Soy consumption inhibits the bioavailability of iron

This study can be accessed at the American Journal of Clinical Nutrition 1992 Sep;56(3):573-8

Study title and authors:
Soy protein, phytate, and iron absorption in humans.

Hurrell RF, Juillerat MA, Reddy MB, Lynch SR, Dassenko SA, Cook JD.
Nestec Ltd., Nestlé Research Centre, Lausanne, Switzerland.

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/1503071

Soy protein isolate can be found in protein bars, meal replacement shakes, bottled fruit drinks, soups and sauces, meat analogs, baked goods, breakfast cereals and some dietary supplements.

Soy is high in a substance called called phytic acid (phytate) which binds to minerals (including calcium, magnesium, iron, and zinc) in your digestive tract and keeps your body from absorbing those minerals.

The study examined the effects of soy-protein isolate on iron absorption. The study included 32 men and women aged 20 to 31 years in which the absorption of soy-protein isolate was compared with egg-whites.

The study found:
(a) Iron absorption increased four- to fivefold when phytic acid was reduced from its normal amount of 4.9-8.4 to less than 0.01 mg/g of soy-protein isolate.
(b) Even relatively small quantities of residual phytate were strongly inhibitory and phytic acid had to be reduced to less than 0.3 mg/g of soy-protein isolate (corresponding to less than 10 mg phytic acid/meal) before a meaningful increase in iron absorption was observed.
(c) Even after removal of virtually all the phytic acid, iron absorption from soy-protein isolate was still only half that of egg whites.

The researchers conclude that phytic acid is a major inhibitory factor of iron absorption in soy-protein isolates but that other factors in the soy also contribute to the poor bioavailability of iron.