Iron supplements are sold under a confusing set of names, and most of them tell you something real if you know how to read them. Here is what the words mean, without anybody's marketing attached.
Every iron supplement is iron bound to something else
You cannot put plain iron in a capsule. It has to be bound to another molecule to be stable and to dissolve in the right place. The second word on the label is that other molecule.
The consequence is that the compound always weighs more than the iron inside it. That ratio — the proportion of the compound that is actual iron — is the single most useful thing to understand about an iron label.
Ferrous sulfate
Iron bound to sulfate. Roughly 20 per cent elemental iron by weight, and the form used in most of the clinical trial literature going back decades, which is why it is usually the comparator when a new form is tested.
Ferrous fumarate
Iron bound to fumaric acid. Around 33 per cent elemental iron, so a smaller tablet for the same dose.
Ferrous gluconate
Iron bound to gluconic acid. Around 12 per cent elemental iron, so it takes a larger amount of compound to deliver the same dose.
Ferrous bisglycinate chelate
Iron bound to two molecules of the amino acid glycine — that is what “bis” and “glycinate” mean. A chelate is simply a molecule that holds a mineral in a claw-like grip. Around 20 per cent elemental iron.
One thing worth knowing about this form, because it is often claimed otherwise: isotope studies in 85 women found that iron from bisglycinate joins the body's ordinary non-haem iron pool and is regulated by your iron status in the same way as any other iron salt. There is no separate pathway and no bypassing of the body's own controls (Pizarro and colleagues, American Journal of Clinical Nutrition, 2002).
Iron polymaltose
Iron held in a carbohydrate complex. Sold as a liquid or tablet, and dosed by elemental iron like the rest.
So which form should you choose?
Here is the answer you will not often see on a supplement website. In 2025 an international consensus panel reviewing the evidence on managing iron status concluded that there is insufficient evidence to support a specific oral iron formulation over another, and that outcomes depend more on the amount of elemental iron than on the type of preparation (Benson and colleagues, The Lancet Haematology, 2025).
That is worth sitting with, because a great deal of supplement marketing depends on you believing the opposite. The honest position is that the form matters less than the dose, and less than whether you actually take it.
Why trial comparisons are harder to read than they look
If you go looking at the research yourself, there is one trap to watch for. A large share of the trials comparing a newer iron form against ferrous sulfate gave the two groups different amounts of elemental iron — often two to eight times less in the newer-form group.
A 2023 systematic review of seventeen randomised trials is a good example: the pregnancy results favoured bisglycinate, but the bisglycinate groups had been given 15 to 30 mg of elemental iron against comparators on 40 to 115 mg (Fischer and colleagues, Nutrition Reviews, 2023). When you see a result like that, the first question is always: were the doses the same?
The three things actually worth checking on a label
- The elemental iron figure. Not the compound weight. If a label only gives you the compound, it is making itself hard to compare on purpose.
- Whether it is on the ARTG. Australian supplements carry an AUST L or AUST R number. It is not a quality rating, but it means there is a sponsor accountable for the product.
- What else is in it. Fillers, colours and flow agents are not a scandal, but you are entitled to know what you are swallowing.
This article is general information, not medical advice. Whether an iron supplement is right for you, and at what dose, is a question for your doctor. Always read the label and follow the directions for use.