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A Primer on Iron

By Andrew

Jul 20, 2010

A good friend of mine — a marathon runner — has taken me up on my free nutritional analysis offer.  I haven’t finished crunching the data yet, but one of the things she mentioned is that she wants to be sure she’s getting enough iron.

Women generally need to be a bit more diligent about their iron intake (particularly during pregnancy and menstruation), and elite athletes (such as marathon runners) likely need to increase their iron intake as well.

Types of Dietary Iron

There are two types of dietary iron.  Heme iron comes from animal foods that originally contained hemoglobin (hence the name), such as red meat, fish, and poultry.  Nonheme iron, therefore, comes from plant sources.

Breakfast cereal and other iron-enriched or iron-fortified foods provide nonheme iron.

Absorption Rates

Many factors influence the rate of absorption.  If your iron stores are low, your body will naturally absorb more iron from the foods you eat.  When iron stores are high, absorption rates will decrease — automatically helping to protect against iron toxicity.  (Too much iron can be a big problem, so be sure to talk to your doctor before taking any iron supplements!)

Heme iron is generally absorbed at a rate of 15% to 35%, whereas nonheme iron absorption ranges from about 2% to 20%.

Dietary factors can influence rates of nonheme iron absorption, but heme iron is usually well absorbed regardless of the foods with which it is combined.

When eaten at the same time, Vitamin C (ascorbic acid) will increase the absorption rate of nonheme iron. Interestingly, eating heme iron-rich foods (meat, fish, or poultry) with nonheme sources will also increase the absorption rate of the nonheme foods.

Conversely, tannins (found in tea), calcium, polyphenols (found in tea, coffee, grain products, herbs such as oregano, and red wine), phytates (found in legumes, rice, and whole grains), vegetable proteins (particularly soy protein) can decrease the absorption rate of nonheme iron.

Contrary to popular belief, spinach is not a great source of iron.  It does have a fair amount of iron (about the same as other green vegetables), but the oxalic acid (often referred to as oxalates) works to block much of the absorption.

Of course, even if a food may reduce the absorption of iron, there may be plenty of other good reasons to eat it — so you don’t necessarily need to eliminate it from your diet!

How much iron do you need?

The Dietary Reference Intake recommendations are the levels that are expected to help maintain proper iron balance.

It’s important to understand that these numbers are the total recommended intake, and already account for partial absorption.  For example, although men need to consume 8 mg per day, only about 1 mg needs to actually be absorbed in order to maintain healthy levels in the body.

These recommended intake levels assume that 75% of the iron intake comes from heme sources. Vegetarians should roughly double these numbers to ensure adequate iron absorption.

* Women who are pregnant should get 27 mg/day.  Lactating adult women should get 10 mg/day, and lactating women aged 14 to 18 should get 9 mg/day.

Good dietary sources of heme iron

• Clams, cooked, 3 ounces: 23.8 mg
• Chicken Liver, cooked, 3½ ounces: 12.8 mg
• Turkey Giblets, with some giblet fat, 1 cup:  11.2 mg
• Chicken Giblets, 1 cup:  10.2 mg
• Duck, meat only, 1/2 duck:  6 mg
• Oysters, raw, 6 medium: 5.6 mg
• Lean Beef, 3 ounces: 3.1 mg
• Turkey, roasted, meat only, 1 cup: 2.5 mg
• Sardines, canned, 3 ounces: 2.5 mg
• Shrimp, 3 ounces: 1.5 mg
• Tuna, light, canned, 3 ounces:  1.3 mg
• Chicken Breast, roasted, no skin, 1/2 breast: 0.9 mg
• Halibut, 3 ounces: 0.9 mg
• Chicken Drumstick, roasted, no skin, one:  0.6 mg

Good dietary sources of nonheme iron

• Fortified breakfast cereal, 3/4 cup: 6 to 18 mg
• Cream of Wheat cereal (fortified), 1 cup: 12.74 mg
• Instant Oatmeal, fortified, 1 packet: 10.55 mg
• Soybeans, boiled, 1/2 cup: 4.4 mg
• White Beans, canned, 1/2 cup: 3.9 mg
• Lentils, cooked, 1/2 cup: 3.3 mg
• Pumpkin, canned, no salt added, 1 cup: 3.4 mg
• Tomatoes, canned, stewed, 1 cup: 3.4 mg
• Blackstrap Molasses, one tablespoon: 3.0 mg
• Kidney Beans, canned, 1/2 cup: 1.6 mg
• Chickpeas (garbanzo beans), canned, 1/2 cup: 1.6 mg
• Pumpkin Seeds, 1 ounce: 2.3 mg
• Cashews, roasted, 1 ounce: 1.7 mg
• Soymilk, unfortified, 1 cup: 1.6 mg
• Hazelnuts (filberts), 1 ounce: 1.3 mg
• Beets, cooked and drained, 1 cup: 1.3 mg
• Tofu, firm, 1/4 block: 1.3 mg
• Sunflower Seeds, 1/4 cup: 1.2 mg
• Pistachios, 1 ounce: 1.2 mg

The Bottom Line

Vegetarians should include fortified cereals and grains, seeds, nuts, and tomatoes — and combine those with foods rich in vitamin C, such as orange or grapefruit juice, oranges, papaya, grapefruit, pineapple, strawberry, red bell peppers, broccoli, brussels sprouts, and peas.

For non-vegetarians, in addition to following the advice above, heme iron is your most useful source, and it’s best to combine heme and nonheme iron in any given meal.

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Sources & References:

National Institutes of Health, Dietary Supplement Fact Sheet: Iron

Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc

USDA National Nutrient Database for Standard Reference, Release 22

Photo by zero g.