Dietary Iron Absorption and Bioavailability

Factors Enhancing Non-Heme Iron Absorption

Non-heme iron, found in plant-based foods, is less readily absorbed than heme iron. Several factors significantly influence its uptake. Ascorbic acid (vitamin C) is a potent enhancer, reducing ferric iron (Fe³⁺) to the more readily absorbed ferrous form (Fe²⁺). The presence of certain amino acids, such as cysteine and histidine, can also improve absorption. Furthermore, consumption of iron-rich foods with meals containing meat, poultry, or fish can increase absorption through a mechanism involving meat-derived factors.

Factors Inhibiting Non-Heme Iron Absorption

Conversely, several substances hinder non-heme iron absorption. Phytates, found in grains, legumes, and nuts, bind to iron, rendering it unavailable for absorption. Oxalates, present in spinach, rhubarb, and chocolate, also reduce absorption. Polyphenols in tea and coffee can also form complexes with iron, diminishing its bioavailability. Calcium and zinc can compete with iron for absorption sites in the gut.

Heme Iron Absorption

Heme iron, derived from animal sources (red meat, poultry, fish), is absorbed more efficiently than non-heme iron. Its absorption is less affected by dietary inhibitors compared to non-heme iron. The efficiency of heme iron absorption is relatively constant, regardless of other dietary components.

Gastrointestinal Physiology and Iron Absorption

Iron absorption primarily occurs in the duodenum and proximal jejunum. Enterocytes, the cells lining the small intestine, play a crucial role in the uptake and transport of iron. The process involves specific transporters, including divalent metal transporter 1 (DMT1) for non-heme iron and heme carrier protein 1 (HCP1) for heme iron. Once inside the enterocytes, iron is either stored as ferritin or transported across the basolateral membrane into the bloodstream via ferroportin.

Iron Deficiency and Absorption

In individuals with iron deficiency, the body enhances iron absorption to compensate for low iron stores. This reflects increased expression of iron transporters and decreased storage of iron within enterocytes. Conversely, individuals with iron overload exhibit reduced iron absorption.

Individual Variation and Other Considerations

Individual variations in iron absorption exist due to genetic factors, health status, and other lifestyle factors. Certain medical conditions, such as celiac disease and inflammatory bowel disease, can significantly impair iron absorption. Gastric acidity also plays a role; reduced stomach acid can decrease iron absorption.