There’s no shortage of nutritional advice out there, and many experts tell us entire food groups are bad. Vegans advise against animal products, while carnivore followers warn against grains and legumes. Yet, leafy greens seem to be universally praised. When it comes to greens, the more you eat, the better off you are, right?
Not necessarily. Compounds called oxalates in leafy greens and other foods might make us think twice about how much we eat.
What Are Oxalates?
Oxalates (oxalic acid) are natural compounds found in many plant foods but are also produced as a waste product by the human body. Our bodies can make it from excess vitamin C, certain amino acids (glycine and hydroxyproline), and glyoxylic acid.
Plants produce oxalates and alkaloids as a defense mechanism to deter predators from eating them. These compounds also protect against viruses and bacteria. However, eating high-oxalate foods or those rich in oxalate precursors can increase our oxalate levels.
The human body typically absorbs only 2-5% of dietary oxalates when eaten as part of a meal. On the other hand, up to 12% of oxalates can be absorbed from high-oxalate foods or drinks consumed on an empty stomach.
The majority of oxalates are excreted via the kidneys in urine. However, when oxalate levels build up to the point that they overwhelm the kidneys, they can form crystals. These crystals can lead to kidney stones or other issues when they build up in tissues.
Are Oxalates in Foods Really A Big Deal?
Oxalate’s impact depends on how they’re bound in foods. In vegetables, oxalates are often bound to minerals like calcium. The mineral-to-oxalate ratios in these foods determine how problematic the oxalates can be.
However, certain conditions can increase oxalate absorption. In those with impaired fat absorption, the fat can bind up calcium, making more free oxalates available for absorption. As a result, these individuals can absorb over 30% of the oxalates they consume.
People with these conditions are especially at increased risk:
- Irritable bowel syndrome (IBS)
- Inflammatory Bowel Disease (IBD – Crohn’s and Colitis)
- Celiac disease
- Short bowel syndrome
- Bariatric surgery
- Medications that block fat absorption, such as Orlistat (Alli)
However, only about 20% of oxalate in the body comes from foods. The rest can come from a variety of other situations.
Health Concerns Linked to High Oxalate Levels
While harmless in small amounts, excess oxalate can be damaging, especially when not properly broken down and excreted. Here are some potential results of high oxalates:
Kidney Stones
Oxalates produced internally or absorbed from foods, can combine with minerals to form oxalate crystals. Calcium salts and oxalate crystals can then combine to make calcium oxalate kidney stones. About 80% of kidney stones are made of calcium oxalate.
Nutrient Deficiencies
While most leafy greens are rich in minerals, these minerals are often bound to oxalates, making them less bioavailable. Like phytates, free oxalates are antinutrients that can bind with minerals, reducing absorption. The negative charge of oxalates causes it to readily bind with positively charged forms of minerals like calcium, magnesium, and iron.
Ingested oxalate can bind up these minerals in the gut, causing them to be excreted with the stool rather than absorbed and used by the body. Calcium deficiency can lead to osteopenia and osteoporosis. Iron deficiency can cause anemia.
However, when balanced with calcium, they’re less likely to steal nutritional minerals from foods.
Pain and UTI-Like Symptoms
High oxalates may not necessarily cause kidney stones, but crystals anywhere in the urinary tract can be problematic. In some women, it causes vulvodynia or vulvar pain. About 1 in 4 women with vulvodynia find that their symptoms significantly improve when they go on a low oxalate diet.
Increased oxalates in the urine can also lead to urinary tract infections. They can cause frequent urination, bladder pain, and an intense urge to urinate. Women with recurrent UTIs often have high oxalate levels in their urine. Of course, UTIs can also occur from antibiotic use, which can kill off healthy bacteria.
Possible Toxicity in Large Amounts
A few case reports show ingesting 4-5 grams of oxalate can cause death in adults. In one case, a 56-year-old man who consumed 16 glasses of black tea daily for years developed kidney failure. In another case report, a man died after consuming 500 grams of sorrel in a soup (6-8 grams of oxalate) in one sitting. It’s hard to say if these deaths were caused by oxalates or other ingredients or toxins in the food. However, it’s worth considering the potential harms.
May Contribute to Other Diseases
Oxalates may contribute to conditions outside the urinary tract. Dr. Isabella Wentz suspects oxalates contribute to hypothyroidism in some people. They may also contribute to joint pain, systemic pain, and depression.
Oxalate injections cause breast cancer in mice, and oxalate calcium salts are found in breast cancer cells.
High Oxalate Foods
Certain leafy greens, tubers, seeds, and—unfortunately—even chocolate are high in oxalates. The oxalate-to-calcium ratio plays a critical role in determining their impact on health. Many leafy greens are high in oxalates and low in calcium.
Here’s the ratio of oxalate to calcium in common foods (per 100 g):
- Spinach: 890-1100 mg oxalates, ~ 4-5:1 oxalate-to-calcium ratio
- Rhubarb: 275-1336 mg oxalates ~ 8-9:1
- Purslane: 910-1679 mg oxalates, ~ 5:1
- Beet greens: 300-450 mg oxalates, ~2.5:1
Many tubers and seeds also contain high levels of oxalate, including:
- Beetroot: 121-450 mg oxalates, ~ 5:1
- Taro: 278-574 mg oxalates, ~ 10-20:1
- Sweet potatoes: 470 mg oxalates, ~ 30:1
- Sesame seeds: 350-1750 mg oxalates, ~ 0.3-2:1
- Cacao: 500-800 mg oxalates, ~ 4-6:1
For sensitive individuals, experts recommend avoiding foods with more than 50 mg of oxalate per 100 grams. Other high oxalate foods to avoid include collards, okra, corn grits, wheat germ, soybeans, blackberries, and oranges.
Benefits of High Oxalate Foods
In extremely high doses, oxalates can be toxic. However, in moderate amounts, they can promote health.
Triggering Hormesis
Small amounts of toxins like oxalates can trigger hormesis, a process in which mild stress promotes healing and strength. It’s much like how lifting weights damages muscles but builds them up stronger. Consuming some oxalates can activate the body’s defense mechanisms, enhancing resilience.
Providing Essential Nutrients
High-oxalate foods like spinach, sweet potatoes, and cacao have essential vitamins and minerals. For example, spinach is an excellent source of vitamin K, folate, and iron. Sweet potatoes are a great source of beta-carotene, which the body converts into vitamin A. Cacao is rich in magnesium, iron, and flavonoids.
Nuts and seeds are rich in omega-3s and monounsaturated fatty acids, and beets provide nitrates. These nutrients are essential for bone health, vision, cardiovascular health, and the immune system.
Providing Antioxidant Support
Many high-oxalate foods are rich in free radical-fighting antioxidants. Spinach and beet greens contain lutein and zeaxanthin, which protect the eyes. Cacao’s flavonoids may lower blood pressure and support heart health. Raspberries and blackberries have vitamin C and anthocyanins, which support the immune response.
Supplying Fiber and Resistant Starch
Seeds, legumes, and tubers like taro and sweet potatoes provide dietary fiber to support gut health. Taro root provides resistant starch, which feeds beneficial gut bacteria and supports metabolism.
Cancer Prevention
Some high-oxalate foods have bioactive compounds that may reduce cancer risk. Cruciferous vegetables like kale have glucosinolates which have anticancer properties. Rhubarb has anthraquinones, compounds known to reduce inflammation and potentially fight cancer.
Should You Go on a Low Oxalate Diet?
Most people can safely enjoy high-oxalate foods in moderation. However, those with the following conditions may want to try a low oxalate diet:
- Kidney stone disease
- Vulva pain (vulvodynia) or pain during intercourse
- Frequent urination
- Frequent urinary tract infections (UTIs)
- Joint pain
- Hypothyroidism
- Mineral deficiencies (calcium or iron, leading to osteopenia, osteoporosis, or iron deficiency anemia)
You should also be careful about oxalate levels if you have a digestive problem that hinders fat absorption. This is especially the case if you can see floating stool or fat in your stool.
If you’re wondering whether you have high oxalate levels, it may be worth getting tested through your healthcare provider. In addition, organic acid tests (offered through Genova or Mosaic) also include urinary oxalate levels.
Eating a low oxalate diet might be a good way to manage your levels if you are sensitive to it. Many (but not all) patients find significant relief in their symptoms when they go on a low oxalate diet.
If you have high oxalates and symptoms of oxalate overload, you may do well to avoid high oxalate greens like spinach. Some people go on a Carnivore Diet to lower oxalate intake and promote oxalate dumping from the tissues.
Other Factors That Can Raise Oxalate Levels in the Body
Aside from leafy greens, chocolate, and other high oxalate foods, it’s possible to have exposure from other causes, including:
- A yeast overgrowth, as some yeast strains can produce oxalates in the body.
- Genetic conditions that increase oxalate production or reduce oxalate breakdown. Examples include primary hyperoxaluria and cystic fibrosis.
- Reduced oxalate-degrading gut bacteria (oxalobacter formigenes).
- Dehydration, which can increase oxalates in the urine, increasing the odds of developing kidney stones.
- Vitamin C supplementation, as vitamin C can be converted into oxalates inside the body
- Low dietary minerals such as calcium, magnesium, and iron. These minerals bind to oxalate in the intestines, preventing absorption.
A vegetarian diet is an increased risk factor for oxalate issues, especially for women, who naturally need more calcium in their diets.
Low Oxalate Vegetables
Some low oxalate vegetables to enjoy on a low oxalate diet include the following:
- Arugula
- Broccoli
- Brussels sprouts
- Cabbage
- Cauliflower
- Cucumbers (peeled)
- Summer squash
- Turnips (roots)
- Zucchini
Medium-oxalate vegetables may be consumed in small amounts. Leeks are medium oxalate, as are parsnips. Lentils are low oxalate legumes that can replace other (higher oxalate) beans in soups.
Ways to Reduce Oxalate Levels
You can reduce oxalates in your diet without eliminating whole food groups. Here are some ways to lower the oxalates absorbed from foods:
- Cook Your Greens: Boiling high-oxalate leafy greens can significantly reduce oxalate levels. The oxalates leach into the water, which is then poured out.
- Soak and Sprout: For grains, nuts, and seeds, soaking, sprouting, or blanching reduces oxalates.
- Add Fermentation: Fermentation also helps — Sourdough bread is lower in oxalates than regular bread.
- Soak and Boil Legumes: For legumes, soaking with lemon juice or vinegar and thoroughly cooking will also help.
- Pair Oxalates with Calcium: Eat calcium-rich foods with oxalate-rich foods to reduce oxalate absorption in the gut. Examples: oatmeal with yogurt or nuts with cheese. Though I do not recommend calcium supplements!
- Stay Hydrated: Drinking enough water dilutes oxalates in the urine, which can help lower the risk of kidney stones and kidney disease.
If dietary changes aren’t enough, potassium citrate supplements are often used. Citrates can help prevent kidney stones by directly increasing urinary citrate levels.
My Thoughts on Oxalates
Foods high in oxalates tend to be high in nutrients and delicious side dishes. If you’re healthy, you probably only absorb a small amount of oxalate from foods and it’s unlikely to be a problem. In these cases, enjoy leafy greens, wild greens, teas, and chocolate in reasonable amounts.
Have you changed your diet to avoid oxalates? Has your health improved as a result? Share with us below!
