Oxalic acid does not influence nonhaem iron absorption in humans:

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Eur J Clin Nutr. 2008 Mar;62(3):336-

41. Epub 2007 Apr 18.

Oxalic acid does not influence nonhaem iron absorption in humans: a

comparison of kale and spinach meals.

genannt Bonsmann SS, Walczyk T, Renggli S, Hurrell RF.

Institute of Food Science and Nutrition, ETH Zurich, Zurich, Switzerland.

OBJECTIVE: To evaluate the influence of oxalic acid (OA) on nonhaem iron

absorption in humans. DESIGN: Two randomized crossover stable iron

isotope absorption studies. SETTING: Zurich, Switzerland. SUBJECTS:

Sixteen apparently healthy women (18-45 years, <60 kg body weight),

recruited by poster advertising from the staff and student populations

of the ETH, University and University Hospital of Zurich, Switzerland.

Thirteen subjects completed both studies. METHODS: Iron absorption was

measured based on erythrocyte incorporation of (57)Fe or (58)Fe 14 days

after the administration of labelled meals. In study I, test meals

consisted of two wheat bread rolls (100 g) and either 150 g spinach with

a native OA content of 1.27 g (reference meal) or 150 g kale with a

native OA content of 0.01 g. In study II, 150 g kale given with a

potassium oxalate drink to obtain a total OA content of 1.27 g was

compared to the spinach meal. RESULTS: After normalization for the

spinach reference meal absorption, geometric mean iron absorption from

wheat bread rolls with kale (10.7%) did not differ significantly from

wheat rolls with kale plus 1.26 g OA added as potassium oxalate (11.5%,

P=0.86). Spinach was significantly higher in calcium and polyphenols

than kale and absorption from the spinach meal was 24% lower compared to

the kale meal without added OA, but the difference did not reach

statistical significance (P>0.16). CONCLUSION: Potassium oxalate did not

influence iron absorption in humans from a kale meal and our findings

strongly suggest that OA in fruits and vegetables is of minor relevance

in iron nutrition.