Iron

by Jack Norris, RD

Contents

• Summary
• Dietary Reference Intakes for Iron
• Iron Content of Plant Foods
• Daily Value
• Vitamin C Content of Plant Foods
• Functions of Iron
• Iron Deficiency
• Athletes
• Screening for Iron Status
• Treatment for Iron Deficiency
• Heme vs. Non-Heme Iron Absorption
• Iron Status of Vegetarians
• Iron RDA and Vegetarians
• Iron and Chronic Disease
• Hemochromatosis
• Bibliography

Summary

Iron is plentiful in many plant foods, but it’s less absorbable than iron in meat. Vegetarian athletes and people who are pregnant, breastfeeding, or menstruating should pay attention to their iron needs. Plant iron absorption is significantly increased by:

• Adding vitamin C at meals (see chart of foods below)
• Avoiding tea and possibly coffee, red wine, and cocoa within an hour of meals
• Avoiding calcium supplements at meals

If someone suspects they have iron deficiency, they should be screened by a physician. A wide range of serious diseases can cause iron deficiency, via internal bleeding, and a physician should determine if additional testing is warranted (Pasricha). It’s important to correct iron deficiency in order to prevent increased manganese absorption (more info: Manganese).

People with milder forms of iron deficiency should add more high-iron foods and follow the above recommendations for increasing plant iron absorption. Serious cases of iron deficiency are treated with high-dose oral or parenteral iron which is normally effective (Pasricha) but should be administered under the supervision of a physician.

Physicians sometimes recommend eating meat to vegetarian clients who have iron deficiency. Meat-eaters with anemia are normally treated with iron therapy rather than being told to eat more meat; similarly, vegetarians can be treated with supplemental iron and vitamin C.

On average, vegetarians have lower iron stores than meat-eaters, but this might be largely a function of reduced inflammation. The iron from meat has been associated with an increased risk of mortality, type 2 diabetes, cardiovascular disease, and colon cancer.

Dietary Reference Intakes for Iron

The table below lists the Dietary Reference Intakes for iron.

Dietary Reference Intakes (DRI) for Iron
Age	US DRI (mg)	Upper Limit (mg)
0–6 mos	0.27	40
7–12 mos	11	40
1–3	7	40
4–8	10	40
9–13	8	40
Male 14–18	11	45
Female 14–18	15	45
Male 19+	8	45
Female 19–50	18	45
Female > 50	8	45
Breastfeeding ≤ 18	10	45
Breastfeeding > 18	9	45
Pregnancy	27	45

Iron Content of Plant Foods

The table below shows a range of plant foods including those richest in iron.

Iron Content of Plant Foods
Food	Preparation	Serving	mg
Grape Nuts cereal		1/2 C	16
Total cereal—whole grain		1/2 C	8.0
Molasses		2 T	3.8
Lentils	boiled	1/2 C	3.3
Spinach	chopped, boiled	1/2 C	3.2
Kidney beans	boiled	1/2 C	2.6
Garbanzo beans	boiled	1/2 C	2.4
Swiss chard	chopped, boiled	1/2 C	2.0
Edamame		1/2 C	1.8
Pinto beans	boiled	1/2 C	1.8
Tempeh	cooked	3 oz	1.5
Dried figs	dried, raw	1/2 C	1.5
Soymilk		1 C	1.0 – 1.5
Rice—white, enriched	cooked	1/2 C	1.4
Raisins		1/2 C	1.4
Almonds	roasted	1/4 C	1.3
Peas – green	boiled	1/2 C	1.2
Pistachios	dry roasted	1/4 C	1.2
Sunflower seeds	dry roasted	1/4 C	1.2
Collard greens	chopped, boiled	1/2 C	1.1
Oatmeal	cooked	1/2 C	1.0
Walnuts	chopped	1/4 C	0.9
Tomatoes	cooked	1/2 C	0.8
Tofu		3 oz	0.8
Sweet potato	baked, w/skin	1/2 C	0.7
Rice—white, unenriched	cooked	1/2 C	0.7
Hummus		2 T	0.7
Bread—whole wheat		1 slice	0.7
Peanut butter		2 T	0.6
Kale	chopped, boiled	1/2 C	0.6
Broccoli	chopped, boiled	1/2 C	0.5
Rice—brown	cooked	1/2 C	0.5
Taken from the USDA National Nutrient Database or food labels.

Daily Value

In the United States, iron amounts listed on a nutrition label are a percentage of the Daily Value for iron which is 18 mg/day. For example, 25% of the Daily Value = .25 x 18 mg = 4.5 mg.

Vitamin C Content of Plant Foods

Eating foods high in vitamin C with meals increases the absorption of plant iron. The table below shows the common plant foods that are high in vitamin C.

Vitamin C in Foods
Food	Preparation	Serving	mg
Orange juice		cup	80
Grapefruit juice		cup	80
Oranges		small	50
Broccoli	chopped, cooked	1/2 cup	50
Strawberries	whole berries	1 cup	85
Grapefruit		1/2 fruit	40-50
Yellow peppers	chopped	1/4 cup	70
Red peppers	chopped	1/4 cup	50
Also found in other green leafy vegetables (kale, collards, Swiss chard, Brussels sprouts), green bell peppers, and cauliflower.

Functions of Iron

Iron’s primary function in the body is binding oxygen. Over half of the body’s iron is part of the hemoglobin in red blood cells which transports oxygen throughout the body (Dev, 2017). Iron is also a component of myoglobin that utilizes oxygen in the heart and skeletal muscles. Iron is part of the enzyme, NADH dehydrogenase, which is part of the electron transport chain that produces ATP for energy. Iron is involved in DNA synthesis. Iron’s pro-oxidation properties are used by the immune system to destroy bacteria.

Iron Deficiency

Iron is stored on the ferritin protein and transported on the transferrin protein; both are used in the diagnosis of iron deficiency.

Pasricha et al. (2021) divide the signs and symptoms of iron deficiency into the three progressive stages of low iron stores, iron deficiency, and anemia:

• Low iron stores are marked by serum ferritin of 15–30 μg/l with a transferrin saturation of >20%. This stage of deficiency is either asymptomatic or mildly symptomatic with physical fatigue or reduced cognitive function.
• Iron deficiency is marked by a transferrin saturation of < 20% and low reticulocyte hemoglobin. It can be further divided into two types:
• Absolute iron deficiency is marked by serum ferritin of < 15-30 µg/l; it can be asymptomatic or involve fatigue, poor concentration, dizziness, tinnitus, headache, pica, or restless leg syndrome.
• Functional iron deficiency is marked by normal or increased serum ferritin (30–100 µg/l) with systemic inflammation and some of the symptoms of iron deficiency.
• Iron deficiency anemia is marked by a reduced mean cell volume (MCV) and hemoglobin concentration. The World Health Organization defines anemia as a hemoglobin level of < 120 g/l for non-pregnant females ≥15 years old, < 110 g/l in pregnancy, and < 130 g/l for males ≥15 years old.

Other consequences of iron deficiency include koilonychia (spoon-shaped nails where the outer edges are raised) , soft nails, glossitis, cheilitis (dermatitis at the corner of the mouth), mood changes, muscle weakness, and impaired immunity (EFSA), and hair loss.

The European Food Safety Authority provides a more comprehensive set of markers for iron deficiency for a variety of age groups (EFSA, Appendix B). There are many types of anemia, including B12-deficient anemia, which should be considered when determining the cause of anemia-like symptoms.

Athletes

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 (American Dietetic Association, 2009; Lukaski, 2004).

The average requirement for iron may be 30–70% higher for those who engage in regular, intense endurance exercise, especially running. This can be due to periods of rapid growth, training at high altitudes, menstrual blood loss, foot-strike hemolysis, intravascular hemolysis, injury, and increased losses in sweat, urine, and feces. The American College of Sports Medicine recommends that vegetarian athletes be screened regularly and aim for iron intakes above the RDA (Thomas, 2016).

Screening for Iron Status

Iron status is easily assessed through blood tests ordered by a physician. Due to concerns with both iron deficiency from menstruation and hemochromatosis (especially among men), it’s prudent to speak to a physician about having iron levels tested at least once during early adulthood. Iron deficiency can be a sign of internal bleeding.

Treatment for Iron Deficiency

Vegans with milder forms of iron deficiency should add high-iron foods and vitamin C to meals while avoiding tea, and possibly coffee, red wine, and cocoa within an hour of meals. More serious cases of iron deficiency are treated with high-dose oral or parenteral iron which is normally effective (Pasricha) but should be administered only under the supervision of a physician.

Iron supplementation can reduce fatigue in premenopausal women (Vaucher, 2012; Verdon, 2003) and improve cognition in adolescent girls with mild iron deficiency.

The tolerable upper limit (UL) for iron is 40 mg for ages ≤ 13 and 45 mg for ages >13. The UL is based on gastrointestinal distress (Institute of Medicine, 2001). The UL is not intended for medically supervised iron therapy. Longterm iron supplementation of < 20 mg/day was not associated with an increased risk of colon cancer among women (Lee, 2004).

Heme vs. Non-Heme Iron Absorption

Iron in foods is categorized as heme and non-heme.

Heme iron comes attached to the heme molecule. Meat is the only natural source of heme iron. The average heme iron content of various cooked meats is 65% for beef, 39% for pork, and 26% for chicken and fish (Balder, 2006). Heme iron is absorbed at a rate of about 25% (EFSA, 2015). The plant-based Impossible Burger is the only plant food that contains heme iron; the heme is produced by a genetically modified yeast (Impossible Foods).

Non-heme iron absorption ranges from approximately 0.7–23% (Collings, 2013). Single-meal laboratory experiments show that absorption of non-heme iron can be reduced by phytate (in legumes and whole grains), polyphenols (in many herbal teas, black tea, green tea, cocoa, and red wine), and calcium (Hurrell, 1999). Avoiding herbal tea within an hour of a meal can ameliorate the impact (Ahmad, 2017).

Siegenberg, et al. (1991) found: various doses of phytate can reduce iron absorption by 10–50%; 50 mg of vitamin C can counteract phytate; 150 mg of vitamin C can increase iron absorption to almost 30%; in the presence of a large dose of tannic acid, 100 mg of vitamin C can increase iron absorption from 2% to 8%.

A high percentage of Indian children were cured of anemia using ​100 mg of vitamin C at two daily meals for 60 days (Seshadri, 1985). Researchers used 500 mg of vitamin C twice daily after meals to increase hemoglobin and serum ferritin in Indian vegetarians; vitamin C was more effective than iron supplements (Sharma, 1995).

The body can adapt to solely relying on non-heme iron. Serum ferritin is the main determinant of non-heme iron absorption: as ferritin decreases, non-heme iron absorption increases (Collings, 2013). When iron absorption decreases, ferritin excretion also decreases (Hunt, 1999).

Cooking tomato sauce in an iron skillet increased the amount of iron in the sauce and improved iron status among Brazilian teenagers and young adult lacto-ovo-vegetarians (Quintaes, 2007). The researchers considered both the acidic and water-based qualities of the tomato sauce to be important.

Beta carotene reduces the inhibitory effects of polyphenols and phytates in iron-rich grains (Garcı́a-Casal, 1998; García-Casal, 2000; Layrisse, 2000). See our chart of food high in beta carotene.

An in vitro study found that garlic and onions might increase iron absorption from grains, but this hasn’t been tested in humans (Gautam, 2010).

Iron Status of Vegetarians

Studies show that among vegetarians (including vegans), most men surpass the iron RDA of 8 mg from food while many women aged 19-50 don’t fully meet the RDA of 18 mg from food alone (Mangels, 2023).

Depending on the cutoff used, vegetarian adults and children can have a higher prevalence of iron deficiency (Haider, 2018; Gorczyca, 2013). Body mass, inflammation, and insulin resistance increase serum ferritin; adjusting for these factors ameliorated the differences between the prevalence of iron deficiency in meat-eating and vegetarian men and non-menstruating women, but not menstruating women (Slywitch, 2021).

It’s not unusual for premenopausal vegetarian women and teenage girls to have iron deficiency and sometimes anemia; it’s important that they eat iron-rich foods with a source of vitamin C at meals.

Iron RDA and Vegetarians

The Institute of Medicine (2001, p. 351) says the “requirement for iron is 1.8 times higher for vegetarians.” The Institute of Medicine bases this view on the following two clinical trials.

Hunt and Roughead (1999) 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. (1991) conducted a clinical trial to study iron absorption among people eating different types of meals: typical, with iron absorption enhancers, and with iron absorption inhibitors. Non-heme iron from meals was absorbed at the rates of 7.2%, 13.5%, and 2.5%, respectively. Over the course of two weeks, average absorption rates were 7.4%, 8.0%, and 3.4%, respectively. The authors say that although iron absorption from meals can vary up to 20-fold, depending on enhancers and inhibitors, population surveys haven’t demonstrated a clear relationship between their intake and iron status.

Because these trials don’t account for the body adapting its iron absorption in response to body stores, they can’t be used to determine needs for vegetarians. Longterm prospective studies on vegetarians are needed to accurately determine their iron needs but there have been none to date.

Iron and Chronic Disease

Iron is a pro-oxidant molecule. In high amounts, it could be a contributor to chronic disease. Because vegetarians have lower iron absorption and stores, their diets could be protective with respect to iron.

The association between serum ferritin levels and chronic disease is confounded by possible reverse causation due to inflammation increasing serum ferritin levels. Studying heme iron intake can be confounded by heme iron being a marker for diets higher in meat and lower in plant foods.

Regardless of the actual cause, research has associated high heme iron intakes or elevated serum ferritin with a higher risk for mortality, type 2 diabetes, cardiovascular mortality, and colon cancer. The research is detailed below.

Mortality and iron intake: A 9-year prospective analysis of the United States National Health and Nutrition Examination Study (NHANES) data found significant trends between higher heme iron intake and an increased risk of all-cause and cardiovascular mortality (Wang, 2021). A 12-year prospective analysis of NHANES II data for people age 30–70 found that iron intake >18 mg/day was associated with increased mortality, but only when combined with elevated transferrin saturation (Mainous, 2004).

Mortality and iron stores: A 12–16 year prospective analysis of NHANES II data 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 (Sempos, 2000). A 12–18 year prospective analysis of NHANES III data found no relationship between serum ferritin and mortality (Kim, 2012; Menke, 2011).

Type 2 diabetes and iron intake: A meta-analysis of prospective studies found that higher intakes of heme iron (5 studies) and iron stores (6 studies) were strongly associated with a higher risk of type 2 diabetes (Bao, 2012). There was no association for higher intakes of non-heme iron.

Type 2 diabetes and iron stores: There’s 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. Six male meat-eaters were given phlebotomies to reduce their serum ferritin levels and their insulin sensitivity increased (Hua, 2001).

Cardiovascular disease and iron intake: A meta-analysis of prospective studies found an increased risk of cardiovascular mortality when comparing highest versus lowest heme iron intake (RR 1.19, 95% CI 1.01–1.39) and for each 1 mg increase in heme iron intake (RR 1.25, 95% CI 1.17–1.33). There were no significant findings for non-heme or total iron intake. Some of the individual studies adjusted for dietary factors known to contribute to cardiovascular disease (Han, 2020).

Colon cancer and iron intake: A meta-analysis of 5 cohort studies found a significant and consistent, but modest, increase in the risk of colon cancer associated with heme iron intake (highest vs. lowest category: RR 1.18, 95% CI 1.06-1.32; Bastide, 2011).

Hemochromatosis

Hemochromatosis is a genetic disorder in which someone absorbs abnormally large amounts of iron, resulting in serum ferritin levels of >300 ng/ml in men, 200-300 ng/ml in postmenopausal women, and >200 ng/ml in premenopausal women (CDC, 2013). Less than 1% of people are homozygous for the hemochromatosis gene. Symptoms, especially for men, normally start to present around age 40–60 and include joint pain, fatigue, abdominal pain, and impotence. If untreated, hemochromatosis can result in liver cirrhosis, liver cancer, heart failure, and other problems.

Last updated November 2021

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