Iron supplements constipation
A common side effect
Of anemia prescriptions

Do you have iron deficiency anemia? If yes, be aware of iron supplements constipation.

You are probably about to start iron supplementation, and constipation is one of the most frequently reported side effects.

In the elderly, it may also lead to a condition called fecal impaction, a serious complication of unresolved constipation.

In this article, we will first discuss the role of iron in human physiology. In a second step, we will talk about iron supplements, contrast them with dietary sources, and explain why they cause constipation.

Finally, we will discuss practical steps you can take to minimize the risk of constipation.

The role of iron

Iron is an essential mineral. It is involved multiple physiological roles.

First, you probably heard that it is a building block of hemoglobin and myoglobin. Hemoglobin, a protein contained in our red blood cells, carries oxygen from the lungs to the different parts of our body. Myoglobin has the same oxygen carrier responsibility, but within our muscles. Simply stated, iron allows our body and our cells to breathe.

Second, it plays a key role in our liver detoxification pathways. As you may know, the liver can be compared to a recycling plant. It breaks down and recycles our own waste materials and hormones, and it is very busy processing pollutants and drugs we have ingested. In other words, iron is a fundamental enabler of detoxification.

Third, it is intimately involved in processes at the cellular level. We find it in the synthesis of DNA and the making of our cells, in the generation of energy (ATP) in our mitochondria, as a cofactor for the production of collagen and elastin, and more.

Needless to say, we all need to ensure we have a proper intake of dietary iron for optimal health. We will discuss dietary sources further down. Building up our reserves, and keeping those reserves at a proper level, is foundational to good health. But of course, too much iron via supplementation can cause iron supplements constipation.

For information, a 70-kg male contains about 3.5 g of iron in his body(1), stored:

  • In the red blood cell hemoglobin (65%)
  • In muscle myoglobin (10%)
  • The remaining stored in the liver, macrophages (they eat up old red blood cells) and bone marrow (the site of red blood cell production).

Who needs iron supplements?

Some of us will need to take supplements as prescribed by our doctor. This usually happens if:

  • You are diagnosed with iron deficiency anemia;
  • You are diagnosed with an iron deficiency, typically measured by a serum ferritin test, with no anemia declared yet (but probably on the way).
  • You are a woman and you are pregnant;
  • You are a woman and you are undergoing heavy menstrual bleeding;
  • You have gastrointestinal issues and you cannot absorb iron properly;
  • You have gastrointestinal problems that are accompanied by regular bleeding (ulcerative colitis for instance);
  • For premature infants and preschool kids deemed at risk of iron deficiency.

Iron deficiency (the stage before anemia) is unfortunately very common in the US and other industrialized countries, probably due to malnutrition.

That being said, don’t rush to the iron supplements without knowing whether or not you are deficient. Adverse effects (including iron supplements constipation) are quite frequent, event at normal recommended dosage levels. It is therefore best that you work with your health care practitioner if you think you need extra iron.

Increasing dietary iron remains the best strategy for most of us, especially for kids. When possible, it is the best approach to avoid iron supplements constipation.

What causes iron supplements constipation?

In order to understand what causes iron supplements constipation, we need to take a close look at our gut flora. We know how important the intestinal flora is to a healthy and well-functioning transit.

Iron greatly impacts our gut flora, and not in the right direction unfortunately. Our gut flora, when in balance, contains a majority of friendly bacteria, and a very small amount of pathogenic (i.e. harmful) bacteria kept in check by the good guys.

Robert Doyle, a Syracuse University researcher, tells us that "Iron is the single most important micronutrient bacteria need to survive". A lot of pathogenic bacteria require iron for growth, and their virulence is enhanced by excess iron. Which is pretty much what happens if you take iron supplements.

Let’s repeat this, it is very important to understand the connection. We take iron supplements, we start to feed a small amount of pathogenic bacteria in our gut, so far kept in check by the friendly flora. They start to reproduce, at the expense of the good flora.

This leads to dysbiosis, a situation where the gut flora balance has been broken, causing an overgrowth of harmful flora. Dysbiosis in turn leads to gastrointestinal troubles, nutrient absorption issues in the gut and a compromised immune system.

And of course, dysbiosis can lead to bloating and constipation.

Dietary iron

To avoid iron supplements constipation, we should obtain the iron our body requires from dietary sources. On average, a healthy adult individual will absorb between 10% and 15% of the dietary iron he or she ingests(3). The actual percentage varies of course, and is influenced by several factors, some dietary and some non-dietary.

The main factor is the level of our iron reserves. Iron absorption increases when our reserves are low. When our reserves are replete, absorption will go down to protect us from the toxic effect of too much iron (note that even though our body has a built-in protection mechanism, one still has to be careful will iron supplements).

But not all dietary iron is created equal. We need to distinguish between two main types:

Plant based iron. This one is called non-heme type, because it is not bound to hemoglobin, and is not as easily absorbed as the meat-based one.

It has a much lower absorption compared to heme type, and is absorbed at a rate 2% to 20%(3). Plant iron is significantly affected by a list of dietary factors that we will discuss further down.

Meat based iron. You remember from an earlier section that iron is a building block of hemoglobin. When we eat red meat, we ingest iron mostly via the hemoglobin and myoglobin of the animal. This is called the “heme” type of dietary iron, and it is much better absorbed that the non-heme type.

Heme-iron from meat is much more bioavailable than the non-heme plant type, and is absorbed at a rate of about 15% to 35%(3). That absorption rate is mostly unaffected by the other components of your diet, which is not the case for the non-heme iron type from plants (fruits, vegetables and grains).

The following table gives a list of heme-iron food sources(3):

Food Milligrams
per serving
% Daily Value
Chicken liver, cooked, 3½ ounces12.870
Oysters, breaded and fried, 6 pieces4.525
Beef, chuck, lean only, braised, 3 ounces3.220
Clams, breaded, fried, ¾ cup3.015
Beef, tenderloin, roasted, 3 ounces 3.015
Turkey, dark meat, roasted, 3½ ounces2.310
Beef, eye of round, roasted, 3 ounces2.210
Turkey, light meat, roasted, 3½ ounces1.68
Chicken, leg, meat only, roasted, 3½ ounces1.36
Tuna, fresh bluefin, cooked, dry heat, 3 ounces1.16
Chicken, breast, roasted, 3 ounces1.16
Halibut, cooked, dry heat, 3 ounces0.96
Crab, blue crab, cooked, moist heat, 3 ounces0.84
Pork, loin, broiled, 3 ounces0.84
Tuna, white, canned in water, 3 ounces0.84
Shrimp, mixed species, cooked, moist heat, 4 large0.74

The following table gives a list of heme-iron food sources(3):

Food Milligrams
per serving
% Daily Value
Ready-to-eat cereal, 100% iron fortified, ¾ cup 18.0100
Oatmeal, instant, fortified, prepared with water, 1 cup10.060
Soybeans, mature, boiled, 1 cup 8.850
Lentils, boiled, 1 cup6.635
Beans, kidney, mature, boiled, 1 cup5.225
Beans, lima, large, mature, boiled, 1 cup4.525
Beans, navy, mature, boiled, 1 cup4.525
Ready-to-eat cereal, 25% iron fortified, ¾ cup4.525
Beans, black, mature, boiled, 1 cup3.620
Beans, pinto, mature, boiled, 1 cup 3.620
Molasses, blackstrap, 1 tablespoon3.520
Tofu, raw, firm, ½ cup3.420
Spinach, boiled, drained, ½ cup3.220
Spinach, canned, drained solids ½ cup2.510
Black-eyed peas (cowpeas), boiled, 1 cup1.810
Spinach, frozen, chopped, boiled ½ cup1.910
Grits, white, enriched, quick, prepared with water, 1 cup1.5 8
Raisins, seedless, packed, ½ cup1.5 8
Whole wheat bread, 1 slice0.9 6
White bread, enriched, 1 slice0.9 6

If you are iron deficient, carefully designing meals that maximize iron intake instead of taking supplements will ensure you never have to encounter iron supplements constipation.

Reducing the risk of iron supplements constipation

If you are suffering from iron deficiency anemia, or if you are at risk of being iron deficient, you will receive an iron supplement prescription from your doctor.

This supplementation may trigger a round of iron supplements constipation.

What to do then?

It would be unwise to stop the supplementation, based on the fact that iron is essential for life, and that your stores are running low. However, here are different points you can consider as a coping strategy. You want to replenish your reserves as fast as possible in order to minimize the length of treatment.

1. Try different forms of iron supplements

Ferrous Sulfate is the cheapest and most commonly prescribed form. It remains the standard first-line prescription of iron-deficiency anemia

It is also the one that seems to cause the most iron supplements constipation issues, and is considered by patients to be a bit harsh on the digestive tract.

Claims of improved tolerability of one iron supplement over another have not been substantiated by clinical trials though. This is the contentious point, because patients do seem to complain more about Sulfate.

Whatever the clinical research says, if one form doesn’t work for you, go for another form, ferrous gluconate or ferrous fumarate, which may provide relief to your iron supplements constipation.

Make sure you get the same amount of "elemental iron" when you switch from one form to another one.

Let us run through an example: Let’s say that a 300 mg tablet of ferrous sulphate contains 60 mg of elemental iron, and that your doctor prescribed 2 tablets a day. That represents a total elemental iron content of 120 mg a day. Now you want to switch to ferrous gluconate. One gluconate 300 mg tablet contains 35 mg of elemental iron content. In order to reach the same amount of elemental iron, you would need to take about 3.5 tablets.

Switching to a non-sulfate supplement type will likely be more costly, but it is worth a try to avoid iron supplements constipation.

2. Increase your iron intake from dietary sources

As mentioned previously, dietary iron does not cause constipation issues. As a result, in order to replenish your iron stores, you should make sure you get as much iron as you can from your food. This will allow you to minimize the length of treatment.

See the previous section covering dietary iron, and try to include as many of those sources as possible, without creating too much of an imbalance in your usual diet (which in itself may be a cause of constipation).

3. Take substances that increase iron absorption

The following substances increase absorption of iron:

  • Vitamin C is a powerful enhancer of absorption of non-heme iron(4), the one coming from plant sources. It also counters the negative effect of the iron absorption inhibitors mentioned in the next section. Therefore, make sure you eat plenty of fruits and vegetables rich in vitamin C, and consider taking a supplement for the duration of your iron supplementation treatment.
  • Animal proteins are enhancers of absorption of non-heme iron. Studies have consistently shown an enhancing effect of meat, fish, or poultry on iron absorption from vegetarian sources(5). This is quite interesting – meat seems to be the iron powerhouse, not only containing iron itself, but also augmenting the absorption of the iron coming from vegetables, fruits and grains in the meal.

Increasing absorption means replenishing your reserves faster, and less exposure to the risk of iron supplements constipation.

4. Avoid substances that decrease iron absorption

The following substances decrease absorption of iron(5):

  • Phytates are constituents contained in cereals and legumes. They will reduce absorption of non-heme iron from plant sources. They bind to minerals in your gut and prevent digestion of those minerals. In order to decrease the amount of phytates, you can soak your cereals or legumes overnight and carefully rinse them before cooking them. You can also use fermentation or germination methods. Buy sourdough or sprouted varieties of bread (e.g. buy Ezekiel bread). Eat fermented versions of soy beans such as tempeh. Search the internet for ways to reduce phytates from food, there are numerous websites covering this subject.
  • Tannins, found in black teas and certain herbal teas (e.g. rosemary, sage, thyme, wood betony, yarrow, etc) and anything that tastes astringent and create a puckering effect in your mouth (unripe fruits for instance).
  • Polyphenols, found in some fruits and vegetables, in some herbal teas and in wine. This one is hard to put in practice, unless you want to go through the effort of finding out what vegetable contains what levels of polyphenols. Not worth it in my opinion.
  • Calcium from dairy products and supplements, which inhibits absorption of both heme and non-heme iron (i.e. inhibits both animal and plant iron).
  • Milk and egg proteins. We saw in the previous section that meat proteins enhance absorption. Milk and egg proteins though will have the opposite effect. Overall, based on this and on the previous point, avoid dairies for the length of your treatment.

Reducing or eliminating substances that lower iron absorption means replenishing your reserves faster, and less exposure to the risk of iron supplements constipation.

5. Experiment with iron dosage and length of treatment

If you have iron supplements constipation, you will have to make compromises.

You may have to lower your daily dosage of iron supplements, and increase the length of your treatment. At the end of the day, you are in the driver’s seat, and you will have to figure it out through experimentation, what is the highest dose you can tolerate without messing up your transit.

Of course, discuss this with your doctor as well. But do take charge.


Iron is essential for life. It is involved in numerous physiological processes. Due to health issues, you may have been diagnosed with iron deficiency, or with a full blown anemia.

This calls for iron supplementation. We have seen in this article that the occurrence of iron supplements constipation is common and can be explained by the interaction of iron with your gut flora.

We have also seen that there are multiple levers you can play with to make sure you replenish your reserves as fast as possible and limit the duration of the treatment.

As we have discussed in the case of calcium and constipation, life is all about compromises. You should not stop taking iron if it is prescribed by your doctor.

But you should not live with iron supplements constipation either. You should try different forms of iron supplements, increase food sources which are much better tolerated, and with your doctor’s agreement, try a dosage that better fits your tolerance.

This is your health, you are in the driver’s seat. Nobody knows you better than yourself. Trust that with perseverance, you will find the right balance.

References for "Iron Supplements Constipation"

(1) M. Munoz, I. Villar, and J. A. Garcia-Erce, “An update on iron physiology”, World Journal of Gastroenterology, vol. 15, no. 37, pp. 4617–4626, 200. (no mention of iron supplements constipation)

(2) Syracuse University. "How Some Bacteria May Steal Iron From Their Human Hosts." ScienceDaily, 31 Jul. 2008. Web. 16 Nov. 2011.

(3) National Institute of Health, Office of Dietary Supplements, Iron Dietary Supplement Fact Sheet located at : one brief mention to iron supplements constipation).

(4) Lynch SR, Cook JD. "Interaction of vitamin C and iron". Ann N Y Acad Sci. 1980;355:32-44.

(5) Hurrell R, Egli I. "Iron bioavailability and dietary reference values." Am J Clin Nutr. 2010 May;91(5):1461S-1467S. Epub 2010 Mar 3.

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