Ferrous bisglycinate vs ferrous sulfate

Ferrous bisglycinate is iron bound to the amino acid glycine, and it is generally better tolerated than ferrous sulfate at the same elemental dose, with fewer stomach complaints and less effect on the bowel. Ferrous sulfate is cheaper, better studied, and the form most likely to cause constipation and nausea. The trade is a small amount of elemental iron for meaningfully better tolerability.

If iron sits badly with you, the chelated form is the one to try. If cost is the constraint, sulfate works and the tolerability is the price. Keel uses 18 mg of ferrous bisglycinate alongside 5 mg of BioPerine, which is the pair the absorption research used.

Why does the iron form matter so much?

Iron is the one mineral where the form you pick changes how the supplement feels more than what it does. All forms deliver elemental iron, which is the only thing your body absorbs. What differs is how much of the compound is actually iron, how much of that survives the gut, and how much argument the rest of it starts on the way through.

Ferrous sulfate is the reference: about 20% elemental iron by weight, cheap to make, and the form used in most of the deficiency trials. It is also the form most likely to constipate, nauseate and darken the stool more than necessary.

Ferrous bisglycinate is a chelate, iron protected inside a glycine ring. It carries less elemental iron per gram, roughly 20% as well but absorbed more efficiently and with far fewer complaints, and it does not appear to compete as strongly with other minerals for absorption.

How do they compare side by side?

	Ferrous sulfate	Ferrous bisglycinate
Elemental iron	About 20% by weight	About 20% by weight
Absorption	Lower, and inhibited by food, tea and coffee	Higher, and less affected by a meal
Stomach upset	Common	Much less common
Constipation	Common	Less common
Stool darkening	Expected	Expected, usually less pronounced
Cost	Low	Higher
Research base	Large, most deficiency trials	Smaller but growing

The food interaction is the underrated one. Sulfate absorption drops sharply when taken with tea, coffee, dairy or a high-fibre meal. The chelate is less affected, which matters when the whole reason you are taking iron is that you have to take it with food to tolerate it.

How much iron do you need, and what is the ceiling?

The reference intake is 8 mg a day for adult men and postmenopausal women, and 18 mg for premenopausal women. The tolerable upper intake for adults is 45 mg a day.

Keel carries 18 mg, the premenopausal reference amount, alongside 5 mg BioPerine. Piperine raises iron bioavailability, and studies of iron co-administered with piperine showed higher serum iron than iron alone, with human work in iron-low people improving haemoglobin and ferritin. Five milligrams is the studied amount, not a marketing dose.

Why does iron need to be taken with food?

Iron is hardest on an empty stomach, and a GLP-1 medication has already slowed your stomach down. Taking iron with a meal that has some substance to it makes it kinder and, for the chelated forms, does not cost you much absorption. It also means the digestive enzymes in the same capsule have something to work on.

What is the safety line on iron?

Accidental iron overdose is dangerous for young children, and it is one of the most common causes of poisoning deaths in that age group. Keep the bottle out of reach, keep the child-resistant cap on, and never describe iron as candy or as a kids' product.

Iron darkens stool. On its own that is expected and harmless. Paired with pain, vomiting or dizziness it stops being a cosmetic effect and becomes a reason to see a clinician the same day.

Who should pick which?

• Iron upsets your stomach, or you have to take it with food anyway. Bisglycinate, which is the case for most people who have already tried sulfate and given up.
• Cost is the constraint and you tolerate sulfate. Sulfate works. Take it away from tea, coffee and dairy.
• Diagnosed iron deficiency. Your clinician sets the dose, and a higher therapeutic dose may come with a different form entirely. Do not self-prescribe above the label.
• No deficiency and no reason to supplement. Nothing. Doubling iron without a ferritin result is not cautious, it is just doubling.

What is in our bottle?

Keel carries 18 mg of ferrous bisglycinate with 5 mg of BioPerine, in a two-capsule serving with food. The detail is on the iron ingredient page, and the BioPerine mechanism is on its own page.

The wider picture, including what else slips when appetite drops, is in our guide to eating and supplementing well on GLP-1 medications.

FAQ

Which iron is gentlest on the stomach? Ferrous bisglycinate. The chelate is better tolerated than sulfate at comparable elemental doses, with less nausea and less constipation, which is the reason it costs more.

Can I take iron with coffee or tea? Not ideally. Both cut absorption substantially, especially of ferrous sulfate. Take iron with a meal, away from tea and coffee, and separate it from calcium supplements too.

Does black stool mean the iron is not working? No. Iron reliably darkens stool and it is harmless on its own. It becomes a concern only alongside pain, vomiting or dizziness.

How much iron do I need if I eat very little? Ask your clinician, and get a ferritin result rather than guessing. The premenopausal reference intake is 18 mg a day, and Keel carries exactly that, which is a sensible top-up rather than a treatment dose.

Sources

• NIH Office of Dietary Supplements, Iron fact sheet
• Shoba et al., Influence of piperine on the pharmacokinetics of curcumin in animals and human volunteers, Planta Medica

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.