Oxalates

What is it?

Oxalate binds calcium, and where it binds it in the kidney it can form calcium-oxalate stones. It is a plant compound: it concentrates in leaves, roots, bran and seeds rather than in flesh grown to be eaten. Spinach, beetroot, rhubarb, almonds, cashews, chocolate, sweet potato and raw taro carry the most; most fruit sits low, and the exceptions are few enough to name — rhubarb, and berries eaten with their seeds.

Calcium in the same meal is most of the advice

Holmes (2001) measured that rather than assuming it. On controlled diets of 10, 50 and 250 mg of oxalate, dietary oxalate accounted for 24 to 42 per cent of what came out in the urine — well above the 10 to 20 per cent believed before — and cutting the calcium of the 250 mg diet from 1,002 to 391 mg raised urinary oxalate by 28 per cent, taking the dietary share to 53. Jaeger (2004) gives the mechanism: only the soluble form of oxalate can be absorbed, and how much is soluble depends on pH and on the calcium and magnesium present, which is why calcium eaten alongside lowers what reaches the blood. Holmes measures urine against diets of known oxalate content and Jaeger is a review and a model, so neither carries an oxalate figure for any single food.

Dairy is on the right side of that mechanism

A cheese brings no oxalate of its own — oxalate is a plant compound and milk is not a plant — and its calcium binds oxalate from the rest of the plate. Cutting dairy to manage stones therefore works against the person doing it.

Cooking and soaking take part of it away

Noonan and Savage (1999) carry the rule most of this axis rests on: soaking and cooking a high-oxalate food leach part of the oxalate into the water. They also give the arithmetic of a whole diet — mean intake 70 to 150 mg a day in English diets, with tea the largest single contributor, ahead of rhubarb, spinach and beet — and conclude that occasional high-oxalate food in an otherwise nutritious diet poses no particular risk. Chai and Liebman (2005) put figures on the leaching: boiling took 30 to 87 per cent of the soluble oxalate out of nine vegetables, steaming 5 to 53 per cent, and the loss turned up in the cooking water. Boil and drain is the stronger method, steaming the weaker, and baking was tested on potatoes alone. Quinteros (2003) found the same for pulses — all cooking methods reduced the soluble oxalate of beans, chickpeas and lentils, with microwave and industrial methods more effective than domestic cooking — without publishing a figure for any of the three. Neither Noonan and Savage nor Chai and Liebman publishes a per-food table, and nine vegetables and three pulses are the whole of those samples.

Refining removes it before the plate

Oxalate sits in the bran, the skin and the germ — exactly the fractions milling and peeling take out — so a refined product sits low where its whole grain does not. Siener (2006) gives the figures: whole grain wheat above 50 mg of total oxalate per 100 g, with Triticum durum at 76.6, T. sativum at 71.2 and T. aestivum at 53.3, and WHEAT BRAN the highest of that study at 457.4 mg/100 g. Israr (2013) measured bran cereals and legumes together with their phytate and minerals and found total oxalate of 33 to 199 mg/100 g and soluble 14 to 79, the highest soluble figure being an oat bran sample at 79. Both cover grain fractions and nothing else — no maize, no rye, no spinach — and Israr reports ranges across its set rather than a value per food.

Spices, where the numbers are large and the portions are not

Ghosh Das (2012) measured ten Indian spices and the spread between them is the finding: green cardamom highest at 4,014 mg of total oxalate per 100 g with 3,977 of it soluble — almost all in the absorbable form — and nutmeg lowest at 194 and 41. Solubility varies as much as content: turmeric is 91 per cent soluble against 6 per cent in cinnamon. Two limits come with those figures. They are for the spice as sold and not for the pinch a dish uses, and they cover ten Indian spices and nothing outside them — no European spice and no blend, since a blend was never assayed as a blend.

Mushrooms, where the unit matters more than the number

Savage (2002) measured total and soluble oxalate in nine cultivars of edible fungi, six from a supermarket and three picked wild. Raw cultivated ones ran 58.9 to 104.1 mg per 100 g of DRY MATTER and the cultivated cultivars carried mostly insoluble oxalate; the wild ones carried only soluble oxalate, 29.3 to 40.2 mg/100 g dry matter. Those are dry-matter figures, and for a food that is mostly water that is a different number from per 100 g as eaten.

Cola: the usual explanation does not survive measurement

The common claim is that cola raises stone risk through its oxalate. Passman (2009) measured that oxalate at under 1 mg per litre by ion chromatography, and in a six-adult crossover on controlled metabolic diets urinary oxalate after cola (26.2 mg/day) did not differ from water (25.6). The paper states that the proposed mechanism was cola's oxalate content, which its own measurement refutes. Knight (2010) closes the other route: controlled diets at 4, 13 and 21 per cent of calories from fructose changed urinary oxalate, calcium and uric acid not at all, and liver cells given labelled fructose showed no conversion to oxalate. Rodgers (1999) nevertheless found urinary oxalate rising after an acute cola load in 45 volunteers, with more calcium-oxalate crystallisation afterwards. That is an acute load rather than habitual intake and the drink's own oxalate was not measured there, so the disagreement stands — but the oxalate explanation for it does not.

One source behind some levels measures no food at all

Merusi (2010) validated capillary electrophoresis for nitrate, nitrite and oxalate at once against HPLC. It names no vegetable and publishes no oxalate value for anything: what it establishes is that the three can be measured together, not what the answer is for any food. A level resting on it is resting on a method.

Source: Holmes R.P., Goodman H.O. & Assimos D.G., Kidney Int 59 (2001) 270-276, controlled feeding trial on the contribution of dietary oxalate to urinary oxalate; Jaeger Ph. & Robertson W.G. (2004), for the solubility mechanism — a review and a model with no per-food figure; Noonan S.C. & Savage G.P., Asia Pac J Clin Nutr 8 (1999) 64-74, for the leaching rule and the dietary arithmetic; Chai W. & Liebman M., J Agric Food Chem 53 (2005) 3027-3030, for the cooking losses across nine vegetables; Quinteros A. et al. (2003), for the effect of cooking on three pulses; Siener R. et al. (2006), for cereals and wheat bran; Israr B. et al. (2013), for bran cereals and legumes; Ghosh Das S. & Savage G.P., Plant Foods Hum Nutr 67 (2012) 186-190, for ten Indian spices; Savage G.P. et al. (2002), for nine mushroom cultivars on a dry-matter basis; Passman C.M. et al. (2009), for cola's measured oxalate and the crossover trial; Knight J. et al. (2010), for closing the fructose route; Rodgers A. (1999), for the acute cola load; Merusi C. et al. (2010), a method validation that publishes no oxalate value for any food

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