by Ocean Robbins: The date: June 18, 1815. The place: Braine-l’Alleud, a municipality 20 miles south of Brussels, Belgium.

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The key players: the British Duke of Wellington, the Prussian general von Blucher, and the resurgent French Emperor, Napoleon I, regarded as the greatest military tactician of the age.

Outgunned and outmaneuvered by the combined British and Prussian forces, Napoleon fled to Paris, abdicated a few days later, and was sent by the British to a miserable exile on St. Helena, where he remained until his death six years later.

Military historians have been arguing about the Waterloo campaign for a couple of centuries now. What, they wonder, could have led to so many tactical errors by the great French general? How could he so underestimate his foes and fall so easily into their traps?

The answer, according to some scholars, may have been kidney stones. Apparently, Napoleon suffered his whole life from kidney disease. And during the Battle of Waterloo, he suffered from a kidney stone that rendered him “lethargic and indecisive” precisely when he needed his full mental vigor.

Need more proof? Well, almonds are one of the foods highest in oxalates — compounds that may contribute to kidney stone formation. And guess what Napoleon’s favorite food was? According to his second valet, “What he especially liked were fresh almonds. He was so fond of them that he would eat almost the whole plate.”

Should You Avoid Oxalates?

closeup of hands holding almonds which contain oxalates
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Which brings me to today’s topic: oxalates in food. Thanks to some prominent “alt-health” voices and the speed of internet rumors, oxalates are sometimes demonized as nutritional villains, along with other supposedly harmful compounds found in plants, such as phytates, lectins, and the alkaloids found in nightshades. The main concern is that eating a diet high in oxalates can lead to the formation of kidney stones. Another fear is that oxalates act as “anti-nutrients,” impairing the absorption of minerals like zinc, calcium, and iron.

But what’s the truth about oxalates? Are they really that bad? Will a fondness for almonds cause you to lose world-shaping military campaigns? And should you avoid oxalate-containing foods, whether you’re prone to kidney stones or not?

What Are Oxalates?

While the terms oxalate and oxalic acid are often used interchangeably, they’re not actually the same. Oxalates are salts that form from oxalic acid, a naturally occurring organic acid found in plants, animals, bacteria, and fungi.

Oxalates allegedly play a role in calcium regulation, ionic balances, and heavy metal detoxification, as well as in a plant’s innate defense against the herbivores that eat them. Some of the oxalates we carry around in our bodies come from eating plant foods that contain them. Our bodies — as you’ll read about below — actually make the rest of them.

What Foods Contain Oxalates?

collection of oxalate-rich plant-based proteins on table
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A variety of plant foods, like nuts, beans, and certain vegetables contain oxalates.

Some of the largest dietary sources of oxalates are listed below, along with the approximate amount of oxalates in a 100-gram serving.

  • Swiss Chard – 700mg
  • Spinach – 600mg
  • Rhubarb – 500mg
  • Peanuts – 150mg
  • Cocoa – 500mg
  • Tofu – 235mg
  • Beets – 300mg
  • Almonds – 122mg
  • Potatoes – 97mg
  • Beans – 76mg
  • Raspberries – 48mg

Despite their high oxalate content, these are some of the healthiest foods on the planet. And the consumption of most, if not all of them, is associated with longer life expectancy.

A well-balanced diet that incorporates a good variety of healthy foods is going to contain its fair share of oxalates and other “anti-nutrients.” But it’s also going to contain an array of healthy compounds and nutrients that can help to counteract any potentially problematic effects.

If these types of foods only contained oxalates, then they would warrant more of a concern. But the presence of potassium, calcium, and a whole range of phytochemicals found in oxalate-rich foods means that dietary oxalates are generally not a problem for most people

Oxalates & Kidney Stones

medical concept graphic of kidney stones
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The kidney stones that contributed to Napoleon’s demise may have come (in part) from his almond consumption. But the medical field wasn’t quite as developed back then as it is today. And in the modern world, research is telling us that while high oxalate foods may contribute to the formation of some kidney stones, there are other factors that may be at least as significant.

It turns out that there are four basic types of kidney stones:

  1. Calcium phosphate kidney stones mainly result from animal protein-rich foods like meat, dairy, and eggs; fruit juices, sodas, and processed foods with added phosphorus; and excess sodium.
  2. Uric acid kidney stones are a result of too much acid in the urine. And they are mainly fueled by animal protein, sugary drinks, and alcohol.
  3. Cystine kidney stones are caused by a hereditary condition that causes cystine to leak into the urine. Research indicates that they can be fueled by drinking too little water, consuming too much sodium, and eating animal protein.
  4. Calcium oxalate kidney stones are the most common kind of kidney stone. They form when calcium in your urine combines with oxalates. Low-oxalate diets are sometimes prescribed for people who are prone to the calcium oxalate form of kidney stones. Adhering to such a diet generally means eating less than 100 mg of oxalic acid per day — which means no spinach (or beet greens or Swiss chard). When this diet is prescribed, it’s usually out of an abundance of caution because the role of dietary oxalates and calcium oxalate kidney stone formation is still inconclusive.

So if you’re worried about kidney stones, there is only one type that may be associated with oxalate consumption. However, oxalates don’t just come from the food you eat. In fact, about half the oxalates in your urine come from endogenous synthesis (which is a fancy way of saying that your body makes it by itself). What causes your body to make oxalates “endogenously”? Researchers believe that salt, animal protein, and excessive vitamin C are all associated with increased oxalate formation in your body, as measured by what winds up in your urine.

Is Oxalate Really an Anti-Nutrient?

hand putting spinach in personal blender
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Oxalates can also bind to certain minerals in the gut, especially calcium, zinc, and magnesium, and prevent them from being readily absorbed. This is why it’s often considered an anti-nutrient. And is one reason why spinach, despite being high in iron and calcium, isn’t usually the best dietary source of these nutrients. Its high oxalate content can reduce the bioavailability of some of the minerals it contains.

So if you’re concerned about your calcium, zinc, or magnesium levels, you may want to moderate your oxalate consumption, and/or make sure you are eating plenty of these minerals from sources that aren’t especially high in oxalates. This is especially true with calcium because it can regulate dietary oxalate absorption. And low-calcium diets have been shown to increase the risk of calcium oxalate kidney stones.

Are Oxalates Dangerous?

Some people have gone so far as to label oxalates a toxin because of their association as an anti-nutrient. And it’s true that in very high amounts, oxalates could cause damage to your esophagus. They could even be fatal by lowering the calcium in your body to critical levels — if you were to consume household cleaning products or antifreeze that contained pure oxalic acid. But I’m going to go out on a limb and say that most people aren’t doing that. Anyway, demonizing spinach because you shouldn’t drink antifreeze is like saying that drinking water must be unhealthy because people drown in floods.

An average single-serving (two cups raw or one cup cooked) of a high oxalate food is not enough to cause problems for most people. However, if you enjoy green smoothies every once in a while, I should note that oxalates do absorb more rapidly in liquid form. And while I certainly advocate eating your greens (and other healthy plant foods), it’s best to make sure that some of them are lower-oxalate greens, too, such as kale, collards, broccoli, arugula, romaine lettuce, parsley — frankly, any green that isn’t spinach, chard, or beet greens.

Who Should Avoid or Limit Oxalates?

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While oxalates included as a part of a balanced diet aren’t going to be problematic for most people, certain populations may benefit from avoiding or minimizing their intake.

People with a history of calcium oxalate kidney stones

First, people with a history of calcium oxalate kidney stones or kidney disease may want to watch their oxalate intake. People who suffer from hyperoxaluria, a genetic condition that leads to too much oxalate in their urine, should also moderate their consumption of high oxalate foods. If their concentration of urinary oxalates becomes too high, people with hyperoxaluria are at risk for calcium oxalate kidney stones.

People with mineral deficiencies

Next, individuals who are at a higher risk for nutritional deficiencies (particularly minerals) may want to minimize oxalate intake. Because oxalates bind to minerals, if you’re deficient in calcium, zinc, or magnesium already, consistently eating a lot of high oxalate foods can worsen your deficiency. This is more of a concern with calcium deficiency, as adequate calcium intake helps with the excretion of oxalates. As for iron, research suggests that oxalate intake doesn’t really seem to impact iron uptake from diet.

People with hyperparathyroid disease

People who have hyperparathyroid disease may also want to avoid a diet rich in oxalates. Hyperparathyroid disease is a condition in which there’s too much parathyroid hormone (PTH) production. Having too much PTH can lead to a loss of calcium. And a diet high in oxalates, which can bind to calcium, can make already low levels of this critical nutrient even lower.

People with gut malabsorption issues

Lastly, individuals with gut malabsorption issues — like short bowel, malabsorptive bowel disease, or Celiac disease — may benefit from a lower oxalate diet. In many cases, too few bacteria may be present in their digestive tract to degrade oxalates sufficiently. This can also be an issue for people who have undergone bariatric surgery for obesity in which alteration of the gastrointestinal system may impact the absorption of certain nutrients.

How to Reduce Oxalates in Food

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If you fall into a category that requires more awareness of oxalate-containing foods in your diet, or just want to exercise caution, here are some tips to reduce dietary oxalates.

1. Eat calcium alongside high oxalate foods.

A normal calcium diet, which contains around 800-1000 mg per day, should be able to offset the potential effects of oxalates that can otherwise lower your calcium levels. For more on calcium, click here.

2. Increase your intake of magnesium.

You ideally want to consume foods that are high in magnesium in some proximity to your intake of high oxalate foods. Magnesium can also help reduce oxalate absorption when taken at around the same time. These effects disappear if your intake of magnesium and oxalate differ by 12 hours or more. You can take a magnesium supplement or potentially increase your intake of magnesium-rich foods, like whole grains, legumes, low-oxalate nuts, seeds, and greens. For more on magnesium, click here.

3. Cook oxalate-rich foods before eating them.

By the time most foods high in oxalates get to our plates, they may no longer contain enough to cause problems. Research suggests that boiling and steaming can significantly reduce oxalate content. One study found that boiling reduced oxalates in raw vegetables by 30-87%, and steaming reduced them by 5-53%. Roasting, grilling, or baking, however, may have little to no effect on oxalates.

4. Soak, sprout, or ferment oxalate-rich foods before cooking them.

A 2018 study found that soaking pulses (that is, the edible seeds of plants in the legume family) before cooking significantly reduced oxalate levels. The researchers found that approximately 24-72% of total oxalates in pulses appeared to be soluble, meaning their concentrations dissolved in water. Soaking taro leaves for 18 hours may reduce their oxalate content by 26%. And another study found that the fermentation of kimchi, using silverbeet (also known as chard), led to a significant reduction in oxalate content. Furthermore, there was a 72.3% reduction in the amount of calcium bound to insoluble oxalate in the kimchi.

Sprouting may also reduce oxalate content by up to 80% in red kidney beans. (However, it’s important to always cook kidney beans well, even if you sprout them, because they contain a type of lectin called phytohaemagglutinin that can be toxic. Fortunately, cooking destroys this and other harmful lectins.)

5. Drink enough fluids.

Staying hydrated with water can also help flush out oxalates and prevent dehydration, which can otherwise play a role in kidney stone formation for some people.

There’s No Need to Fear Oxalates

While some people may need to avoid dietary oxalates, particularly in large amounts, the evidence of their potential adverse effects on health is inconclusive. And for most people, many of the foods that contain oxalates are beneficial as part of a healthy, balanced diet. In fact, many oxalate-containing foods are some of the healthiest foods out there.

Regardless, if you’re worried about oxalates in your food, you have a number of ways to reduce their prevalence and absorption that still allow you to enjoy the healthy foods in which they’re found.

In other words, you can enjoy almonds and still conquer all of Europe, although I can’t imagine why you’d want to. Instead, perhaps, eat oxalate-containing foods in moderation (sticking to a standard serving), and work for a sustainable and just food system for all.

Source: Food Revolution