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

Eur J Clin Nutr. 2008 Mar;62(3):336-41. doi: 10.1038/sj.ejcn.1602721. Epub 2007 Apr 18.


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 advertizing 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.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Biological Availability
  • Brassica / chemistry*
  • Cross-Over Studies
  • Dose-Response Relationship, Drug
  • Erythrocytes / chemistry
  • Erythrocytes / metabolism
  • Female
  • Humans
  • Intestinal Absorption / drug effects
  • Iron Isotopes
  • Iron, Dietary / pharmacokinetics*
  • Oxalic Acid / pharmacology*
  • Spinacia oleracea / chemistry*


  • Iron Isotopes
  • Iron, Dietary
  • Oxalic Acid