"Total dietary iron intake in vegetarian diets may meet recommended levels; however that iron is less available for absorption than in diets that include meat. Vegetarians who exclude all animal products from their diet may need almost twice as much dietary iron each day as non-vegetarians because of the lower intestinal absorption of nonheme iron in plant foods.These are the current RDA's:
-NIH, Office of Dietary Supplements: Iron
By these guidelines, premenopausal women who exclude animal food from their diets may need almost 36 mg of iron each day, adult men 16 mg.
That's a lot of iron. As a gauge, a 3 ounce serving of beef chuck provides about 2.7 mg iron; a half cup of cooked soybeans provide about 4.4 mg iron; a half cup of cooked lentils provide about 3.3 mg iron. And those foods are good sources, as servings go. (Notable in these figures is that red meat doesn't provide more iron than plant food. The argument that the iron in meat is of a form, heme iron, which is more available for absorption, is tricky for two reasons. One, most of the iron in meat is of the nonheme variety. "Less than 40% of the iron in meat, poultry, and fish is in the heme form." Two, other foods eaten can substantially inhibit or enhance absorption of iron, whether heme or nonheme.)
Maybe there's a mechanism, like the one we saw with calcium, that has us absorb more iron when our stores are low. This paper describes just that mechanism:
Bioavailability Of Iron, Zinc, And Other Trace Minerals From Vegetarian Diets, The American Journal of Clinical Nutrition, September 2003
"Because of apparent upregulation of nonheme iron absorption, nonheme iron contributes more than heme iron to the total amount of iron absorbed in people with low body iron stores. Thus, the generally less well absorbed nonheme iron in vegetarian diets is more responsive than heme iron to differences in body iron status: nonheme iron absorption can be more completely limited by those with high iron stores, while being nearly as well absorbed as heme iron by those with very low iron stores."Not only do we absorb more iron when our stores are low, but we discard it if our stores are high - more so if it's plant-derived nonheme iron.
So, of the two points above that address iron absorption - iron type (heme vs. nonheme) and "other foods eaten" - it appears that the "other foods eaten" has a greater impact. Well, it appears...
Below is a table from Hunt which shows enhancers and inhibitors of iron absorption:
Ascorbic acid, or vitamin C, is found in higher amounts in peppers, cabbage, broccoli, oranges, grapefruit, berries, and lots of other vegetables and fruits. Carotenes are found in orange (pumpkin, carrot, sweet potato), and green (kale, spinach) vegetables. Phytic acid is found in higher amounts in the bran of nuts, seeds, and cereal grains. Sesame seeds, almonds, walnuts, peanuts, corn, oats, rice, wheat, soybeans, chick peas ... these all have higher amounts of phytic acid and will inhibit iron absorption. Soaking, sprouting, and cooking reduce phytic acid to some degree. It looks like red wine can both enhance iron absorption because of its alcohol and inhibit it because of its polyphenols. Probably a wash that drink.
So, if vegetarians do things like combining iron-rich foods with vitamin C and carotene sources, using iron cookware, drinking coffee and tea between meals, prepping foods to reduce phytic-acid content, will it have a significant impact on iron absorption? Hunt says probably not, but if the diet is suitably varied, it doesn't matter: "Although vegetarians tend to have lower iron stores than omnivores, they appear to have no greater incidence of iron deficiency anemia."
I still can't get my mind wrapped around that 36 mg/day recommendation above. It's almost impossible to do without supplementing. Hunt says the same:
"The suggested modification of dietary iron recommendations for vegetarians might imply a need for routine iron supplementation for vegetarian women of fertile age, but the long-term benefit of such supplementation has not been tested."And she gives these reasons for being careful:
- Iron supplementation reduces the efficiency of iron absorption from the diet.
- Iron supplementation must be continuous to have a long-term influence on serum ferritin of women with low iron stores.
- Iron supplementation may be associated with increased oxidative stress by the unabsorbed iron in the lower bowel.
- Excess iron has been linked to increases in colorectal cancer risk.
- Greater serum ferritin of meat eaters has been associated with reduced insulin sensitivity.
- Increased risk of heart disease has been observed in those with hemochromatosis (an iron storage disorder affecting ~9.5% of Caucasians).
After all this, what have I learned? Getting enough iron on a vegan diet doesn't seem that much different from getting enough iron on a diet that includes meat.