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

Heme iron is the most readily absorbed form of iron, and it’s found in shellfish, red meat, poultry, and fish. On average, people absorb between 15-35% of the heme iron they consume (Insel et al 2003).

Nonheme iron

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 particularly effective. 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