One question I often get asked is whether we should avoid leafy green vegetables, and other natural foods, which are high in oxalic acid.
Problems with oxalates in the body are two-fold: first, the restriction of mineral availability, hence depriving the body of essential minerals and second, the potential to form kidney stones in susceptible individuals.
In this article I’m going to elucidate what oxalic acid is, what foods contain oxalic acid, whether we should eat foods high in oxalates if health issues occur from consuming greens high in oxalic acid, and what quantities are optimum for health.
What is oxalic acid?
Oxalic acid is a colorless, organic compound that occurs naturally in plants, animals and in humans. It is made by the human body from other substances such as vitamin C. It can also be ingested from food and readily combines with calcium, which is why some people have concerns. Likewise, our bodies convert many of the things we consume into oxalates. Organic oxalic acid is an important – even essential – element to stimulate and maintain the peristaltic motion in our bodies, so it is not all bad.
What foods contain oxalic acid?
Oxalic acid is found in greens of the Amaranthaceae family such as spinach, Swiss chard, beet tops, lambs quarters and amaranth plus sorrel, parsley, purslane and rhubarb. It imparts a sharp taste to beet greens and chard that is felt in the throat. Oxalic acid generally increases as foods mature, producing increasingly bitter vegetables. Young, fresh vegetables such as baby spinach are less likely to have oxalic acid.
Other foods high in oxalates are almonds, cashews, buckwheat, unhulled sesame seeds, tea, coffee, chocolate, textured soy protein, beets.
Low oxalate greens are lettuce, celery, chickweed, watercress, escarole, asparagus, dandelion, and the members or the brassica family or cruciferous such as kale, bokchoy, collards, mustard greens, turnip tops, cabbage and arugula.
High Oxalic acid/100g food Med oxalic vegetables Low oxalic vegetables
Lambsquarters (highest) Radish (0.48 g) Chicory (0.21 g)
Parsley (1.7 g) Collards (0.45 g) Turnip (0.21 g)
Chives (1.48 g) Beans, snap (0.36 g) Broccoli (0.19 g)
Purslane (1.31 g) Brussels sprouts (0.36 g) Celery (0.19 g)
Amaranth (1.09 g) Lettuce (0.33 g) Eggplant (0.19 g)
Spinach (0.97 g) Watercress (0.31 g) Cauliflower (0.15 g)
Beet leaves (0.61 g) Sweet potato (0.24 g) Asparagus (0.13 g)
Endive (0.11 g)
Carrot (0.5 g)
Cabbage (0.1 g)
Onion (0.05 g)
Pea (0.05 g)
Tomato (0.05 g)
Turnip greens (0.05 g)
Parsnip (0.04 g)
Pepper (0.04 g)
Rutabaga (0.03 g)
Cucumbers (0.02 g)
Kale (0.02 g)
Squash (0.02 g)
Coriander (0.01 g)
U.S. Department of Agriculture Nutrition Data Laboratory. oxalic acid in vegetables
Absorption of minerals
Oxalic acid present in greens and other foods has the potential to bind with calcium, magnesium, iron, zinc and copper in our intestines to form oxalates, insoluble salts, thus interfering with the absorption of these minerals and making them unavailable for the body to use. It doesn’t, however, affect the absorption of calcium in other foods consumed at the same meal.
Studies have shown less absorbability of calcium from spinach. The absorption of calcium in spinach is only 5% compared to 30% absorption in kale, a low oxalate greens. The bio availability of calcium from kale is actually better than milk at 27%.
In another study, about 27 percent of the magnesium from spinach was absorbed compared to 37 percent of the magnesium in kale.
However the oxalic acid on iron absorption varies from person to person and meal to meal, and the body absorbs more iron from oxalic acid-rich foods when you are iron-deficient compared to when your iron stores are plentiful. So you get iron from eating spinach if you need iron.
How does cooking affect oxalic acid?
The oxalic acid is lowered by boiling and steaming but so are the minerals and vitamins and phytochemicals which leach in the water. If you discard that water you reduce the oxalates remaining in the food but at the same time you also lose the vitamins, minerals and other phytochemicals. So you are better off consuming your greens raw if you want to benefit from the whole spectrum of nutrients.
Health problems with oxalates
Oxalic acid forms insoluble salts such as calcium oxalates which are found in kidney stones. When oxalates become too concentrated in body fluids, they can crystallize and may cause health problems such as kidney stones.
There is also a large degree of genetic variability in the ability to detoxify the chemicals that produce oxalates. Perhaps twenty percent of the population has a genetic variance that increases their likelihood of producing oxalates, even when not consuming a high-oxalate diet. For this reason, certain individuals need to be careful about their intake of oxalic acid – those who have existing and untreated kidney or gallbladder problems, a vulnerability to kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain. People with hyperoxaluria (a genetic predisposition to this disorder occurs in less than 1 percent of the population) should highly minimize their intake of oxalates.
For others, the extent to which foods high in oxalic acid are a potential health problem varies from person to person. For normal, healthy persons, that risk is almost negligible provided that great amounts of oxalic acid are not consumed on a continuing, long-term basis.
For those who have a healthy digestive tract, good bacterial flora, who chew their foods well in a relaxed state, and minimize animal protein, will have the potential to lower oxalate absorption/urinary excretion from foods containing oxalic acid.
Although calcium oxalates play a part, it is the consumption of animal protein that is the main culprit in the formation of kidney stones. The acidification of the urine and increased animal protein are linked with the most common stone — uric acid stones. A plant-based lower protein diet will prevent the formation of kidney stones.
Being well hydrated and a daily dose of omega-3 fatty acids are also very effective in preventing oxalate deposition.
How much to eat?
It all depends on whether you have trouble excreting oxalic acid or not. Generally I recommend that high oxalic food be a smaller portion of your leafy green intake. Stone formers should look at minimizing those portions.
However we don’t have to avoid high oxalate foods completely. Spinach, beet leaves, lambsquarters and purslane have much to offer nutritionally: they’re an excellent source of folic acid, phytonutrients carotenoids and lutein, antioxidants, other minerals as well as vitamin K, E and C.
Don’t base your smoothies on spinach or Swiss chard. It is large amounts of consumption over several months that you should be aware of. I know people who put spinach in their smoothies all day and all year long without varying their greens. They are depriving themselves of calcium and other minerals more readily available in other low oxalate greens.
Rotate your high oxalate greens with low oxalate greens. The best spinach substitute in smoothies is baby bokchoy. It is mild in flavor, has soft fiber and a high calcium level.
Mix high oxalic greens with a variety of low oxalic greens. For example instead of using 2-3 cups of spinach in a smoothie, cut it down to half and make up the other half with other greens low in oxalic acid. This way you won’t ingest too much oxalate and will benefit from both worlds.
You can put spinach in your salad and soups but make sure the spinach is less than half of your total consumption of greens.
In general you should not eat more than 8 ounces of spinach a day, juiced or raw. If cooked, 10 ounces is not too much, but eating it every day can’t be as good as consuming a mix of other greens throughout the week.
And if you do experience a ‘burning’ sensation in your throat, however mild, you should stop eating those greens immediately, rather than ignoring it. Once I made a green smoothie containing Swiss chard stems and leaves. The smoothie tasted so strong that I couldn’t even have a sip without feeling an intense burn in my throat. I had to throw away the smoothie. On another occasion I made the same smoothie with Swiss chard and it tasted great. Not all chards are the same. Likewise with raw beets. Some beets burn my throat while other beets don’t. Don’t ignore those messages from your body. A food that burns your throat is not beneficial.
Continue to eat your greens in green smoothies, soups and salads and you’ll benefit from a plethora of nutrients which will help your body functioning at its peak. Rotate your greens and keep at the back of your mind which foods have high oxalic acid and minimize their consumption.