4 forms with different bioavailability + tolerance · canon bisglycinate · avoid sulfate if tolerance OK · fumarate second option.
Patented Fe²⁺ × 2 glycine chelate
Best for
Ideal use: any 2026 pro Fe supplementation · pregnancy · athletes · RLS.
Advantages
Bioavailability 2-4x sulfate via PEPT1 di-peptide. Exceptional digestive tolerance (Milman 2024 5x better vs sulfate). No constipation. No metallic taste.
Disadvantages
Expensive (~12-20 €/month at 25 mg). Label must specify elemental Fe (not total bisglycinate weight).
Historic classic form
Best for
Ideal use (limited): only if bisglycinate unavailable · extreme budget.
Advantages
Extreme economy (~3-5 €/month). Vehicle of classic RCTs. Universal OTC availability.
Disadvantages
Nausea, constipation, black stools in 50-60% subjects. Poor compliance. NOT superior bisglycinate in efficacy · only more adverse.
Fe²⁺ with fumaric acid
Best for
Ideal use: intermediate bisglycinate/sulfate alternative.
Advantages
Bioavailability better than sulfate (~33% vs ~10%). More tolerable than sulfate but less than bisglycinate.
Disadvantages
Still ~30% adverse rate. Bisglycinate superior overall.
Microparticulate Fe⁰ metallic
Best for
Ideal use: pediatric young children · accidental prevention.
Advantages
Slow release HCl-dependent (autoregulation). Lower accidental pediatric toxicity risk.
Disadvantages
Bioavailability inferior to bisglycinate. Slow effect (not acute).
Ideal use: any 2026 pro Fe supplementation · pregnancy · athletes · RLS.
Ideal use (limited): only if bisglycinate unavailable · extreme budget.
Ideal use: intermediate bisglycinate/sulfate alternative.
Ideal use: pediatric young children · accidental prevention.
4 forms with different bioavailability + tolerance · canon bisglycinate · avoid sulfate if tolerance OK · fumarate second option.