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Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals



To evaluate the influence of oxalic acid (OA) on nonhaem iron absorption in humans.


Two randomized crossover stable iron isotope absorption studies.


Zurich, Switzerland.


Sixteen apparently healthy women (18–45 years, <60 kg body weight), recruited by poster advertizing from the staff and student populations of the ETH, University and University Hospital of Zurich, Switzerland. Thirteen subjects completed both studies.


Iron absorption was measured based on erythrocyte incorporation of 57Fe or 58Fe 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.


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).


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.

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The help of Eberhard Denk, Ralf Biebinger and Ines Egli in performing the human studies is greatly appreciated. Special thanks go to Christophe Zeder for preparing the isotope solutions and helping with data analysis, to Karin Hotz for meal iron and calcium determination, to Charlotte Züllig and Marlies Krähenbühl for taking blood samples, and to the volunteers for participating in this study.

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Correspondence to S Storcksdieck genannt Bonsmann.

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Contributors: SSgB was involved in the planning of the study, carried out the study, measured iron status parameters, analysed the data and wrote the first draft of the manuscript. TW contributed to the study design, supervised the meal administration and revised the manuscript draft. SR performed the mass spectrometric iron isotope measurements. RFH supervised the project, contributed to the study design and critically revised the manuscript draft.

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genannt Bonsmann, S., Walczyk, T., Renggli, S. et al. Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals. Eur J Clin Nutr 62, 336–341 (2008).

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  • iron absorption
  • oxalic acid
  • organic acids
  • stable isotopes


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