Oxaalzuur een antinutriënt?
Oxaalzuur is een stof die in heel veel planten zit, vooral in spinazie en rabarber. Het wordt ook in het lichaam zelf aangemaakt, onder meer uit ascorbinezuur (vitamine C) en uit aminozuren.
Oxaalzuur wordt in verband gebracht met nier- en blaasstenen. Hoge concentraties oxaalzuur in de urine worden beschouwd als een risicofactor voor de vorming van calciumoxalaat stenen, die ongeveer 75% van alle nierstenen uitmaken.
Oxaalzuur wordt daarom een ‘antinutriënt’ genoemd. Andere zogenaamde antinutriënten zijn fytinezuur, trypsine remmers, lectinen, saponinen en tanninen.
Het is opvallend dat deze stoffen vooral te vinden zijn in voeding die als de meest gezonde voeding beschouwd wordt: groenten, fruit, peulvruchten, noten, zaden, kruiden, thee, enz. Het is zelfs zo dat steeds meer van de zogenaamde antinutriënten ondertussen ‘bioactieve stoffen’ of ‘nutraceuticals’ genoemd worden, waarvan men aangetoond heeft dat ze essentieel zijn voor onze gezondheid.
Gunstige effecten van oxaalzuur
Het lichaam heeft oxaalzuur nodig. Indien er niet voldoende in de voeding aanwezig is, maakt het lichaam zelf oxaalzuur aan uit ascorbinezuur.
bevordert de darmperistaltiek
haalt calcium uit de vaatwanden (plaque)
wordt omgezet in waterstofperoxide (H2O2) om ziekteverwekkende micro-organismen te vernietigen
doodt kankercellen, virussen en bacteriën
is een antioxidant
voorkomt lipidperoxidatie (oxidatie van vetten)
voorkomt oxidatie van ascorbinezuur
regelt de pH (spijsvertering, enzymatische reacties)
bindt zware metalen en verwijdert ze
is nodig voor de aanmaak van uracil (RNA) en orootzuur (vitamine B13)
Er zijn echter een aantal factoren die van oxaalzuur een antinutriënt maken
Verstoorde darmflora: darmbacteriën kunnen oxaalzuur afbreken, zodat het niet opgenomen kan worden. Uit onderzoek blijkt dat suppletie met probiotica de oxaalzuurconcentratie in de urine aanzienlijk doet dalen.
Te weinig galzouten: oxaalzuur bindt met calcium in de darm en wordt vervolgens met de stoelgang verwijderd. Bij een gebrek aan galzouten worden vetten niet goed opgenomen en is er een grote hoeveelheid onverteerde vetten in de darm aanwezig. Calcium zal in dat geval eerder binden met de vetten dan met oxaalzuur. Het oxaalzuur blijft daardoor ongebonden en kan gemakkelijk opgenomen worden.
Darmziekten: mensen met darmziekten, zoals de ziekte van Crohn, korte darm syndroom en lekkende darm en mensen die darmoperaties ondergaan hebben, breken oxaalzuur niet goed af en absorberen het gemakkelijker.
Nitraten: spinazie bevat van nature een enzym dat oxaalzuur afbreekt. Nitraten (meststoffen) inactiveren het enzym.
Koken van voedsel: koken zet oxaalzuur (zuur) om in oxalaten (zouten). Het zijn deze zouten die verantwoordelijk zijn voor de vorming van calciumoxalaat stenen.
Factoren die de concentratie oxaalzuur in de urine verlagen
een gezonde darmflora, probiotica
organisch calcium en magnesium binden met oxaalzuur in de darm
oxaalzuurrijke voeding in combinatie met andere voedingsmiddelen
taurine en vitamine C: zijn nodig voor de aanmaak van galzouten
rauwe voeding i.p.v. gekookte voeding
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’.
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
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 solubleoxalic 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!?
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/.