The effects of organic acids, phytates and polyphenols on the absorption of iron from vegetables

Abstract

1. Non-haem iron absorption from a variety of vegetable meals was studied in parous Indian Women, using the erythrocyte utilization of radioactive Fe method. 2. The studies were undertaken to establish whether Fe absorption could be correlatedwith the chemical composition of the foodstuff. 3. Addition of the following organic acids commonly found in vegetables, improved the geometric mean Fe absorption from a basic rice meal as follows: from 0·028 to 0·085 with 1 g citric acid, from 0·031 to 0·081 with 15 mg ascorbic acid, from 0·048 to 0·095 with 1 g L-malic acid, from 0·041 to 0·096 with 1 g tartaric acid. The only exception was oxalic acid; the addition of 1 g calciumoxalate to cabbage (Brassica oleraceae) was associated with some depression in Fe absorption from 0·320 to 0·195. 4. There was a marked inhibition of the geometric mean absorption when 500 mg tannic acid was added to a broccoli (Brassica oleraceae) meal (0·015 v. 0·297). Sodium phytate (2 g) caused a similar, though less profound inhibition (0·035 to 0·152). 5. When 3 mg ferrous sulphate was added to different vegetables the geometric mean absorption varied widely. Vegetables of low Fe bioavailability were wheat germ (Triticum aestivum) 0·007, aubergine (Solanum melongena) 0·007, butter beans (Phaseolus lunatus) 0·012, spinach (Spinacea oleraceae) 0·014, brown lentils (Lens culinaris) 0·024, beetroot greens (Beta vulgaris) 0·024 and green lentils (Lens culinaris) 0·032. In contrast, bioavailability was moderate or good with carrot (Daucus carota) 0·098, potato (Solanum tuberosum) 0·115, beetroot (Beta vulgaris) 0·185, pumpkin (Cucurbita mixta) 0·206, broccoli 0·260, tomato (Lycopersicon esculentum) 0·224, cauliflower (Brassica oleraceae) 0·263, cabbage 0·320, turnip (Brassica rapa) 0·327 and sauerkraut 0·327. 6. All the vegetables associated with moderate or good Fe bioavailability contained appreciable amounts of one or more of the organic acids, malic, citric and ascorbic acids. 7. Poor Fe bioavailability was noted in vegetables with high phytate contents (e.g. wheat germ 0·007, butter beans 0·012, brown lentils 0·024 and green lentils 0·032). 8. The fact that a number of vegetables associated with low Fe-absorption turned bluish-black when Fe was added to them, suggested that the total polyphenol content in them was high. The vegetables included aubergine spinach, brown lentils, green lentils and beetroot greens. When the total polyphenol content in all the vegetables tested was formally measured, there was a significant inverse correlation (r 0·859, P < 0·001) between it and Fe absorption. The inverse correlation between the non-hydrolysable polyphenol content and Fe absorption was r 0·901 (P < 0·001). 9. The major relevance of these findings is the fact that the total absorption of non-haem-Fe from a mixed diet may be profoundly influenced by the presence of single vegetables with either marked enhancing or inhibiting effects on Fe bioavailability.