Iron absorption in anthropological perspective
Back in the Paleolithic, iron deficiency was probably uncommon (Eaton et al 1999).
That’s because the Paleolithic diet included
• animal muscle tissue (from mammals, birds, fish, and/or shellfish)
• plant-based sources of iron (e.g., green leafy vegetables), and
• foods rich in vitamin C (which can triple the bioavailability of iron)
Today, many people around the world are iron deficient. They can improve their iron status by changing their diets. But how does this work? Here is an overview of iron absorption--the factors that enhance it and the substances that inhibit it.
Heme versus nonheme iron
The bioavailability of iron depends on several factors. People who are iron-replete absorb iron less easily than people who are iron-deficient.
In addition, absorption depends on the specific form that iron takes.
Heme iron is the most readily absorbed form of iron, and it’s found in shellfish, red meat, poultry, and fish (see the chart in this article about iron rich goods). On average, people absorb between 15-35% of the heme iron they consume (Insel et al 2003).
Nonheme iron is found in plant foods, as well as in eggs, milk, and meat.
Compared with heme iron, it’s less easily absorbed by the body.
Moreover, sources of nonheme iron often contain phytates, which bind to iron and carry it through the digestive tract unabsorbed.
As a result, the foods with high iron content aren’t necessarily the best sources of iron. By weight, soybeans have roughly twice the iron of beef. But only about 7% of the iron in soybeans is absorbed. Spinach is also high in iron, but less than 2% of the iron in cooked spinach is absorbed (Scrimshaw 1991).
Iron absorption enhancers
Nonheme iron is harder to absorb. But this doesn’t mean it’s unimportant. What’s needed is a way to improve the body's absorption of nonheme iron, and you can do this by adding one or more of these iron absorption enhancers to your meals:
• Vitamin C (Teucher et al 2004; Fidler et al 2009)
• Beef (Johnson and Walker 1992; Englemann et al 1998)
• Poultry (Hurrell et al 2006)
• Salmon (and, presumably, some other kinds of fish: Navas-Carretero et al 2008)
• Pork (Engle-Stone et al 2005)
• Citric acid (Hallberg and Rossander 1984)
How much difference do these additives make?
Vitamin C may be a particularly powerful iron absorption enhancer. One study reported that adding just 63 mg of vitamin C to a meal rich in nonheme iron yielded a 2.9-fold increase in iron absorption (Fidler et al 2009).
Meat can also make a big difference. Experiments suggest that adding 50 to 85 grams of meat to a meal results in a 1.5- to 4-fold increase in iron absorption (Baech 2003; Baynes and Bothwell 1990; Cook et al 1976; Engle-Stone et al 2005; Navas-Carretero 2008).
Beef may more effective than other meats. In one experiment, beef protein enhanced iron absorption 80% better than did chicken protein (Hurrell et al 2006).
Iron absorption inhibitors
The flip side of the coin are substances that inhibit the absorption of iron:
• phytic acid (found in grains, legumes, and other plant foods)
• egg protein (from both the white and the yolk)
• minerals that compete with iron for absorption: calcium, zinc, magnesium, and copper
• tannic acid (in tea)
• certain herbs, including peppermint and chamomile
Notice that many nutritious, healthful foods contain iron absorption inhibitors. It’s not desirable to cut these out of your child’s diet--your child needs calcium, zinc, phosphorus, and fiber.
However, if your child is iron deficient, it might be helpful to avoid some of these foods just before, during, or after a meal that contains iron rich foods.
For instance, experiments have demonstrated that people absorb much less iron from bread when their meals include egg protein, tea, peppermint, chamomile, or coffee (Hurrell et al 1999; Hurrell et al 1988).
Similar experiments have demonstrated that calcium interferes with iron absorption (Hallberg 1998; Perales et al 2006).
What about phytic acid? Phytates, which are found in most plant foods, can reduce iron absorption by up to 80%. But vitamin C—consumed along with a meal--can counteract the effect. In one study, preschoolers with iron deficiency anemia were given vitamin C supplements twice a day—100 mg at each of two phytate-rich meals. After two months, most of the kids were no longer anemic (Seshahdri et al 1985).
This underscores the importance of vitamin C. Many grains and legumes (including soy) can be good sources of iron--if you consume them with vitamin C.
Iron-fortified foods really can help. Iron-fortified cereals--consumed with vitamin C--can be an effective way to improve your child’s iron status. So can iron-fortified soy sauce (Chen et al 2005).
Cooking with iron cookware can add iron to your diet, particularly if you cook acidic foods at high temperatures (Kuligowski and Halperin 1992).
Although egg protein inhibits the absorption of iron, a few egg yolks might not hurt. In a study of healthy, iron sufficient weanling infants, researchers fed 6-month-old babies egg yolks from eggs enriched with n-3 fatty acids. Babies ate 4 cooked yolks a week for 6 months. At the end of the treatment, their iron status was similar to that of controls (Makrides et al 2002). The study didn’t describe whether the eggs were eaten alone or in the context of a meal.
Learn more about iron rich foods
For more information about improving iron absorption, see this article about iron rich foods.
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Image of red blood cells by OpenStax College, Anatomy & Physiology, Connexions website. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.