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

European Journal of Clinical Nutrition (2008) 62, 336341 & 2008 Nature Publishing Group All rights reserved 0954-3007/08 $30.00 www.nature.com/ejcn

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

S Storcksdieck genannt Bonsmann, T Walczyk, S Renggli and RF Hurrell

Institute of Food Science and Nutrition, ETH Zurich, Zurich, SwitzerlandObjective: 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 (1845 years, o60 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 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.

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 (P40.16). 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. European Journal of Clinical Nutrition (2008) 62, 336341; doi:10.1038/sj.ejcn.1602721; published online 18 April 2007 Keywords: iron absorption; oxalic acid; organic acids; stable isotopes

Introduction Oxalic acid (OA) is a common constituent of plant foods such as green leafy vegetables, rhubarb, parsley, beetroot, carrots, potatoes, cocoa and tea (Zarembski and Hodgkinson, 1962; Chai and Liebman, 2005). Concentrations vary

depending...