Food sources of iron come in two forms - heme and non-heme. Heme iron is found in animal products such as dark meat poultry and red meat. Non-heme iron is found in foods such as spinach, iron-fortified cereals, beans and raisins. Heme iron is absorbed a lot better by the body - between 10 and 30 percent is absorbed. Non-heme iron, on the other hand, is only absorbed 2 to 10 percent of the time. Natural compounds called phytates in these plant foods prevent iron-uptake. Phytates are phosphorus-containing compounds that block iron absorbance. Natural occurring polyphenols and tannins found in non-herbal teas and coffee block in a similar fashion.
Heme sources (per 3 ounce serving) - Beef liver (6mg), beef (3.5mg), pork (3.4mg), shrimp (2.6mg), dark meat turkey (2mg), chicken breast (1mg), tuna (1mg), flounder (1mg)
Non-heme sources - Iron-fortified cereal (2-18mg/ounce), kidney beans (3mg/0.5 cup), molasses (2.3mg/1 Tbsp), baked beans (2mg/0.5 cup), cooked spinach (2mg/0.5 cup), enriched bread, pasta, rice (~2mg/slice or 1 cup)
Other Sources of Iron (Heme and Non-heme)
Although non-heme iron absorbancy is compromised, these foods can still be great sources of dietary iron. Consider eating non-heme sources with heme sources for best results. For example, include beans in a lean ground beef chili, or add spinach to a shrimp pasta. Also, cooking these foods in a cast iron skillet helps. Eat all iron-rich foods with a source of vitamin C. The vitamin helps chelate the iron, making it more soluble in the intestines. Good sources of vitamin C include citrus fruits, bell peppers, tomatoes, melon, papaya, broccoli, spinach, Brussel sprouts, strawberries, kale, and cauliflower.
Proton pump inhibitors such as prilosec used for acid reflux can be detrimental to iron absorbancy. The pH of the stomach influences aborbance in the intestines. Other nutrients can compete for absorbance as well. Zinc, manganese, and calcium all naturally exist with a +2 charge (review your chemistry!), so they will all compete for the same ion channels in the body. In other words, do not eat milk, dairy, oysters (zinc) , or pineapple (manganese) with iron-rich foods.
When looking for an iron supplement, look for iron in the ferric (+2) form. Ferrous (+3) iron is not absorbed as well, and must be first oxidized to its +2 form. Look for a supplement with at least 100% of the daily value (18mg for adult females, 10mg for males) and take separate from other supplements. Consider taking with a vitamin C (ascorbic or citric acid) supplement. If iron causes constipation, which it can, include more fiber in your diet or take a fiber supplement (cellulose, flax seed, psyllium husk, pectin) but remember to take at a different time than the iron since most fiber-rich food contains phytates.
Fact: On my test to become a dietitian, they asked what the best vegetarian source of iron was. Answer according to the test: Baked beans! Double check the sugar content or make your own. Most canned baked beans have lots of added sugars.