Quick Summary
- Three factors matter when choosing iron: absorption efficiency by form, elemental iron content, and tolerability profile.
- Elemental iron content varies by form — 130 mg of ferrous sulfate contains only ~42 mg elemental iron.
- Bisglycinate achieves higher fractional absorption than conventional salts in controlled studies, meaning more iron delivered and less reaching the colon.
- The best iron supplement is the one you'll actually take consistently — tolerability drives compliance.
Choosing an iron supplement comes down to three pharmacokinetic questions: (1) Does the iron form minimise colonic free-iron exposure? (2) Is the elemental iron dose within the DMT1 absorption window? (3) Does the formulation account for hepcidin timing?
If you understand these three factors, you can evaluate any iron supplement on the market — regardless of branding.
Why iron form matters more than iron dose
A key reason iron supplements cause GI side effects is that more iron enters the colon than the duodenum can absorb. The unabsorbed fraction can catalyse the Fenton reaction — free Fe²⁺ reacts with hydrogen peroxide to produce hydroxyl radicals (OH·), one of the most reactive oxygen species in biology.
These radicals can damage the colonic mucosa and disrupt the gut microbiota, contributing to the nausea, cramping, and constipation associated with oral iron therapy. A meta-analysis of 43 studies (Tolkien et al. 2015) found that GI side effects with ferrous sulfate were significantly higher than with placebo or IV iron.
The solution is not "less iron." It is more efficient absorption per milligram — so less iron reaches the colon in the first place.
A pharmacokinetic checklist for choosing iron
Here is the practical summary in five checks — enough to work from in the pharmacy. The sections below then explain each one in mechanistic detail.
- Iron form: Chelated forms like bisglycinate achieve higher fractional absorption than ionic salts in controlled studies, and may also access a PepT1 secondary pathway (still under investigation) — resulting in less colonic free-iron spillover
- Elemental dose: most mild-to-moderate cases need somewhere around 25–40 mg; severe cases are for a doctor to determine
- Vitamin C (helpful addition): Can enhance Fe³⁺→Fe²⁺ reduction and improve absorption in some contexts — useful if not already in the formula, especially with conventional salts
- Dosing frequency: Once daily or alternate-day? Split dosing significantly reduces second-dose efficiency due to elevated hepcidin
- Supporting nutrients (if co-deficient): Some formulations include active B12, folate, and B6 — relevant primarily if you also have deficiencies in these vitamins
DMT1 absorption and the chelation advantage
Traditional iron salts (ferrous sulfate, fumarate, gluconate) dissociate in gastric acid to release free Fe²⁺ ions. These ions are absorbed exclusively through the DMT1 transporter (divalent metal transporter 1) on the apical membrane of duodenal enterocytes.
DMT1 is saturable. It handles a fixed throughput of Fe²⁺ per unit time. When the dose exceeds DMT1 capacity, the excess free iron transits to the colon unabsorbed.
The chelation advantage: Ferrous bisglycinate keeps iron bonded to two glycine molecules through gastric transit, protecting it from free-ion release in the stomach. Mechanistic studies have proposed that this chelated structure may also access the PepT1 peptide transporter as a secondary absorption pathway — though how much PepT1 contributes in humans at typical doses remains under investigation. What is established is that bisglycinate achieves higher fractional absorption than conventional iron salts in controlled studies — including in infants and young children (Pineda & Ashmead 2001) — consistent with less free iron reaching the colon.
Elemental iron: the number that actually matters
Supplement labels show compound weight (e.g., "130 mg ferrous sulfate"), but absorption depends on elemental iron content — the actual absorbable iron fraction:
| Iron Form | Compound Weight | Elemental Iron |
|---|---|---|
| Ferrous sulfate | 130 mg | ~42 mg |
| Ferrous fumarate | 200 mg | ~66 mg |
| Ferrous gluconate | 300 mg | ~36 mg |
| Ferrous bisglycinate | 130 mg | 36 mg |
The key insight is not a fixed calculation but a directional one: conventional iron salts typically achieve lower fractional absorption than chelated forms, meaning a larger proportion of the dose reaches the colon as free Fe²⁺. Chelated bisglycinate achieves comparatively higher fractional absorption in controlled studies, so more iron is taken up in the duodenum and less spills into the colon as reactive free iron. The exact percentages vary by individual iron status, food intake, and other factors.
Higher fractional absorption = more iron delivered per milligram, less free iron reaching the colon.
The hepcidin timing window
Even with the right form and dose, when you take iron matters. Hepcidin — the liver-produced peptide hormone that regulates iron absorption — rises 6–8 hours after an iron dose and stays elevated for ~24 hours (Moretti et al. 2015).
During this window, hepcidin binds ferroportin on enterocytes, blocking iron export to the bloodstream. A second dose taken within this window encounters a partially closed gate — absorption is approximately 35–45% lower than the first dose (Moretti et al. 2015; Stoffel et al. 2017).
Practical implication: Once-daily dosing is pharmacokinetically superior to split dosing. Alternate-day dosing allows full hepcidin reset and has shown comparable haemoglobin outcomes in clinical trials. The iron form should be efficient enough to deliver adequate iron in a single moderate dose.
How Hemascore applies this framework
Hemascore was designed around these pharmacokinetic principles:
- 36 mg elemental iron from ferrous bisglycinate — higher fractional absorption than conventional salts in controlled studies, with less free iron reaching the colon
- Vitamin C included — enhances Fe³⁺→Fe²⁺ reduction at the enterocyte brush border
- Active B-vitamin cofactors: P5P (B6), Methylcobalamin (B12), 5-MTHF (Folate) — supporting erythropoiesis downstream of iron absorption
- Once-daily after food — for tolerability; splitting doses does not improve absorption and reduces efficiency due to hepcidin rise
To see the full ingredient list, visit the Hemascore product page.