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 . 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.
It can affect the balance of gut bacteria .
It may also affect bowel habits, frequently leading to constipation.
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' .
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
Iron bisglycinate
Ferrous fumarate/sulfate
Chelated iron
Iron salt
Gentler on digestion
More commonly associated with side effects such as constipation
Better absorbed
Lower absorption compared to chelated iron
Often effective at lower doses
Often used at higher doses
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 . 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 . While Vitamin C helps with food-based iron, its impact on high-quality supplements may be less significant . Emerging research also suggests certain probiotics may support iron conversion in the gut .
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.