Iron
Ferrous bisglycinate at 18 mg, a chelated form that is usually kinder to the stomach than oxide or sulfate at the same elemental amount.
Written by R. Kessler · About Knurle · Editorial policy
The short answer
Iron is needed to make haemoglobin, and deficiency shows up as fatigue, breathlessness on stairs and pale skin. Ferrous bisglycinate is iron bound to the amino acid glycine, and it is generally better tolerated than ferrous sulfate or ferrous oxide at the same elemental dose.
The form, and why it matters
Ferrous bisglycinate is a chelate, which protects the iron from binding to other compounds in the gut and is associated with fewer gastrointestinal complaints than sulfate. Ferrous sulfate is the cheapest and the best-studied, and the one most likely to cause constipation and nausea.
Typical dose
The reference intake is 8 mg a day for adult men and postmenopausal women, and 18 mg for premenopausal women. The tolerable upper intake is 45 mg for adults. Keel carries 18 mg, the premenopausal reference amount, alongside 5 mg BioPerine.
What the research shows
Iron status responds to dose and form, and the chelated forms trade a small amount of elemental iron for meaningfully better tolerability. Accidental iron overdose is a leading cause of poisoning deaths in young children, which is why the warning on the label is written straight and never softened.
Who should be careful
Iron darkens stool, which is expected and harmless on its own. Paired with pain, vomiting or dizziness it stops being cosmetic and becomes a reason to see a clinician. Do not supplement iron without a ferritin result, and keep bottles away from children.
Where it sits in our formulas
| Product | Form on the label | Amount per serving | % Daily Value |
|---|---|---|---|
| Keel | as ferrous bisglycinate | 18 mg | 100% |
Sources
Figures checked against the sources above on 21 September 2026.
Vitamin B12
Methylcobalamin at 525 mcg. Both forms raise B12; neither has been shown clearly better for topping up a dietary gap in a healthy adult.
Read itBioPerine®
Black pepper fruit extract at 5 mg. It raises the bioavailability of co-taken compounds, including curcumin and iron, and 5 mg is the studied amount.
Read itMagnesium
200 mg as oxide plus citrate. Magnesium draws water into the bowel, which is why the oxide form is the one that helps constipation.
Read itFerrous bisglycinate: the iron that is gentle
Why Keel uses 18 mg ferrous bisglycinate plus 5 mg BioPerine rather than a cheap oxide. What black stool actually means, why with food, and when to check ferritin instead of guessing.
Read itFerrous bisglycinate vs ferrous sulfate
Ferrous sulfate is the cheapest and best-studied and the most likely to cause constipation and nausea. Ferrous bisglycinate is a chelate that trades a little elemental iron for much better tolerability.
Read itEating less on a GLP-1? What runs low first
Appetite drops, portions shrink, and the multivitamin you took with breakfast may no longer cut it. Which vitamins people run low on during GLP-1 use, why absorption shifts, and what a single daily habit can sensibly cover.
Read itEvery milligram, printed.
Each amount on the label is the amount in a full daily serving, and it matches what is on this page.
