Iron is one of the most abundant minerals on the earth and is found in all living organisms. The mineral has been used in manufacturing as far back as 3500BC. It is possible that the name iron comes from the words meaning “holy metal” because iron was used to make swords during the Crusades.
In biology, iron has numerous functions. In plants, it plays an integral role in chlorophyll development, energy transfer, plant respiration, plant metabolism, and nitrogen fixation. In animals, iron is part of enzymes which aid in digestion and metabolism. However, iron’s most notable and important role in animals is delivering oxygen to cells.
Iron is a mineral that our bodies need for many functions. The body needs iron to make the proteins hemoglobin and myoglobin. Hemoglobin is found in red blood cells and myoglobin is found in muscles. They help carry and store oxygen in the body. Iron is also part of many other proteins and enzymes in the body.
Table of Contents
- 1 Iron and Red Blood Cells
- 2 Iron and Myoglobin
- 3 Types of Dietary Iron
- 4 Iron Absorption in the Body
- 5 Foods which Impact Iron Absorption:
- 6 Recommended Dietary Intake of Iron
- 7 Symptoms of Iron Deficiency
- 8 Who is At Risk of Iron Deficiency?
- 9 How to Take Iron Supplements
- 10 Side Effects of Iron Supplements
- 11 Iron Overdose
- 12 Best Vegetarian Sources of Iron
- 13 Why Spinach isn’t the Great Source of Iron You Thought It Was
- 14 Does this mean that spinach is a bad source of iron?
- 15 How to Meet Iron RDA on a Vegetarian Diet
Iron and Red Blood Cells
Red blood cells are the primary cells in the human body, making up about 25% of our cells. The main function of RBCs is to deliver oxygen from the lungs to the rest of the body. Hemoglobin is a protein in RBCs which holds the oxygen for transport. One-third of red blood cells is made up of hemoglobin.
Nearly all of the oxygen transported by blood is held in hemoglobin proteins. The rest is transported in plasma, which is very ineffective in delivering oxygen to cells compared to hemoglobin.
Every hemoglobin protein consists of 4 atoms of iron. Each iron atom is capable of carrying 1 oxygen molecule. It is this iron in hemoglobin which gives blood its red color.
Iron and Myoglobin
The iron in red blood cells carries oxygen to the muscles. Once the oxygen arrives, the oxygen molecules bind to another protein called myoglobin. Like hemoglobin, myoglobin also helps transport oxygen. However, it only transports oxygen to muscles and is not found in the bloodstream. Because oxygen is integral for muscle movement, iron is needed to support all muscle function.
The iron in myoglobin is what gives raw meat its red color. When meat is cooked, the iron becomes oxidized and gets a darker color. The iron in myoglobin is also what makes meats a good source of iron as opposed to plants which do not have myoglobin.
Types of Dietary Iron
There are two types of iron: heme iron and non-heme iron. Heme iron comes from hemoglobin in red blood cells and thus is not vegetarian. Non-heme iron comes from non-animal sources.
Nutritionally, heme iron is far superior to non-heme iron because it is much more readily absorbed by the body. This is the reason that vegetarians, particularly vegans, are at risk of iron deficiency.
Iron Absorption in the Body
Animal-sourced heme iron is much more absorbable than vegetarian-sourced non-heme iron. Studies put the absorption of heme iron at about 15-35% compared to 2-20% with non-heme iron. Because vegetarian sources of iron are so poorly absorbed, vegetarians may not be able to get their daily intake of iron even when meeting the listed recommendations. According to the Institute of Medicine, vegetarians may need to consume nearly double the amount of dietary iron as meat-eaters because of how poorly non-heme iron is absorbed.
Foods which Impact Iron Absorption:
- Vitamin C: Vitamin C has shown to significantly improve iron absorption. Vegetarians should include vitamin C when eating iron-rich foods to maximize absorption.
- Calcium: Calcium has shown to block absorption of iron.
- Tannins and Polyphenols: These substances have shown to block absorption of iron. They are particularly abundant in tea and wine.
- Phytic Acid: Phytic acid, or phytates, are antioxidants found in certain foods. There are studies which show that they block absorption of iron. Legumes, grains, nuts and rice are particularly high in phytic acid.
- “Junk Foods”: Foods high in calories but low in nutritional value can reduce iron absorption. Soda is an example of one of these iron-blocking junk foods.
Recommended Dietary Intake of Iron
The daily recommended intake of iron and the tolerable upper intake level for iron are as follows:
|Age||RDA (mg/day)||Upper Limit (mg/day)|
|Males 14-18 years||11||45|
|Males 19+ years||8||45|
|Females 14-18 years||15||45|
|Females 19-50 years||18||45|
It is important to note that these recommendations are based on the assumption t that ¾ of the iron comes from animal sources. Since vegetarian sources of iron are much less absorbable, the RDAs for vegetarians and vegans may be nearly twice as much!
The Institute of Medicine recommends adjusting the iron RDA for vegetarians as follows:
Parents raising vegetarian children should consult their doctors about the recommended intake of iron. Blood tests to detect hemoglobin levels or red blood cell count may be given to determine whether the vegetarian children are getting adequate dietary iron.
Symptoms of Iron Deficiency
Iron deficiency is the leading nutritional disorder in the world. According to one report, as much as 80% of the world may have an iron deficiency.
Because iron is critical for oxygen transportation, iron deficiency can result in symptoms such as:
- Hair loss and brittle fingernails
- Irritable mood
- Inflammation of the tongue
Severe or prolonged iron deficiency leads to iron deficient anemia, a condition in which there is inadequate levels of red blood cells and/or hemoglobin in the body. In addition to the above, symptoms of anemia may include:
- Bruising easily
- Restless leg syndrome
- Pale gums
- Lack of appetite
- Mouth lesions
- Unusual heartbeat
- Heavy menstrual periods
If iron deficiency is not treated, it can result in serious problems such as heart problems. In children, iron deficiency can lead to impaired growth and developmental problems. In pregnant women, iron deficiency can cause problems with the child, such as premature birth or low birth weight.
Diagnosing an iron deficiency typically involves a blood test. A hemoglobin test will measure the levels of hemoglobin in the blood whereas a hematocrit test will measure the percent of red blood cells in the blood.
Who is At Risk of Iron Deficiency?
Vegetarians are particularly at risk of iron deficiency because non-heme (i.e. vegan) sources of iron are not as absorbable as heme (animal) sources. Additionally, these groups are also at risk of iron deficiency:
- Pregnant women
- Babies and toddlers
- Teenage girls
- People with gastrointestinal problems or those who take antacids regularly
- People with celiac disease
Vegetarians and vegans who also fall into the at-risk groups listed above should have their iron levels checked to make sure they aren’t deficient.
How to Take Iron Supplements
Iron supplements are often taken by vegetarians to prevent iron deficiency. However, even those who are extremely at risk of an iron deficiency (such as a vegan athlete) should be very cautious about taking iron supplements as too much iron can result in iron toxicity. It is recommended that you first have your iron levels checked to determine your need for vegetarian iron supplements.
Here are some guidelines on how to take vegetarian iron supplements for best results:
- Take iron supplements on an empty stomach as this will increase absorption.
- Take iron supplements with vitamin C to increase absorption.
- Do not take iron supplements with tea, nuts, raw vegetables, or uncooked grains as these decrease absorption.
- For best results, take iron in several small doses throughout the day (as opposed to one large daily dose). This will increase the amount of iron that your body absorbs and decrease the likelihood of side effects.
- Avoid taking iron tablets, particularly time-released tablets, as these are not as effective as liquid versions of vegetarian iron supplements.
Note that vegetarian iron supplements come in two main forms. The first is called ferric iron and it is NOT recommended because it is not as absorbable as the other form, ferrous iron.
Ferrous iron comes in three forms: ferrous sulfate, ferrous fumarate, and ferrous gluconate. Of these, ferrous sulfate is most commonly used in iron supplements. However, it is NOT the best absorbable. Ferrous fumarate is the most absorbable with approximately 33% elemental (absorbable) iron.
The percentages of elemental iron are as follows:
Side Effects of Iron Supplements
Vegetarian iron supplements may cause these common side effects:
- Constipation or diarrhea
- Darkened stools
You can reduce the likelihood of side effects by starting supplementation with lower dosages and then working up to the full, recommended dose. Taking smaller dosages spaced throughout the day can also reduce symptoms. Vegetarians may want to increase the amount of fiber in their diets while taking iron supplements as well to reduce the likelihood of constipation or diarrhea.
Vegetarians should also note that iron supplements can impact absorption of zinc. They may wish to increase consumption of zinc-rich foods while taking iron.
Taking too much iron can result in iron toxicity (also called iron poisoning or iron overload). The excess amounts of iron build up in organs, particularly the liver, and can lead to serious damage or organ failure.
The body naturally protects itself from iron overload by decreasing the amount of iron absorption when it has excess. However, a significant amount of people (about 0.5%) have a genetic condition called hemochromatosis in which the body absorbs higher levels of iron. These people are at risk for iron overload.
Symptoms of iron overload are similar to those of iron deficiency. The symptoms also resemble the common side effects of iron supplements, such as stomachache. People who are taking iron supplements who have severe or long-lasting side effects should contact their doctors as it may be a sign of iron overdose.
Symptoms of severe iron overload include: seizures, low bread pressure, and coma.
Best Vegetarian Sources of Iron
|Rice bran, uncooked, ½ cup||11.0|
|Soybeans, cooked, ½ cup||4.4|
|Pumpkin seeds, 1oz||4.2|
|Cashews, roasted, ½ cup||3.9|
|White beans, cooked, ½ cup||3.9|
|Lentils, cooked, ½ cup||3.3|
|Potato with skin, baked||3.2|
|Spinach, boiled, ½ cup||3.2|
|Wheat bran, uncooked, ½ cup||3.0|
|Bran, ½ cup||3.0|
|Red kidney beans, cooked, ½ cup||2.6|
|Quinoa, cooked, ½ cup||2.4|
|Garbanzo beans, cooked, ½ cup||2.4|
|Hearts of palm, canned, ½ cup||2.3|
|Prune juice, ¾ cup||2.3|
|Lima beans, cooked, ½ cup||2.2|
|Spirulina, 1 tbsp.||2.0|
|Swiss chard, boiled, ½ cup||2.0|
|Tahini, 1 tbsp.||1.3|
|Sun dried tomatoes, 5 pieces||1.0|
|Whole grain bread, 1 slice||0.9|
|Green beans, cooked, ½ cup||0.8|
|Peanut butter, 2 tbsp.||0.6|
|Apricots, dried, 3||0.6|
|Zucchini, cooked, ½ cup||0.3|
Why Spinach isn’t the Great Source of Iron You Thought It Was
The nutrient contents of foods are listed as-is. They don’t take into account the bioavailability of the nutrient – i.e. the amount of the nutrient which our bodies actually absorb and put to use. So, if a food contains 10 grams of a nutrient, it will be listed as such – even if our bodies only use a fraction of this amount. This is the case with spinach and iron.
Like with many other vegetables, spinach contains a chemical called oxalic acid. Oxalic acid has gotten a very bad reputation because it binds with iron (as well as calcium, magnesium, and potassium) in the intestines, thus decreasing the absorption of these nutrients.
Does this mean that spinach is a bad source of iron?
According to Dr. Philip Kern, M.D. from the Department of Endocrinology/Metabolism at UAMS, the oxalic acid in spinach prevents more than 90% of iron from being absorbed.
The RDA for adult males is 8mg/day and 18mg/day for menstruating females. Assuming that only 10% of the iron from spinach is absorbed, adult males would need to eat 12.5 cups of cooked spinach per day to meet RDAs and adult women would need to eat 28 cups per day!
But (I reiterate), nutritional facts aren’t quite so straight-forward as this. There are other factors which can influence iron absorption.
Studies show that vitamin C can increase the absorption of iron, even in the presence of oxalic acid. One study published in the British Journal of Nutrition showed that 1 gram of citric acid was able to increase iron absorption by over 300%. Thus, vegetarians who rely on spinach and other greens as a source of iron are encouraged to consume vitamin C along with the greens (which can easily be achieved by squeezing some lemon juice into your water).
Along with oxalic acid, these substances can also decrease iron absorption:
- Phytic acid (found in a lot of nuts, grains, and legumes)
- Tannins and polyphenols (such as found in tea)
- Junk foods
How to Meet Iron RDA on a Vegetarian Diet
Iron is divided into two types: heme iron which comes from animal sources and non-heme iron which doesn’t come from animal sources. Even in foods which are low in oxalic acid, non-heme iron has a much lower bioavailability than heme iron. Studies show that heme iron has an absorption of about 15-35% compared to just 2-20% with non-heme iron.
To make sure that they are getting enough iron on a plant-based diet, the Institute of Medicine recommends that vegetarians consume double the amount of iron that is recommended for the general public.
Based on this advice, the modified iron RDA for vegetarians and vegans would be as follows:
- Adult men: 16mg/day
Spinach may not be the great source of iron that it is often hyped-up to be. However, it is still a great source of other nutrients such as vitamin A, vitamin E, beta carotene, and antioxidants. Further, young spinach has lower amounts of oxalic acid than mature spinach, so its iron content will be better absorbed.
Other good vegetarian sources of iron include:
- Rice bran, uncooked: ½ cup = 11mg
- Soybeans, cooked: ½ cup = 4.4mg
- Pumpkin seeds: 1oz = 4.2mg
- Cashews, roasted: ½ cup = 3.9mg
- White beans, cooked: ½ cup = 3.9mg
- Lentils, cooked: ½ cup = 3.3mg
- Blackstrap molasses: 2 teaspoons = 2.4mg
*Note that these amounts also don’t reflect the bioavailability of the iron!
Thanks to the numerous vegetarian supplements which are available, there is no need for anyone to fall short of their iron RDA. However, because it is possible to overdose on iron, it is best to consult with a doctor and have a blood test to measure iron levels before taking any iron supplement.
It is always best to get your nutrients from food first. But, if you really think you aren’t getting enough iron, here are some of my favorite vegan iron supplements. I like the Garden of Life iron best because it is raw, but the other two options are also good, especially for people on a low budget. You can also check out our chart of the best vegan vitamins which includes some vegan multivitamins with iron.