by Jack Norris, RD | Last updated: June 2013
- Functions of Iron
- Iron Deficiency
- Iron Deficiency & Manganese
- Iron in Meat vs. Plants
- Table 1: Iron Amounts in Plant Foods
- Absorption of Plant Iron
- Table 2: Vitamin C in Foods
- Iron Status of Vegetarians
- The Food and Nutrition Board and Vegetarian Diets
- Table 3: Dietary Reference Intake (DRI) for Iron
- Conditions that can Cause or Aggravate Iron Deficiency
- Tolerable Upper Intake Level
- Iron and Chronic Disease
You do not need to worry about iron if you are otherwise healthy and eat a varied vegetarian or vegan diet. If you think your iron stores might be low, you can increase iron absorption by:
- Adding a source of vitamin C at meals (see Table 2 of good vitamin C sources below).
- Avoiding tea and coffee at meals.
- Increasing legume (peanuts, beans, lentils, peas) intake.
- Cooking foods (especially water based acidic foods like tomato sauce) in cast iron skillets.
If your concerns persist, you should have a doctor measure your iron status. If your iron stores are too low, your doctor might suggest eating meat or taking an iron supplement. Anemia in meat-eaters is normally treated with large doses of supplemental iron, not with eating more meat. Similarly, vegetarians with anemia do not need to start eating meat but can also be treated with supplemental iron and vitamin C. If your doctor insists that you eat meat, you might want to show him or her this article.
It is important for any vegan with iron deficiency to correct it because during iron deficiency, the body has a tendency to absorb too much manganese. Luckily, vitamin C increases iron absorption but does not increase manganese absorption.
The major functions of iron are:
- Energy Production
The majority of iron in the body is involved in energy production. The largest fraction is found in the hemoglobin of red blood cells and is necessary for oxygen transport throughout the body. Iron also serves as part of myoglobin for oxygen supply to muscles. Iron is involved in the electron transport system and is part of an important energy-producing enzyme, NADH dehydrogenase.
- Required for DNA synthesis
Iron has pro-oxidation properties used by the immune system to destroy bacteria.
Iron deficiency is the most common nutrient deficiency in the U.S. There are three stages to iron deficiency:
1. Storage Iron Depletion
Storage iron depletion is typically measured by a serum ferritin (the protein on which iron is stored) of less than 18 ng/ml. When serum ferritin drops below 12 ng/ml, iron stores are completely depleted.
Iron is transferred around the body by the transferrin protein. Elevated total iron binding capacity is a measure of open spots for iron on transferrin molecules and can be used to help diagnose storage iron depletion.
Note that ferritin levels can be raised, even in the presence of iron deficiency, in states of inflammation, infection, liver disease, weight gain, significant alcohol consumption, and elevated plasma glucose.
2. Early Functional Deficiency
Early functional iron deficiency is when red blood cell formation starts to become impaired, but not enough to cause a measurable anemia. This is indicated by low serum transferrin saturation, increased erythrocyte protoporphyrin, and/or increased soluble serum transferrin receptor concentration.
Two studies from Switzerland have shown that iron supplementation can reduce fatigue in premenopausal women (18, 53) whose hemoglobin levels are above 120 g/l (and thus not diagnosed with anemia). The most recent, from 2012 (53), was a double-blinded, randomized controlled trial in which 80 mg of ferrous sulfate (an iron supplement) per day for twelve weeks increased hemoglobin in women who had average serum ferritin levels of 22.5 µg/l. This increase in hemoglobin was matched with a 50% reduction in symptoms of fatigue (compared to only 19% for placebo). Improvements in hemoglobin were seen after 6 weeks.
Cognition in adolescent girls (19) has also been improved by iron supplements in those with early functional deficiency.
3. Iron Deficiency Anemia
The Centers for Disease Control defines iron deficiency anemia as iron deficiency with a low hemoglobin value, typically less than 120 g/l. It is characterized by small red blood cells due to a lack of hemoglobin. Low values for hemoglobin concentration in blood, red blood cell count, hematocrit (the percentage, by volume, of red blood cells in whole blood), low mean corpuscular volume (the size of the average red blood cell), and erythrocyte hemoglobin concentration are all potentially indicative of iron deficiency anemia.
Because iron deficiency is not the only cause of anemia, multiple measures of iron status should be taken to determine if an anemia is truly from iron deficiency. Although there are dozens of types of anemia (including anemia caused by low vitamin B12 levels), in this article anemia refers to iron deficiency anemia.
Symptoms of Iron Deficiency
Many iron deficiency symptoms are related to tissue oxygen deprivation: fatigue, rapid heart rate, palpitations, rapid breathing on exertion, and increased lactic acid production. Anemia symptoms include pale skin, brittle fingernails, koilonychia (spoon-shaped fingernails where the outer edges are raised), weakness, loss of appetite, apathy, hair loss, impaired immunity, angular stomatitis (irritation and fissuring in the corners of the lips), glossitis (inflammation of the tongue), chronic gastritis, pica, abnormal temperature regulation, and delayed psychomotor development in children.
It is important for vegans to resolve iron deficiency because it can increase manganese accumulation in the brain. In iron deficiency, manganese is absorbed instead of iron and vegans have high manganese intakes. The good news is that while phytate decreases both iron and manganese absorption, vitamin C increases only iron absorption.
For references and more information, see the VeganHealth article, Manganese.
|Table 1. Iron in Plant Foods|
|Broccoli||chopped, boiled||1/2 C||0.52|
|Spinach||chopped, boiled||1/2 C||3.2|
|Kale||chopped, boiled||1/2 C||0.59|
|Collard greens||chopped, boiled||1/2 C||1.1|
|Swiss chard||chopped, boiled||1/2 C||2|
|Sweet potato||baked, w/skin||1/2 C||0.7|
|Rice (white, long-grain, unenriched)||cooked||1/2 C||0.7|
|Rice (white, long-grain, enriched)||cooked||1/2 C||1.4|
|Bread – whole wheat||1 slice||0.68|
|Soymilk||1 C||1.0 - 1.5|
|Kidney beans||boiled||1/2 C||2.6|
|Pinto beans||boiled||1/2 C||1.8|
|Garbanzo beans||boiled||1/2 C||2.4|
|Peas – green||boiled||1/2 C||1.2|
|Peanut butter||2 T||0.6|
|Tahini||2 T||.7 - 2.6|
|Pistachios||dry roasted||1/4 C||1.2|
|Sunflower seeds||dry roasted||1/4 C||1.2|
|Dried figs||dried, raw||1/2 C||1.5|
|Grape Nuts||1/2 C||16|
|Total, whole grain||1/2 C||8|
|Taken from the USDA National Nutrient Database or food labels.|
Iron is prevalent in a wide variety of plant foods, especially beans and grains. In fact, vegans' iron intakes are as high or higher than non-vegetarians. Table 1 shows the iron content of some plant foods.
In meat, 40% of iron is bound to the heme molecule (from hemoglobin and myoglobin) (54), which is relatively easily absorbed. Beef contains about 3 mg of heme iron per serving, while chicken and pork contain about 1 to 2 mg per serving, and fish contains about 1 mg per serving (20). The rest of the iron in meat and all iron in plants is non-heme iron (3). Non-heme iron requires being released from food components by hydrochloric acid and the digestive enzyme pepsin in the stomach (3). Non-heme iron needs to be shuttled from the digestive tract into the bloodstream by a protein called transferrin.
Summary: Phytates, polyphenols, and calcium supplements all inhibit non-heme iron absorption. Vitamin C and lysine increase absorption.
Phytates, found in legumes and grains, can inhibit the absorption of plant iron. Calcium supplements can also inhibit iron absorption if taken with meals.
Polyphenols, which include tannic acid, can inhibit iron absorption, and are found in coffee, cocoa, and black, green and many herbal teas. You should avoid these foods at meals if you are trying to increase iron absorption (14). One study showed that, over four weeks, green and black tea lowered iron levels primarily in people with serum ferritin levels less than 20 µg/l (15).
Vitamin C is a strong enhancer of plant iron and can overcome the inhibitors in plant foods. One study found that various doses of phytate reduced iron absorption by 10 to 50%. But adding 50 mg of vitamin C counteracted the phytate, and adding 150 mg of vitamin C increased iron absorption to almost 30%. Similarly, in the presence of a large dose of tannic acid, 100 mg of vitamin C increased iron absorption from 2 to 8% (13).
It is important to note, when assessing studies on iron absorption, that a person's serum ferritin level is the main determinant of non-heme iron absorption. Serum ferritin is inversely proportional to absorption (47).
|Table 2. Vitamin C in Foods|
|broccoli||chopped, cooked||1/2 cup||50|
|strawberries||whole berries||1 cup||85|
|yellow peppers||chopped||1/4 cup||70|
|red peppers||chopped||1/4 cup||50|
|Vitamin C is also found in other green leafy vegetables (kale, collards, Swiss chard, Brussels sprouts), green bell peppers, and cauliflower.|
In another study, vegetarian children with anemia and low vitamin C intakes in India were given 100 mg of vitamin C at both lunch and dinner for 60 days. They saw a drastic improvement in their anemia, with most making a full recovery (2).
Researchers used 500 mg of vitamin C twice daily after meals to increase hemoglobin and serum ferritin in Indian vegetarians. They concluded that vitamin C was more effective at increasing iron status than iron supplements (12).
Cooking foods in cast iron pans can increase iron consumption. A 2007 study in Brazil showed that cooking tomato sauce in an iron skillet increases the amount of iron in the sauce and also increased iron status among teen-aged and young adult lacto-ovo vegetarians (9). The authors considered it important for the food cooked to be both acidic and water-based, such as tomato sauce.
The amino acid, L-lysine, plays a part in the absorption of iron and zinc. Among plant foods, L-lysine is found in high amounts mainly in legumes (peanuts, beans, lentils, peas) and quinoa, and a vegan who doesn't eat many legumes could find themselves falling short on lysine. In some women, iron supplementation does not lead to an increase in iron stores. In one study of such women, adding the amino acid L-lysine (1.5 - 2 g/day for 6 months) to iron supplementation did increase iron stores (46).
Summary – Vegetarian men, boys, and postmenopausal women have shown little problem with iron deficiency. Just as in the greater population, it is not unusual for premenopausal vegetarian women and teenage girls to have iron deficiency and sometimes even anemia. There is anecdotal evidence that some premenopausal women who become vegetarian develop iron deficiency and such women should make sure they are paying attention to the advice above about increasing iron absorption.
The American Dietetic Association's Position Paper on Vegetarian Diets says, "Incidence of iron deficiency anemia among vegetarians is similar to that of nonvegetarians. Although vegetarian adults have lower iron stores than nonvegetarians, their serum ferritin levels are usually within the normal range (8)." This statement is based on cross-sectional studies. The iron status of vegetarians or vegans on self-selected diets has not been followed through time.
Cross-sectional studies show that average iron intakes of male vegetarians (including lacto-ovo vegetarians and vegans) range from 14-18 mg/day from food and 23 mg/day including supplements (21, 22). Male vegetarians' average serum ferritin levels range from 30-75 µg/l (22, 23, 24). In the one study that explicitly stated it, no vegan men were iron deficient or anemic (22).
Cross-sectional studies show that female vegetarians' average iron intakes range from 12-15 mg/day from food (21, 25, 26, 27), although one study measured it at 20 mg/day from food (11) and another at 26 mg/day from food and 42 mg/day from food and supplements (22).
Female vegetarians' average serum ferritin levels range from 11-35 µg/l (11, 23, 24, 25, 27). White vegetarian women have high rates of iron deficiency ranging from about 25-50%, although omnivores' rates of deficiency ranged from 20-60% in those same studies (11, 22, 25, 26), possibly suggesting that women with iron deficiency issues are more likely to take part in studies on iron deficiency. Vegan women over 50 had a deficiency rate of only 12% (11). In three studies measuring hemoglobin, two had no female vegetarians with anemia (26, 27), while another had 2 out of 15 (22), and one had 3 out of 75 (11).
One study included Indian, female, lacto-ovo vegetarians living in Britain. Fifteen out of the 19 were iron deficient and two had anemia (26).
There has been one prospective study on iron status using a vegetarian diet (28) in which men aged 59 to 78 were placed on either a lacto-ovo vegetarian or omnivorous diet for 12 weeks during which they also participated in resistance training. After 12 weeks, serum ferritin in the vegetarian group went from 95 to 72 µg/l, while the omnivores' ferritin levels stayed the same. Other iron parameters stayed about the same, with no change in vegetarians' hemoglobin.
A 2013 study from Poland measured iron intakes and iron status of vegetarian children (17). The study compared 22 vegetarian children (5 ate fish, none were vegan) to 18 omnivores, aged 2 to 18 years old. Of the vegetarian girls of menstruating age, 2 of the 5 had iron deficiency anemia, whereas none of the 4 omnivore menstruating girls had iron deficiency anemia. The researchers noted that their anemia was not due to menstrual period disorders, and that they had been trying to lose weight for "quite a long time." Of the vegetarians, eight (36%) had iron deficiency compared to only two (11%) of the omnivores. You can read more on this study in Iron Deficiency in Polish Vegetarian Children.
I have met many ex-vegetarian women (and a few men) who claimed to become anemic after becoming vegetarian. In most cases, they did not have a doctor diagnose them but assumed they were anemic because they were tired. I do know of one woman who became iron deficient, verified by laboratory testing, after becoming vegetarian. She cured her deficiency by adding more legumes and vitamin C to her diet.
The Food and Nutrition Board (FNB) of the Institute of Medicine sets the RDA for nutrients. The FNB suggests that iron in vegetarian diets is absorbed at a rate of 10% compared to 18% from omnivorous diets and that iron absorption could be as low as 5% for vegans. The FNB does not explicitly give a separate RDA, but says that the "requirement for iron is 1.8 times higher for vegetarians (5)."
|Table 3. Dietary Reference Intake (DRI) for Iron|
|Age (years)||DRI (mg)||Upper limita (mg)|
|0 - 6 mos||.27||40|
|7 - 12 mos||11||40|
|1 - 3||7||40|
|4 - 8||10||40|
|9 - 13||8||40|
|aThe Upper Limit for iron intake is set to prevent gastrointestinal distress rather than to prevent any possible chronic diseases from iron overload.1|
Those who engage in regular, intense exercise may need an additional 30%.5
Iron amounts listed on a nutrition label are based on 18 mg/day. For example, 25% of the Daily Value = .25 x 18 mg = 4.5 mg.
The FNB bases their recommendations on two clinical trials:
Hunt and Roughead (44) performed a crossover study in which participants spent 8 weeks on a typical lacto-ovo vegetarian diet and 8 weeks on an omnivorous diet. Iron absorption on the lacto-ovo vegetarian diet was 1.1% compared to 3.8% on the omnivorous diet.
Cook et al. (45) divided people without anemia into 3 groups: those eating a normal diet, those eating a diet with iron absorption enhancers, and those eating a diet with inhibitors. Over the course of two weeks, non-heme iron from meals was absorbed at the respective rates of 7.2%, 13.5, and 2.5% whereas the absorption rates for the entire two weeks was 7.4%, 8.0%, and 3.4%. The authors said that although iron absorption from meals can vary up to 20-fold within the same meal, depending on enhancers and inhibitors, large population surveys have not demonstrated a clear relationship between iron status and daily consumption of such factors.
These trials do not account for the body adapting its absorption levels over longer periods of time (such as a year or more) or using iron absorption enhancers, especially vitamin C, in vegetarian diets.
Until prospective studies investigate iron levels in free-living vegetarians over a period of years, it is impossible to know exactly what a vegetarian diet might do to iron levels or what increased rates of iron deficiency or anemia a vegetarian diet might cause, if any.
Iron deficiency, with or without anemia, can impair muscle function and limit work capacity. Performance has been shown to improve with iron supplementation in athletes who are iron-deficient but not anemic (42, 43).
The average requirement for iron may be 30% to 70% higher for those who engage in regular, intense endurance exercise, especially running. This is due to increased gastrointestinal blood loss after, and red blood cell destruction during, running (1). This does not necessarily mean that the RDA for runners should be 30 to 70% higher as the RDA provides a buffer above the average iron requirement.
According to the American College of Sports Medicine, "Athletes who are vegetarian or regular blood donors should aim for an iron intake greater than their respective RDA," and "Athletes, especially women, long-distance runners, adolescents, and vegetarians should be screened periodically to assess and monitor iron status." They add that this is especially true during adolescence and pregnancy (42).
Any disease or medication that causes bleeding, including internal, or any disease of the digestive tract, could potentially aggravate or cause iron deficiency. If you have a case of stubborn iron deficiency, have your doctor consider such conditions and medications before assuming it is simply due to a low iron intake or poor absorption of plant iron.
Here are a couple conditions worth mentioning:
Celiac disease is the cause of some cases of unexplained iron deficiency anemia (10). Celiac disease is a condition in which gluten (from wheat, barley, and rye) causes an autoimmune reaction against the intestinal cells. Often, someone has severe diarrhea, vomiting, and other problems, but other times celiac disease goes undetected. It occurs in about 1 in 133 people in the USA (10).
Proton Pump Inhibitors
Proton Pump Inhibitors (PPIs) are widely prescribed to treat gastrointestinal diseases. Research has shown that they can cause iron deficiency anemia (16). If you take PPIs and find that you have anemia, talk to your doctor about a possible connection.
Table 3 lists the tolerable upper intake level (UL) for iron. It is 45 mg for adults.
There is a concern that high iron intakes could contribute to chronic disease. As of 2001, the last time iron recommendations were updated by the Food and Nutrition Board (FNB), there was not enough information to base a UL on these concerns and the they used gastrointestinal distress for determining the UL.
The UL may not protect people with hereditary hemochromatosis, chronic alcoholism, alcoholic cirrhosis, or inborn errors of iron metabolism. The UL is not intended for people being treated for iron deficiency under close medical supervision.
Iron deficiency anemia is normally treated with 100 to 200 mg/day for 4 to 6 months. These large amounts can cause nausea, diarrhea, or constipation, and should only be taken under a doctor's care. Taking supplements with food can often alleviate such problems.
Iron overdose is a common cause of poisoning death in children. Symptoms of acute toxicity occur at iron intakes of 20-60 mg/kg of body weight, and death occurs at approximately 200-250 mg/kg. Iron overdose is an emergency situation because the severity of the toxicity is related to the amount of iron absorbed which will increase over time. Symptoms can subside but then return 12 to 48 hours after ingestion.
Hemochromatosis is a genetic disease that results in people absorbing too much iron, resulting in very high serum ferritin levels. People with hemochromatosis can suffer from heart failure, liver disease and other problems. This has led to a concern that high iron stores in people without hemochromatosis might also cause long-term damage, and some early research found that high serum ferritin levels were associated with heart disease. Because vegetarians usually have lower serum ferritin levels, it was suggested that vegetarians might have a lower risk for chronic disease based on these lower levels.
Since then, there has been a lot of research on the association between iron intakes and stored iron levels with chronic disease (in the absence of hemochromatosis). A quick summary is:
- High iron stores and higher intakes of heme iron are associated with a higher risk of type 2 diabetes.
- Heme iron intake is associated with colon cancer, while non-heme iron is not.
- Iron supplementation of < 20 mg/day is not associated with colon cancer (studied in women).
- High iron serum ferritin levels are not associated with cardiovascular disease or increased mortality.
- Transferrin saturation has been found to be associated with mortality in various ways and at different levels, though it is not clear why or what can be done about it.
More details are below for those who are interested in how these conclusions were drawn.
Less than 1% of people of northern European descent are homozygous for the hemochromatosis gene and have abnormally high iron absorption that can start to cause problems, especially for men, around age 40 to 60. If untreated, hemochromatosis can result in liver cirrhosis, liver cancer, heart failure, and other problems. Symptoms include joint pain, fatigue, abdominal pain, and impotence. People should talk to their doctors about their risk factors.
Hemochromatosis can lead to serum ferritin levels of > 300 ng/ml in men, 200-300 ng/ml in postmenopausal women, and > 200 ng/ml in premenopausal women (52).
Type 2 Diabetes
There is evidence that the beta cells of the pancreas, which produce insulin, are particularly susceptible to oxidation from iron due to their weak antioxidant defense mechanisms. A 2012 meta-analysis of prospective studies found that higher iron stores (6 studies) and higher intakes of heme iron (5 studies) at baseline were strongly associated with a higher risk of type 2 diabetes (49). Higher intakes of non-heme iron were not associated.
A cross-sectional study from the USA found lower ferritin levels in lacto-ovo vegetarians (35 µg/l) than meat-eaters (72 µg/l). The vegetarians also had higher insulin sensitivity. Upon giving phlebotomies to 6 male meat-eaters to reduce their ferritin levels, their insulin sensitivity increased. The authors suggested that the lower ferritin levels could be a reason why vegetarians had greater insulin sensitivity (48).
It is possible that the lower risk of type 2 diabetes in vegetarians (see Type 2 Diabetes and the Vegan Diet), which has been shown to be independent of body mass index, could be partially explained by their lower iron stores.
Many studies have looked for an association between iron stores, iron intakes, and colon cancer, and the results have been mixed (31, 32, 33, 34). One study found that high iron intake was associated with colon cancer only when combined with a high fat diet (32).
The Nurses Health Study and Health Professionals Follow-up Study found no relation between iron intake or iron supplements and risk of colorectal cancer. The highest quintiles of iron intake for men and women were, respectively, > 24.6 and > 22.7 mg/day with median supplemental iron intake at 10 and 15 mg/day (35).
The Iowa Women’s Health Study analyzed iron supplement intake levels, fermentable substrates (fiber plus resistant starch), and colon cancer. They hypothesized that the acidic environment created by fermentable substrates in the colon could interact with the iron supplements, possibly increasing colon cancer. Women taking ≥ 50 mg/day had a significantly increased risk of distal (but not proximal) colon cancer if they also were above the median for fermentable substrates (26 g/day). Supplements of 1–19 mg/day did not appear to increase risk (36).
As distinct from total iron or non-heme iron intakes, heme iron intake has been consistently associated with a greater risk of colon cancer. A 2011 meta-analysis of 5 cohort studies found a significant and consistent but modest increase in the risk of colon cancer associated with high heme iron intake, with a risk of 1.18 (1.06– 1.32) for subjects in the highest category of heme iron compared with the lowest (50). The researchers did not believe they could separate this finding from red meat intake, but believed there are plausible mechanisms to suggest it is the heme iron in red meat causing colon cancer.
A meta-analysis of observational studies (29) found that serum ferritin levels of 200 µg/l were not associated with coronary heart disease compared to levels below 200 µg/l. A more recent systematic review found the association between iron stores and cardiovascular disease to be mixed, with the majority of studies showing no association (4).
Damage might not be caused by high storage levels of iron, but rather through repeated bouts of toxic exposure that would not necessarily be evident in serum ferritin levels (30).
The question is not settled, and it may be that levels somewhat higher than 200 µg/l are more indicative of damage. It could also be that tissues are protected against oxidative damage by iron when the iron is bound to storage and transfer proteins.
Mortality and Iron Intake
A 12-year prospective study from the United States National Health and Nutrition Examination Study (NHANES) II examined the relationship between iron intake, transferrin saturation, and mortality among people aged 30 to 70 years at baseline. High iron intake led to an increased mortality risk when, and only when, it was combined with elevated transferrin saturation. High vs. low iron intake was ≤ 18 vs. > 18 mg/day (40).
Mortality and Iron Stores
A prospective report from NHANES II (33) found no relationship for mortality among white men, white women, or black men when comparing serum ferritin levels of 100-200 µg/l or > 200 µg/l with 50-100 µg/l.
In a large sample of US adults without hemochromatosis and who were not taking iron supplements, from NHANES III, serum ferritin and transferrin saturation were not associated with mortality (37). However, another analysis from NHANES III limited to adults 50 years and older found that higher transferrin saturation was associated with lower all-cause and cardiovascular mortality in both men and postmenopausal women. Men also showed the inverse association between transferrin saturation and cancer mortality (51). High transferrin saturation was > 30-35% compared to < 15-18%. No association with mortality for elevated serum ferritin was found in NHANES III (37, 51).
Using data from NHANES I, Mainous et al. (38) found transferrin saturation above 55% to be associated with a 60% greater risk of mortality.
Using data from NHANES II, Wells et al. (39) found neither elevated LDL or elevated transferrin saturation (> 55%) to be independently associated with mortality, but when combined were strongly associated with mortality.
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