Iron: What is it?
Iron, one of the most abundant metals on Earth, is essential to most life forms and to normal human physiology. Iron is an integral part of many proteins and enzymes that maintain good health. In humans, iron is an essential component of proteins involved in oxygen transport [1,2]. It is also essential for the regulation of cell growth and differentiation [3,4]. A deficiency of iron limits oxygen delivery to cells, resulting in fatigue, poor work performance, and decreased immunity [1,5-6]. On the other hand, excess amounts of iron can result in toxicity and even death [7].
Almost two-thirds of iron in the body is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues. Smaller amounts of iron are found in myoglobin, a protein that helps supply oxygen to muscle, and in enzymes that assist biochemical reactions. Iron is also found in proteins that store iron for future needs and that transport iron in blood. Iron stores are regulated by intestinal iron absorption [1,8].
What foods provide iron?
There are two forms of dietary iron: heme and nonheme. Heme iron is derived from hemoglobin, the protein in red blood cells that delivers oxygen to cells. Heme iron is found in animal foods that originally contained hemoglobin, such as red meats, fish, and poultry. Iron in plant foods such as lentils and beans is arranged in a chemical structure called nonheme iron [9]. This is the form of iron added to iron-enriched and iron-fortified foods. Heme iron is absorbed better than nonheme iron, but most dietary iron is nonheme iron [8]. A variety of heme and nonheme sources of iron are listed in Tables 1 and 2.
Table 1: Selected Food Sources of Heme Iron [10]
| Food | Milligrams per serving |
% DV* |
|---|---|---|
| Chicken liver, cooked, 3? ounces | 12.8 | 70 |
| Oysters, breaded and fried, 6 pieces | 4.5 | 25 |
| Beef, chuck, lean only, braised, 3 ounces | 3.2 | 20 |
| Clams, breaded, fried, ? cup | 3.0 | 15 |
| Beef, tenderloin, roasted, 3 ounces | 3.0 | 15 |
| Turkey, dark meat, roasted, 3? ounces | 2.3 | 10 |
| Beef, eye of round, roasted, 3 ounces | 2.2 | 10 |
| Turkey, light meat, roasted, 3? ounces | 1.6 | 8 |
| Chicken, leg, meat only, roasted, 3? ounces | 1.3 | 6 |
| Tuna, fresh bluefin, cooked, dry heat, 3 ounces | 1.1 | 6 |
| Chicken, breast, roasted, 3 ounces | 1.1 | 6 |
| Halibut, cooked, dry heat, 3 ounces | 0.9 | 6 |
| Crab, blue crab, cooked, moist heat, 3 ounces | 0.8 | 4 |
| Pork, loin, broiled, 3 ounces | 0.8 | 4 |
| Tuna, white, canned in water, 3 ounces | 0.8 | 4 |
| Shrimp, mixed species, cooked, moist heat, 4 large | 0.7 | 4 |
Table 2: Selected Food Sources of Nonheme Iron [10]
| Food | Milligrams per serving |
% DV* |
|---|---|---|
| Ready-to-eat cereal, 100% iron fortified, ? cup | 18.0 | 100 |
| Oatmeal, instant, fortified, prepared with water, 1 cup | 10.0 | 60 |
| Soybeans, mature, boiled, 1 cup | 8.8 | 50 |
| Lentils, boiled, 1 cup | 6.6 | 35 |
| Beans, kidney, mature, boiled, 1 cup | 5.2 | 25 |
| Beans, lima, large, mature, boiled, 1 cup | 4.5 | 25 |
| Beans, navy, mature, boiled, 1 cup | 4.5 | 25 |
| Ready-to-eat cereal, 25% iron fortified, ? cup | 4.5 | 25 |
| Beans, black, mature, boiled, 1 cup | 3.6 | 20 |
| Beans, pinto, mature, boiled, 1 cup | 3.6 | 20 |
| Molasses, blackstrap, 1 tablespoon | 3.5 | 20 |
| Tofu, raw, firm, ? cup | 3.4 | 20 |
| Spinach, boiled, drained, ? cup | 3.2 | 20 |
| Spinach, canned, drained solids ? cup | 2.5 | 10 |
| Black-eyed peas (cowpeas), boiled, 1 cup | 1.8 | 10 |
| Spinach, frozen, chopped, boiled ? cup | 1.9 | 10 |
| Grits, white, enriched, quick, prepared with water, 1 cup | 1.5 | 8 |
| Raisins, seedless, packed, ? cup | 1.5 | 8 |
| Whole wheat bread, 1 slice | 0.9 | 6 |
| White bread, enriched, 1 slice | 0.9 | 6 |
*DV = Daily Value. DVs are reference numbers developed by the Food and Drug Administration (FDA) to help consumers determine if a food contains a lot or a little of a specific nutrient. The FDA requires all food labels to include the percent DV (%DV) for iron. The percent DV tells you what percent of the DV is provided in one serving. The DV for iron is 18 milligrams (mg). A food providing 5% of the DV or less is a low source while a food that provides 10-19% of the DV is a good source. A food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet. For foods not listed in this table, please refer to the U.S. Department of Agriculture’s Nutrient Database Web site: http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl.