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Iron Supplements by Jane Cronin

Clinicians Iron SupplementsIron Supplements, which form of iron is best absorbed and what are the side effects of each form. An article by Naturopath Jane Cronin, inspired by her many clients whom experience adverse side effects from Iron Supplements.

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Do iron supplements have to upset your digestive system?Iron Supplements

I was inspired to write about this topic as I have recently seen a number of clients in my clinic who have had this issue.  They come in with all the signs of low iron such as fatigue, dizziness, weakness, poor immunity and generally feeling quite fed up.  They have been diagnosed with low iron levels, been prescribed iron tablets, but stop taking them due to digestive upsets.  Generally non-compliance is due to severe constipation, however some people get loose bowels, nausea and stomach pains.  The result is they take the tablets for a while, but when the digestive effects get too much they stop and don’t finish the course.   So they never really get their iron levels up to scratch.

What is it about iron that causes these side effects?

According to research sources the higher the level of available iron in the stomach and small intestines the more irritation it causes.  When higher doses of iron are given to people suffering from anaemia (clinically low iron) they tend to suffer worse side effects.  The uptake of iron from the digestive tract is also important in this equation as obviously the longer the iron stays in the system the more irritation can be caused. 

The body is programmed to be very careful in its absorption and uptake of iron, as getting rid of excess is difficult for the body and toxicity can be fatal.  Regulation is actually at a cellular level in the digestive tract and the cells that absorb the iron only last for 48-72 hours before they are shed, taking any unabsorbed iron with them.  Unabsorbed iron transits through the digestive tract and is removed via the bowel.  This increased amount of iron in the bowel adds to the constipation and causes bowel motions to be darker in colour.

So does all iron have this effect and what should for we look for when selecting iron?

When looking at different forms of iron we need look at a number of areas:

How much iron is actually available in a particular form?

The majority of iron comes as salts in the forms of ferric or ferrous iron.  Generally these salts are formulated with other substances to make them more absorbable e.g. amino acids, sulphur, aspartic and citric acid etc.  There is also carbonyl iron that is not a salt, but is pure elemental iron.  This means that it is not bound to another chemical and its small particle size adds to its bioavailability

When you see the amount of iron listed you may see for example 320mg, but this is the amount of the whole iron salt molecule.  The actual amount of iron available from the molecule is known as the elemental iron content and is usually much lower.  If you look at the table below you can see the percentage of available iron in different forms.  The amount of elemental iron should be displayed on all product labels.  You can see from the table below the amount of iron available from different forms.

Iron Percentage

Form of iron:

% of elemental iron :

Ferrous gluconate


Ferrous sulphate


Ferrous fumarate


Carbonyl Iron


How well is a particular form of iron absorbed? 

The digestive system only absorbs iron in the ferrous form, so if you see ferric iron you know it has to be converted first before it is usable.  One research paper shows ferrous iron is absorbed three times more readily than the ferric form.

Vitamin C taken at the same time can help to increase the absorption of iron.  This is why doctors often suggest taking iron supplements with orange juice.  Many iron supplements available often come with vitamin C in the preparations.

Percentage Absorbed

Form of iron:

Elemental iron content absorbed:

Carbonyl Iron


Ferrous fumarate


Ferrous sulphate (monohydrate)


Ferrous citrate


Ferrous carbonate


Ferrous sulphate


Ferrous gluconate



What about slow release iron tablets?

There are slow release forms of iron that tend to contain a higher dose, which breaks down more slowly.  This means that the tablet or capsule gradually breaks down, so at one time there is only a small amount of available iron in the digestive tract to cause irritation.  This however goes on for a longer period of time.

Iron toxicity

Did you know that iron toxicity is one of the leading causes of poisoning in children?  Hence pack size and iron content are limited.   Tablets that are slowly absorbed can be better as there is more time for medical intervention should accidental consumption occur.  There is good safety data around carbonyl iron as it is slow to absorb and to date there are no fatalities with this form of iron.

Iron supplementation below the tolerable intake level of 40 – 45mg per day (depending on age) is well tolerated.  Taking iron above this level should be done under health professional supervision after laboratory assessment of iron status and monitored regularly.

Keep iron supplements out of reach from children.

How are you going to choose an iron that will cause the least digestive discomfort?

Taking into account the factors discussed when trying to avoid major side effects your best bet is to use an iron supplement with a high amount of elemental iron that is easily absorbable.  This means that it does not stay in the digestive tract any longer than necessary causing irritation.  Having a tablet that is slower release can also help to avoid discomfort

If you are find that your current iron supplement is causing discomfort don’t just write off all iron supplements.  Look for other gentler and more effective forms.  All irons medications are not the same and iron is important for your physical, mental, emotional and immune health and growth in children.

Read more about Iron Deficiency Anaemia


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Showing 14 comments


What about heme iron supplements? I read heme based animal heme) iron supplements have better absorption than non heme from plant sources. The lower pill dose and better absorption greatly reduces the side effects and amount of iron sitting in your system.


Hi Catherine,
Thank you for your comment. Generally we talk about iron as heme and non-heme when we look at the absorption of iron from foods and indeed heme iron is better absorbed, which is why vegans and vegetarians can be at risk of iron deficiency. In New Zealand iron supplements are not vegetable protein based, but come from mineral salts. As you will see from the article these salts have varying degrees of absorption.
^Jane - Clinicians Naturopath

Dr V s Sachan

Sodium feridetate is tolerated best, good taste, better absorption, most anaemic pt,s have gastritis or peptic ulcers, there fore it better for good compliance. But non of pharma co. has given the syp with folate, zinc, n methicobalamin .I think they do it not to spoil taste n smell of thier Brand.


I am speechless that doctors would tell patients to take iron that makes them sick. The human body cannot use the Ferrous form of iron and so the symptoms after taking the supplement are the same as taking arsenic. But this iron is in almost every food we eat. And we continue to wonder, why are we all getting sicker and more anemic while at the same time we consume iron and other vitamins in almost every food we eat. Humans can only benefit from heme iron from meat and non heme iron from plants, everything else is just causing oxidative stress and makes us rust inside. But if you take a look at Harvard Institute of Health Food Pyramid you’’ll see they recommend whole grains not the stuff with the ferrous iron added to it because that is the only thing that will keep us alive. The whole enrichment program is nothing more than government imbecilism and for some reason nobody is stopping it.

michael James

The vast majority of people who take Ferrous Iron salts, do tolerate it well and it remedies Iron deficiency in most. The reason it is so widely prescribed in addition to its efficacy and tolerability is it’’s cheap.


Hi I have been put on iron tablets and I have been taking them for over 3weeks. Just wanted some advice is it normal to have a gurgling stomach, mucas coming from my bottom and to be pooing more than 5times a day with them wasnt sure if they was upsetting my stomach and if I should go to the doctors about this?

Jane Naturopath

Hi Kirsty,

I guess the question is was your stomach doing this before you started taking the iron tablets? The prescription iron tablets agrevate the stomach and I have know people to get loose bowels as well as constipation.

michael James

Dear Jane.
I’’d appreciate you letting me know your sources for the absorption rates. The only paper I could find gave rates of between 5% and 20%.
I am looking for a better Iron source for my mother, as she is unable to tolerate her Ferrous Sulphate.
Yours sincerely.

Michael James

Dear Jane.
My earlier comment/question was in relation in particular to Carbonyl Iron being 69%absorbed.

Jane Clincians Naturopath

Hi Michael, If you could please send your email address to I will send you some documents. only have the research papers in PDF.

Thet Htar

Can Carbonyl iron cause constipation more than other iron forms?

Jane Cronin Clinicians Naturopath

Hi Thet, on the contrary the article is explaining that carbonyl iron is the least likely form to cause constipation.


I have been on iron supplements for 55 days, I am not constipated, but have more of a loose stool and soreness after bowel movements. I am a runner and can feel the positive effects of increased iron levels, but these side effects are becoming hard to bear. Additionally, although I feel better when running breathing better, leg pain normal), afterward fatigue is still present. Is that normal?

Jane Cronin Clinicians Naturopath

Hi Kathy,

Which type of iron have you been supplementing with? The side effect of some types of iron can be constipation, but loose bowels can also occur in some people.. This is because some of the iron salt formulas generally irritate the gut. If you are taking these kinds you should consider carbonyl iron as it is very gentle on digestion. With regards to fatigue it could be to do with lots of things, but I would need to know more about you running. Feel free to email me at or better still call for a chat on 0800 622533. That way I will be able to help you better.
Dietary supplements are not a replacement for a balanced diet. Always read the label. Use as directed. Do not exceed the recommended daily dose. If symptoms persist, see your health professional.

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