Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals.
Level 2 - randomized trial
Randomized crossover trial in humans
PubMed 17440529 · doi:10.1038/sj.ejcn.1602721
What was done
Two randomized crossover stable iron isotope studies evaluated nonhaem iron absorption in 16 healthy women (18–45 years, <60 kg; 13 completed both studies) in Zurich, Switzerland. Iron absorption was determined via erythrocyte incorporation of 57Fe or 58Fe 14 days after labelled meals. Study I compared wheat rolls with 150 g spinach (1.27 g native oxalic acid) against wheat rolls with 150 g kale (0.01 g native oxalic acid). Study II compared wheat rolls with 150 g kale plus a potassium oxalate drink (total 1.27 g oxalic acid) against the spinach reference meal.
What was found
After normalization to the spinach reference meal, geometric mean nonhaem iron absorption from kale alone (10.7%) did not differ significantly from kale with 1.26 g added oxalic acid (11.5%, P=0.86). Absorption from the spinach meal was 24% lower than from the kale meal without added oxalic acid, but this difference was not statistically significant (P>0.16).
Why it matters
Contrary to common nutritional assumptions, oxalic acid in vegetables appears to have minimal influence on human nonhaem iron bioavailability.
Limits
The sample size was small (16 enrolled, 13 completed) and restricted to healthy, low-weight (<60 kg) adult women, limiting generalizability to males, iron-deficient individuals, or broader populations. Differences in other inhibitors between spinach and kale (such as higher calcium and polyphenols) complicate direct whole-food comparisons.
Cited by
- supports Iron from spinach is poorly absorbed by the body compared to heme-based iron from meat.