Home of edible weed walks in Melbourne, and the publication:
The Weed Forager's Handbook: A Guide to Edible and Medicinal Weeds in Australia

Some notes on oxalic acid for foragers

In writing the book we took pains to research all the potential health hazards related to foraging.  Here is our fully-referenced extended remix of our information on oxalic acid.  The version in the book is shorter and gets more quickly to the relevant strategies.

What is oxalic acid?

Oxalic acid is an organic substance (with the chemical formula H2C2O4) that occurs naturally in high levels in many common foods, including almonds, chocolate, bananas, rhubarb, parsley, tea, beer and spinach.  Oxalic acid forms various salts – known as oxalates – when it binds with minerals such as calcium and magnesium. We’ll refer to pure oxalic acid and its oxalates collectively as simply ‘oxalic acid’.  In some of these forms it tastes pleasantly sour, and in others it’s essentially tasteless.  Because it binds with some nutrients making them unavailable, oxalic acid is often described as an ‘anti-nutrient’.

What’s the relationship to foraging?

Many wild edibles are high in oxalic acid. These include amaranth, dock, fat hen, oxalis and purslane.

What are the concerns?

We don’t believe oxalic acid poses a health threat if you’re reasonable about your consumption and it’s no reason not to forage, but it may be worth knowing a little about it.  There are some easy strategies to allow you to take advantage of these nutritional powerhouse plants while not overdoing the oxalic acids.

As it happens, even if we don’t eat oxalic acid, our bodies create it internally (from vitamin C, amino acids and other sources). Most of the oxalic acid in our bloodstream comes from these internally generated sources, with dietary oxalic acid accounting for only 10-15% of the total in usual circumstances. Some researchers have suggested that oxalic acid plays some as-yet-little-understood beneficial roles in the immune system and toxin scavenging in the body.1

However, in high doses oxalic acid can be toxic. Poisoning from food sources is extremely rare, but in 1989 a 53-year-old man (who had diabetes, and was a heavy smoker and drinker) died after eating approximately 6-7 grams of oxalic acid in a soup containing around 500 grams of sorrel.2

In lower doses, oxalic acid can be considered an anti-nutrient, limiting the absorption of some nutrients, particularly calcium and iron. When oxalic acid combines with calcium and some of these other minerals it creates oxalate crystals, which can contribute to kidney stones, gout, vulvodynia and rheumatoid arthritis in some people. High intake of vitamin C may also be a concern if you have any of these ailments.3 4 Although it’s not clear how much dietary oxalic acid influences these conditions, as a precaution doctors advise that pregnant women and anyone with these conditions to limit their intake of oxalic acids.

Strategies for eaters and foragers

Foods high in both calcium and oxalic acid are less of a concern than those merely high in oxalic acid, since the calcium bonds with the oxalic acid in the stomach. 5 The gritty feeling you get in the mouth if you drink milk while eating rhubarb is this process of crystal formation in action. Once crystals form, they become insoluble and they mostly pass harmlessly through us. Most of the weeds mentioned (with the exception of dock6 and possibly oxalis) also have high calcium content. Combining high oxalic acid foods with high calcium foods binds these soluble forms of oxalic acid to the calcium, making them insoluble. Yoghurt is a great combination food, and purslane tatziki is a classic dip, and one in which most oxalic acid is harmlessly bound up.7 (We have a recipe for that in The Weed Forager’s Handbook).

Another method of limiting oxalic acid consumption is to blanch. Contrary to what some books say, cooking does not destroy oxalic acid. However, blanching your greens for a few minutes and disposing of the water leaches out roughly one third of the oxalic acid.8 That’s one third of total oxalic acid but most of the soluble oxalic acid. The insoluble ones that remain pass right through us. So although you may also lose some nutrients, we can recommend this method. (Consider watering some plants with the cooled cooking water to keep those nutrients in the system!)

Probiotics is another strategy. Naturally occurring gut flora bacteria Oxalobacter formigenes break down oxalates as a food source.9 Many antibiotics may kill these beneficial bugs, increasing the risk of kidney stones and the other mentioned symptoms, so if you’ve ever taken antibiotics and have these symptoms, you might consider consulting your doctor to see if specific Oxalobacter probiotics are available. Fermented foods and off-the-shelf probiotics can help also. The common Lactobacillus acidophilus bacteria as found in yoghurt and sauerkraut can also break down oxalic acid.1011

One foraging note is that oxalic acid tends to be in higher concentrations in plants during dry conditions.12 How it relates to plant age is not so clear – in some plants such as spinach and beets there is an increase during the early stages of development, then a decrease as they mature.

So in conclusion we suggest:

  • not eating unreasonably huge quantities of high oxalic foods in one sitting,
  • blanching and discarding water, or food combining with high calcium foods if eating a lot of high-oxalate foods,
  • if you’ve ever had antibiotics, take probiotics and unpasteurised fermented foods to recolonise your gut,
  • eating a well-rounded healthy diet, of which weeds can play a staring role.

Level of oxalic acid in various foods

For your interest, below is a table with the results of tests on various vegetables and wild edibles for their oxalic acid content. The results are for total oxalic acid, and don’t specify what percentage of the oxalic acids are soluble and insoluble – so their health effects might be quite different. Different studies also report some wildly divergent figures. So who knows what to make of it all!?

Vegetable

or Fruit

Oxalic Acid Content

as a percentage

USDA13

Dr. Duke

Litholink

Guil et al. 14

Amaranth

1.09

0.96

Beans

0.36

0.03

0.02

Beets

0.04

0.68

Beet Greens

0.61

Cabbage

0.10

0.04

0.00

Carrots

0.50

0.01

0.00

Celery

0.19

0.02

Chard, Swiss

0.65

Chickweed

0.37

Chicory

0.21

Chives

1.48

0.00

Dandelion Greens

0.02

Dock, Curly

0.62

Kale

0.02

0.01

Fat Hen (Lambsquarters)

1.10

Lettuce

0.33

0.01

0.00

Mustard Greens

0.13

0.01

Parsley

1.70

0.10

Peas

0.05

0.01

0.00

Peppers, Sweet

0.04

0.12

0.02

Plantain, Broad Leaved

0.07

Potatoes

0.05

0.02

0.00

Purslane

1.31

1.68

Radishes

0.48

0.01

0.00

Rhubarb (stems)

1.34

0.86

Sorrel

0.30

Sow Thistle (S. tenerrimus)

0.06

Spearmint

0.00

Spinach

0.97

0.66

0.68

Squash

0.02

0.02

Sweet Potatoes

0.24

0.10

0.06

Tomato

0.05

0.03

0.00

Watercress

0.31

0.01

 

Hat tip to oxalicacidinfo.com for compiling the first three columns.15

 

References:

1 Mahmut Çalişkan, “The Metabolism of Oxalic Acid,” Journal of Zoology 24 (2000): 103–106.

2 Farré, Mercè, Judith Xirgu, Antonio Salgado, Ramón Peracaula, Ramón Reig, and Pere Sanz. 1989. “Fatal Oxalic Acid Poisoning from Sorrel Soup.” The Lancet 334 (8678–8679) (December 30): 1524. doi:10.1016/S0140-6736(89)92967-X.

3 Alessandra Calabria Baxmann, Claudia De O G Mendonca, and Ita Pfeferman Heilberg, “Effect of vitamin C supplements on urinary oxalate and pH in calcium stone-forming patients,” Kidney Int 63, no. 3 (March 2003): 1066-1071

4 TRAXER, OLIVIER, BEVERLEY HUET, JOHN POINDEXTER, CHARLES Y.C. PAK, and MARGARET S. PEARLE. 2003. “Effect of Ascorbic Acid Consumption On Urinary Stone Risk Factors.” The Journal of Urology 170 (2, Part 1) (August): 397–401. doi:10.1097/01.ju.0000076001.21606.53.

5 Gary C Curhan et al., “Comparison of Dietary Calcium with Supplemental Calcium and Other Nutrients as Factors Affecting the Risk for Kidney Stones in Women,” Annals of Internal Medicine 126, no. 7 (1997): 497-504.

6 José Luis Guil et al., “Oxalic Acid and Calcium Determination in Wild Edible Plants,” J. Agric. Food Chem. 44, no. 7 (1996): 1821-1823.

7 Moreau, A.-G., and G.P. Savage. 2009. “Oxalate Content of Purslane Leaves and the Effect of Combining Them with Yoghurt or Coconut Products.” Journal of Food Composition and Analysis 22 (4) (June): 303–306. doi:10.1016/j.jfca.2009.01.013.

8 Shashi Kala Yadav and Salil Sehgal, “Effect of domestic processing and cooking on selected antinutrient contents of some green leafy vegetables,” Plant Foods for Human Nutrition (Formerly Qualitas Plantarum) 58, no. 3 (September 1, 2003): 1-11.

9 Troxel, S.A., and R.K. Low. 2003. “Intestinal Oxalobacter Formigenes Colonization in Calcium Oxalate Stone Formers and Its Relation to Urinary Oxalate.” Journal of Endourology 17 (3): 173–176.

10 Campieri, C., M. Campieri, V. Bertuzzi, E. Swennen, D. Matteuzzi, S. Stefoni, F. Pirovano, et al. 2001. “Reduction of Oxaluria After an Oral Course of Lactic Acid Bacteria at High Concentration.” Kidney International 60 (3): 1097–1105.

11 Lieske, J. C, D. S Goldfarb, C. De Simone, and C. Regnier. 2005. “Use of a Probioitic to Decrease Enteric Hyperoxaluria.” Kidney International 68 (3): 1244–1249.

12 Bsc, Sc Noonan. 1999. “Oxalate Content of Foods and Its Effect on Humans.” Asia Pacific Journal of Clinical Nutrition 8 (1): 64–74. doi:10.1046/j.1440-6047.1999.00038.x.

13 USDA Nutrient Data Laboratory. “Oxalic Acid Content of Selected Vegetables.” National Agricultural Library. http://www.nal.usda.gov/fnic/foodcomp/Data/Other/oxalic.html

14 Guil, José Luis, María Esperanza Torija, Juan José Giménez, Ignacio Rodríguez-García, and Antonio Giménez. 1996. “Oxalic Acid and Calcium Determination in Wild Edible Plants.” J. Agric. Food Chem. 44 (7): 1821–1823. doi:10.1021/jf950472a.

15 The Owlcroft Company. “Oxalic Acid and Foods.” http://oxalicacidinfo.com/.