There’s no shortage of controversy in the field of nutrition. No wonder! It’s not an exact science and many studies show an association between two things but can’t show causation – that A causes B. For this, we need randomized, controlled, double-blind trials, a type of study that’s costly and difficult to run in the area of nutrition.
Due to these limitations, it’s not surprising that nutritional information makes its rounds that have little or no reliable data to support it. Therefore, it’s hard to draw conclusions unless you see repeated studies showing that same thing. That’s why nutritional information changes so much and, so often, a new study comes out and negates the previous ones.
One claim you may have heard is that oxalate-rich foods are bad for you and you should eliminate them from your diet. Before buying into this idea, you should understand what oxalates are, the effect they have on the human body, and what foods you find them in. Then, we’ll see if there is research to support this claim.
What Are Oxalates?
Oxalates are organic acids found in many plant-based foods, including healthy fare such as leafy greens. The oxalate content of green, leafy vegetables varies from substantial amounts in beet greens to very little in watercress. One food that has a very high quantity of oxalates is rhubarb. In fact, there’s so much oxalate in rhubarb leaves that they’re toxic. However, rhubarb leaves have an unusual quantity of oxalates.
The purpose of oxalates is to protect plants against predators. Since they have a bitter taste, bugs, and other critters that devour plants shy away from them. Who likes the taste of bitter things? That’s a good thing for the plant, but what about your health?
Diet isn’t the only source of oxalates. Your body can make oxalates of its own from vitamin C. In fact, about 80% of the oxalates in your body are synthesized internally and only about 20% comes from food.
Oxalates Can Cause Problems for Certain People
Since your body makes oxalates, they’re not foreign to the human body. However, they get a bad rap because they bind to minerals such as iron and calcium and reduce their absorption from the digestive tract. In addition, when oxalates bind to calcium, they form calcium oxalate crystals, the cause of calcium oxalate kidney stones. This is the most common type of painful stone that forms in the kidneys. In fact, 60 to 80% of kidney stones are made of calcium oxalate.
For this reason, doctors recommend that people with a history of kidney stones limit the amount of oxalate-rich foods in their diet. Since the human body can make oxalates from vitamin C, they also don’t recommend taking high doses of vitamin C in supplement form.
The average person who doesn’t have a strong family history of kidney stones or a personal history probably won’t develop calcium oxalate kidney stones if they eat foods that contain oxalates since they are better at metabolizing and eliminating these organic acids.
For people who are poor metabolizers, oxalates have an “irritating” effect on the body and may increase the risk of kidney stones and even worsen some medical conditions. Some experts believe that oxalates aggravate conditions like fibromyalgia. The theory is that calcium oxalate stones deposit in tissue in the body and cause irritation and inflammation. Some research also suggests that a diet high in oxalates worsens a condition called vulvodynia.
A Healthy Person Who Eats a Varied Diet Shouldn’t Have a Problem with Dietary Oxalates
If you don’t have a history of kidney stones, autoimmune conditions, or fibromyalgia and your gut is healthy, eating foods that contain oxalates is unlikely to cause problems. A healthy gut is important since damage to the lining increases the absorption of oxalates from the gut into the bloodstream.
Remember, dietary oxalates make up only 20% of the oxalates in the human body. People who are sensitive to oxalates produce too much oxalate and don’t metabolize these compounds well. Plus, research shows that people who form calcium oxalate stones are “hyper-absorbers” of oxalates from food: they take up more than a healthy person and this increases their body’s oxalate burden.
Unfortunately, foods that contain oxalates are some of the healthier foods you can eat, including black tea, beans, some leafy vegetables and fruits, nuts, whole grains, buckwheat, quinoa, and dark chocolate. To strip these foods out of your diet when you don’t have a problem metabolizing oxalates would be shortsighted. Plus, it’s difficult to eliminate all oxalates from your diet. Even if you do, your body will produce a certain quantity on its own.
However, you can reduce the oxalate content of your diet by making smarter choices. For example, Swiss chard, beet greens, and spinach are quite high in oxalates while kale, arugula, lettuce, and watercress are leafy greens low in oxalates. If you’re a nut lover, skip high-oxalate nuts, like almonds and Brazil nuts, and eat ones lower in oxalates, like macadamia nuts. You can also reduce the number of oxalates in vegetables, like leafy greens, by boiling them. However, boiling also increases nutrient loss, including vitamin C and some B-vitamins.
The Bottom Line
If you’re healthy and don’t have a history of calcium oxalate stones, eating foods that contain oxalates shouldn’t be a problem. Even urologists who treat kidney stones now only recommend restricting oxalates in people who have a high level of oxalates in their urine. However, if you do have a family history of kidney stones or have an autoimmune condition, talk to your doctor about whether to limit the oxalate content of your diet. Don’t be too quick to strip these foods from your diet: many are too nutrient-rich to give up.
References:
- 2016 Aug; 44(4): 289–297.Published online 2016 Mar 22. doi: 10.1007/s00240-016-0868-7.
- Am J Clin Nutr. 1989 Oct;50(4):830-2. doi: 10.1093/ajcn/50.4.830.
- Korean J Urol. 2014 Dec; 55(12): 775–779.Published online 2014 Nov 28. doi: 10.4111/kju.2014.55.12.775.
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