Soy has a pronounced inhibitory effect on the absorption of iron

This study was published in the American Journal of Clinical Nutrition 1981 Dec;34(12):2622-9

Study title and authors:
The inhibitory effect of soy products on nonheme iron absorption in man.

Cook JD, Morck TA, Lynch SR.

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/7198374

Three separate studies were performed in 36 men ranging in age from 18 to 50 years to determine the effects of various proteins on iron absorption.

The studies comprised of:
(i) Study I compared the effects of different proteins (egg protein, milk protein and isolated soy protein) on iron absorption.
Each meal contained 29.4 grams of protein.
(ii) Study II examined the influence of egg protein and various soy products (full fat soy flour, textured soy flour and isolated soy protein) on iron absorption.
Each meal contained 14.7 grams of protein.
(iii) Study III evaluated the effect on iron absorption of adding soy protein to meat.
Meal 1 included 100 grams of ground (minced) beef.
Meal 2 included 100 grams of ground (minced) beef and 30 grams of textured soy flour. (A 3:1 meat/soy ratio).
Meal 3 included 70 grams of ground (minced) beef and 30 grams of textured soy flour. (A 2:1 meat/soy ratio).

The results of the studies were as follows:
(a) In study I, those who consumed the isolated soy protein meal absorbed 81% less iron than those who consumed the egg protein and milk protein meals.
(b:i) In study II, those who consumed the full fat soy flour meal absorbed 82% less iron than those who consumed the egg protein meal.
(b:ii) In study II, those who consumed the textured soy flour meal absorbed 65% less iron than those who consumed the egg protein meal.
(b:iii) In study II, those who consumed the isolated soy protein meal absorbed 92% less iron than those who consumed the egg protein meal.
(c:i) In study III, those who consumed the meal with the extra soy with the 100 grams of beef (meal 2) absorbed 61% less iron than those who consumed the 100 grams of beef only meal.
(c:ii) In study III, those who consumed the meal with the extra soy and depleted beef (meal 3) absorbed 53% less iron than those who consumed the 100 grams of beef only meal.

The results of these studies show that soy products have a pronounced inhibitory effect on the absorption of iron.