Best Form of Iron Supplement: Iron Bisglycinate vs Ferrous Sulfate

Have you ever been told your iron levels are low, or advised to take an iron supplement after a blood test?

Iron deficiency is common, affecting up to 30% of the global population (1). Supplements are often used to restore iron levels or support increased needs during certain life stages.

But many people quickly notice that not all iron supplements feel the same. Some can cause nausea or constipation, whereas others are easier to tolerate.

So what's the difference?

In this article, we'll explore how iron is absorbed, why some forms are harder on digestion, and how to choose the most suitable option for you.

Why Iron Status Can Become Low

Low iron is not always just about what you eat. In many cases, it comes down to how well your body absorbs iron and how much it needs at that time.

Iron from food exists in two main forms: haem and non-haem iron.

• Haem iron (from animal foods like meat and fish) is generally well absorbed
• Non-haem iron (from plant foods like legumes, grains and leafy greens) is less easily absorbed and more affected by the rest of your diet

This is because non-haem iron must first be converted into a form your body can absorb. At the same time, certain compounds in foods can reduce how much iron you take in. These include:

• Phytates (found in wholegrains and legumes)
• Polyphenols (in tea, coffee and some plant foods)
• Calcium, when consumed alongside iron

On the other hand, nutrients like vitamin C can help support the absorption of non-haem iron from food (3).

Digestive health also plays a role. Stomach acid helps release iron from food. When stomach acid is reduced—through ageing or medications like PPIs—iron absorption may be affected.

Why Iron Supplements Often Cause Digestive Symptoms

One of the main reasons people stop taking iron supplements is digestive discomfort. A big part of the issue is that not all the iron you take is absorbed. The remaining portion stays in the gut and may cause side effects in the following ways:

• It can irritate the gut lining, contributing to nausea or stomach discomfort (4).
• It can affect the balance of gut bacteria (5).
• It may also affect bowel habits, frequently leading to constipation (12).

Types of Iron Supplements

Iron supplements generally fall into two main categories: iron salts and chelated iron.

Iron salts, such as ferrous sulfate and ferrous fumarate, dissolve quickly and release iron rapidly. However, the iron that remains unabsorbed stays in the digestive tract, which is why these forms are often described as ‘harsh’.

Chelated iron works differently. Rather than existing as a free mineral, iron is bound to amino acids, which helps protect it during digestion and reduces unwanted interactions in the gut.

What Is Iron Bisglycinate?

Iron bisglycinate is a form of chelated iron bound to two molecules of the amino acid glycine. This structure helps stabilise it as it moves through the digestive system, making it a 'gentle iron' (7).

If you'd like to explore this further, read our article on how chelated minerals influence absorption.

Iron Bisglycinate vs Ferrous Fumarate vs Ferrous Sulfate

Is More Iron Always Better?

Iron absorption is tightly regulated by a hormone called hepcidin. After taking iron, hepcidin levels increase, which can reduce the absorption of additional iron for up to 24 hours (8). This is why taking very high doses daily isn't always the most effective strategy.

How to Choose the Right Iron Supplement

Consider the form (bisglycinate vs sulfate), the dose of elemental iron, and your current status. Those who may benefit from a gentler form include:

• Women of reproductive age & pregnant women
• Those following plant-based diets
• People with sensitive stomachs
• Endurance athletes

Factors That May Support Iron Absorption

Separate tea, coffee, and calcium from your supplement to maximize uptake (2). While Vitamin C helps with food-based iron, its impact on high-quality supplements may be less significant (10). Emerging research also suggests certain probiotics may support iron conversion in the gut (11).

Final Thoughts

The best iron supplement isn't necessarily the one with the highest dose, but the one you can take consistently without discomfort.

Struggling With Side Effects from Iron Supplements?

If you've been prescribed iron and find it hard to stick with because of side effects, you're not alone, and you don't necessarily have to push through.

For confirmed iron deficiency, standard oral iron salts are often used first line. However, if side effects are difficult to tolerate, guidelines support adjusting the dosing strategy or considering alternative forms.

If you find standard iron supplements hard to tolerate, it may be worth discussing alternative forms, such as iron bisglycinate, with your GP or a qualified practitioner.

Final Thoughts

Iron is one of those nutrients that is easy to overlook until something feels off. For many people, the best option is not simply the highest dose, but the form that feels manageable to take and can be used consistently.

FAQs

What is iron bisglycinate?

Iron bisglycinate is a chelated form of iron bound to the amino acid glycine. It is often described as being well-tolerated and gentle on digestion.

Is iron bisglycinate better than ferrous sulfate?

Both forms can be effective, but iron bisglycinate is often better tolerated and may be suitable for individuals who experience digestive side effects with iron salts.

Why do iron supplements cause constipation?

Not all iron is absorbed, and the remaining iron can affect the gut environment. This may influence gut motility, particularly at higher doses.

Does chelated iron absorb better?

Recent research found that iron bisglycinate may offer higher bioavailability than conventional iron salts and was associated with fewer gastrointestinal side effects.

Is Liquid Iron Better?

Many liquid iron products still use traditional iron salts, so the format (liquid vs capsule) matters less than the form of iron used.

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The Evidence

1. National Institute for Health and Care Excellence (NICE). (2025). Anaemia – iron deficiency: How Common Is It?
2. Hurrell, R. and Egli, I. (2010). ‘Iron bioavailability and dietary reference values’. The American Journal of Clinical Nutrition.
3. Loganathan, V., et al. (2023). ‘Treatment efficacy of vitamin C given as co-intervention with iron for anemia’. Clinical Nutrition ESPEN.
4. Tolkien, Z., et al. (2015). ‘Ferrous sulfate supplementation causes significant gastrointestinal side-effects’. PLOS One.
5. Puga, A.M., et al. (2022). ‘Iron supplementation at the crossroads of nutrition and gut microbiota’. Nutrients.
6. Dentand, A.L., et al. (2024). ‘Current iron therapy: are we prescribing too much iron?’. Critical Reviews in Clinical Laboratory Sciences.
7. Fischer, J.A., et al. (2023). ‘The effects of oral ferrous bisglycinate supplementation: a systematic review’. Nutrition Reviews.
8. Stoffel, N.U., et al. (2017). ‘Iron absorption from oral iron supplements given on consecutive versus alternate days’. The Lancet Haematology.
9. von Siebenthal, H.K., et al. (2023). ‘Alternate day versus consecutive day oral iron supplementation’. EClinicalMedicine.
10. Li, N., et al. (2020). ‘The efficacy and safety of vitamin C for iron supplementation’. JAMA Network Open.
11. Puga, A.M., et al. (2022). ‘Iron supplementation and gut microbiota: the state of the art’. Nutrients.
12. Bloor, S.R., et al. (2021). ‘Oral iron supplementation—gastrointestinal side effects and the impact on the gut microbiota’. Microbiology Research.