Iron Supplementation: Absorption and What Blocks It

Iron from animal tissue is heme iron, absorbed intact through a dedicated transporter at roughly 25% efficiency. Iron from plants and supplements is non-heme, absorbed through DMT1 as ferrous ions at roughly 5-12% efficiency and heavily influenced by what else is in the gut.

That difference explains why a vegetarian can eat a diet with adequate iron content and still run low ferritin. It also explains why the enhancers and inhibitors below matter so much.

Vitamin C is the most powerful enhancer. Ascorbic acid reduces ferric iron to the absorbable ferrous form and chelates it, keeping it soluble as it transits the alkaline small intestine. Adding 100-500 mg can multiply non-heme absorption several-fold.

Heme iron itself enhances non-heme absorption. A small serving of meat, poultry or fish alongside plant iron raises uptake of the plant iron in the same meal.

Polyphenols in coffee and tannins in tea bind iron and can reduce absorption by 40-70% when consumed at the same time. This is the single most common self-inflicted absorption problem: coffee with breakfast, iron with breakfast.

Calcium competes directly through DMT1 and inhibits iron absorption dose-dependently. Dairy combines this calcium effect with casein and whey inhibition. Phytates in grains and legumes chelate iron, though vitamin C largely overcomes this.

Iron absorption is saturable, so a single large dose wastes most of what it contains. Two doses of 25 mg absorb more total iron than one dose of 50 mg, and cause less nausea.

Bisglycinate chelate is the best-tolerated form. Ferrous sulfate is the most studied and the cheapest, but causes constipation and nausea in a substantial minority. Take it with vitamin C rather than with food if you can tolerate it.