Iron Absorption

What is iron?

Iron is an essential element of various metabolic processes in humans, including DNA synthesis, electron transport, and oxygen transport. Unlike other minerals, iron levels in the human body are controlled only by absorption. The mechanism of iron excretion is an unregulated process arrived at through loss in sweat, menstruation, shedding of hair and skin cells, and rapid turnover and excretion of enterocytes [1].

Types of Iron

here are two types of absorbable dietary iron: heme and non-heme iron.

• Heme iron, derived from hemoglobin and myoglobin of animal food sources (meat, seafood, poultry), is the most easily absorbable form (15% to 35%) and contributes 10% or more of our total absorbed iron.
• Non-heme iron is derived from plants and iron-fortified foods and is less well absorbed.

Despite its relative abundance in the environment and the relatively low daily iron requirements (10 mg ingested/1 mg absorbed) of humans, iron is often a growth-limiting nutrient in the human diet [1].

Low intake of iron accounts for most anemia in developed countries and is responsible for nearly half of the anemias in non-industrialized nations [1].

Inhibitors and enhancers of Iron

• Inhibitors of iron absorption include phytate, which is a compound found in plant-based diets that demonstrate a dose-dependent effect on iron absorption. Polyphenols are found in black and herbal tea, coffee, wine, legumes, cereals, fruit, and vegetables and have been demonstrated to inhibit iron absorption [1].  

Unlike other inhibitors such as polyphenols and phytates, which prevent only non-heme iron absorption, calcium inhibits both heme and non-heme iron at the point of initial uptake into enterocytes. Animal proteins such as casein, whey, egg whites, and proteins from plants (soy protein) have been shown to inhibit iron absorption in humans. Oxalic acid is found in spinach, chard, beans, and nuts and acts to bind and inhibit iron absorption [1].  

• Enhancers of iron absorption are dominated by the effect of ascorbic acid (vitamin C), which can overcome the effects of all dietary inhibitors when it is included in a diet with high non-heme iron availability (usually a meal heavy in vegetables).  Ascorbic acid forms a chelate with ferric (Fe3+) iron in the low pH of the stomach, which persists and remains soluble in the alkaline environment of the duodenum [1].

Conditions that will limit the iron absorption

• Celiac disease
• Tropical sprue
• Crohn’s disease
• Duodenal cancer
• Duodenal ulcers
• Familial adenomatous polyposis

What is Anemia?

Anemia of chronic disease is a normochromic, normocytic anemia that shows characteristically elevated ferritin stores but lower total body iron. Inflammatory states increase cytokine release (IL-6), which stimulates hepcidin expression in the liver. Hepcidin causes decreased iron absorption through ferroportin degradation and decreases the release of iron from macrophages. The iron that accumulates in cells in anemia of chronic disease is stored as ferritin [1]. 

What is Iron deficiency anemia?

Iron-deficiency anemia is a hypochromic, microcytic anemia caused by hemorrhage (most commonly through trauma or gastrointestinal lesions), decreased dietary iron, or decreased iron absorption [1]. Menstruating women of reproductive age require twice the amount of iron as similarly-aged men. Pregnancy and breastfeeding also significantly increase the iron requirements of women, helping to make iron deficiency the most common dietary deficiency in the world [2,3].