccharomyces cerevisiae and Lactobacillus plantarum could be cancelled out by the creation of oxalates by Bacillus spp.  This could be why the total oxalate levels remained relatively the same as fermentation progressed after day 3.  


C) Levels Pre and post drying

After 7 days of fermentation, the cacao seeds were then dried using 3 different methods: sun drying, oven drying, and freeze drying.  The only difference worth noting was that the oven dried raw cacao powder contained more soluble oxalates, but only slightly more. There didn’t appear to be much of a difference pre/post drying.

table three oxalates.png

2. Study comparing oxalate levels in commercial made chocolate and cocoa powders

In this study, chocolate and cacao products were collected from Germany, Netherlands, New Zealand, and Sweden.  Their oxalate levels in 15 cocoa powder products and 34 chocolate products were measured and compared.  They also looked at where the cacao was grown, as well as the variety of Theobroma cacao the products were made from. 

The 15 cocoa powder products analyzed

The 15 cocoa powder products analyzed

The 34 dark chocolate products analyzed

The 34 dark chocolate products analyzed

The researchers responded to previous studies that used what is now considered outdated and unreliable methods for collecting oxalate data.  These older studies often reported only total oxalate and disregarded soluble oxalate, which we now know is responsible for the formation of kidney stones.  As well, the researchers here observed higher levels of oxalate content in the products they tested compared to levels listed in older studies, likely due to more efficient extraction techniques used today.  They suggested that  perhaps there may also be a difference in the way chocolate and cacao is processed today compared to the past.  

Chocolate vs Cocoa

Cocoa powder appears to have a great deal more oxalate than dark chocolate. The total oxalate found in 100 g of chocolate was much lower than that for 100 g of cocoa powder (ex: 39-254 mg for chocolate vs 216-729 mg for cocoa).  It is believed that this is due to the fact that chocolate is diluted with other ingredients (sugar, cocoa butter). They extrapolated the mean cocoa content of chocolate bars to 100%, and the levels of oxalates (51.7 mg - 335 mg) was still much lower for chocolate than for cocoa powder.  They suggest that during the processing of making chocolate, oxalate levels are reduced, since the production of chocolate is much different than for cocoa powder post the roasting stage.  

Oxalate levels of cocoa powders sorted by highest to lowest total oxalate (mg/100g DM).

Oxalate levels of cocoa powders sorted by highest to lowest total oxalate (mg/100g DM).

However, oxalates are found in the cocoa solids portion of the cocoa bean.  A cocoa bean is roughly 50% cocoa butter (or fat), and 50% cocoa solids. Cocoa powder is essentially ground up cocoa beans with much of the fat (cocoa butter) pressed out of it.  Therefore, it contains a much higher concentration of cocoa solids (roughly above 80% depending on the manufacturer).  A 100% chocolate bar, which is essentially cocoa beans (50% fat, 50% solids) ground up and processed into a bar, doesn’t have the fat removed from it.  That 100% chocolate bar will never have as high a cocoa solid concentration as cocoa powder does because it is "diluted" with its own cocoa fat (which doesn't contain any oxalates).  

Oxalate levels of chocolate bars sorted by highest to lowest total oxalate (mg/100g DM).

Oxalate levels of chocolate bars sorted by highest to lowest total oxalate (mg/100g DM).

There was a relationship between higher cocoa content and higher oxalate levels.  Generally, the chocolate with the lowest amount of cocoa content also contained the lowest levels of soluble oxalate, and the chocolate with the highest amount of cocoa content contained the highest levels of soluble oxalate.  

However, there were some exceptions, with some chocolate bars containing lower cocoa content yet containing higher levels of soluble oxalate. For instance, sample 24, a 77% cocoa content chocolate made from Arriba Nacional cacao grown in Ecuador had a very low level of soluble oxalate (157 mg) compared to another chocolate bar at 76% cocoa content containing 212 mg of soluble oxalate.  Same level of cocoa content, yet a much lower oxalate level.   


Urinary oxalate:

This study also looked at the oxalate levels in the urine of 14 female participants. The participants avoided oxalate rich foods preceding and during the testing period.  Before testing, their mean baseline for oxalate output was 6.82 mg/6h (between 3.88 and 11.7 mg).  For the experiment, they consumed 68 g of dark chocolate in the morning before eating breakfast after fasting all night, and followed by a low oxalate lunch (they were given a list of high oxalate foods to avoid).  


Their urine was collected over 6 hours post ingestion of the dark chocolate. The urine samples were positive for greater levels of oxalate. After consuming the dark chocolate, their mean output was 11.1 mg/6h (between 5.56 - 16.7 mg).  Therefore, the mean increase in total oxalate output found in their urine was 4.24 mg/6h more (between 1.17 to 8.86 mg) than their baseline sample. This means that consuming the dark chocolate increased their urinary oxalate output by 69%.  


Conclusion: What to take from this.

Oxalates are found in many beneficial foods, including chocolate.  It's not something to worry about, but is worth being informed about.  Visit the websites listed in this article to learn more about oxalates and kidney stones.  If you're a healthy individual who hasn't suffered from kidney stones, you likely don't need to shift your diet unless for some reason you're eating bars of chocolate a day!  If you have or do suffer from kidney stones, and you love eating dark chocolate, it's worth keeping an eye on how much you are consuming.  Ask your doctor or registered nutritionist about oxalate rich foods and inform yourself.  

Many people want to lift chocolate to "superfood" status.  This concept of superfoods fools people into the idea that they can overindulge, since more superfood means more health benefits.  Even though most of us know this not to be true, and understand the idea of moderation, we still can be fooled into this way of thinking without realizing it.  Any health benefits that are derived from consuming dark chocolate are likely canceled out if we pass the threshold of what is a healthy amount and what isn't.  Perhaps another reason to reach out for fine chocolate.  Eating a few bites of flavour packed fine bean to bar chocolate will satisfy your cravings while also satisfying your overall chocolate experience.  Then you won't feel the urge to overindulge! 


  1. Hodgson, J.M., Devine, A., Burke, V., Dick, I. M., Prince, R.L. 2008. Chocolate consumption and bone density in older women. Am. J. Clin. Nutr. 87(1) 175-180.  

  2. R.P. Holmes, J. Knight, D.G. Assimos, Lowering urinary oxalate excretion to decrease calcium oxalate stone disease, Urolithiasis 44 (2016) 27-32

  3. Hà V. H. Nguyễn 1,* , Hùng M. Lê 2 and Geoffrey P. Savage. Effects of maturity at harvesting and primary processing of cocoa beans on oxalate contents of cocoa powder. Journal of Food Composition and Analysis, 67 (2018) 86-90. 

  4. Theresa Schroder, Leo Vanhanen, Geoffrey P. Savage. Oxalate content in commercially produced cocoa and dark chocolate. Journal of Food Composition and Analysis 24 (2011) 916–922.