Iron Bisglycinate Dosing for Low Ferritin with Constipation
For an adult with ferritin of 16 ng/mL who struggles with constipation, start with iron bisglycinate 25 mg elemental iron once daily on alternate days (every other day), taken in the morning on an empty stomach with 250-500 mg vitamin C. 1, 2
Why Iron Bisglycinate Is Optimal for Constipation-Prone Patients
Iron bisglycinate chelate offers a critical advantage over ferrous sulfate in patients with gastrointestinal intolerance:
Ferrous bisglycinate provides 25 mg elemental iron per tablet and demonstrates significantly better tolerability than ferrous sulfate, with gastrointestinal adverse events comparable to placebo. 3, 4
Bioavailability of iron bisglycinate chelate reaches 90.9% compared to 26.7% for ferrous sulfate, meaning lower doses achieve equivalent iron repletion while minimizing GI side effects including constipation. 5
In direct comparison studies, iron bisglycinate chelate produced superior ferritin increases at lower doses (15 mg/day bisglycinate vs 40 mg/day ferrous sulfate) with markedly better compliance—73% adequate adherence with bisglycinate versus only 35% with ferrous sulfate. 6
Optimal Dosing Strategy to Minimize Constipation
Alternate-day dosing is superior to daily dosing for both absorption and tolerability:
Taking iron every other day increases fractional iron absorption by 33% (21.8% vs 16.3%) compared to consecutive daily dosing, while simultaneously reducing gastrointestinal side effects. 2
Daily iron doses ≥60 mg trigger hepcidin elevation that persists 24-48 hours, blocking absorption of subsequent doses by 35-45% and increasing constipation without improving efficacy. 1, 2
A single morning dose of 25-50 mg elemental iron on alternate days optimizes the balance between absorption, tolerability, and compliance. 1, 2
Specific Regimen for Your Patient
Given ferritin of 16 ng/mL (severely depleted) and constipation history:
Start: Iron bisglycinate 25 mg (one tablet) every other day in the morning on an empty stomach. 1, 4
Add: 250-500 mg vitamin C with each iron dose to enhance absorption by forming a soluble chelate. 1
Timing: Take 1-2 hours before meals to maximize absorption; if intolerance occurs despite alternate-day dosing, may take with a small amount of food. 1
Avoid: Tea, coffee, calcium supplements, or dairy products within 1-2 hours of the iron dose, as these markedly reduce absorption. 3, 1
Expected Response and Monitoring
Check hemoglobin at 2 weeks: An increase of ≥10 g/L predicts treatment success with 90% sensitivity and 79% specificity. 1
Recheck ferritin at 4 weeks: Ferritin should show measurable improvement; if not, this indicates need for dose adjustment or consideration of IV iron. 1
Continue therapy for 3 months after hemoglobin normalizes to fully replenish iron stores, resulting in total treatment duration of approximately 6-7 months. 1
When to Escalate to Intravenous Iron
Consider IV iron if:
Ferritin fails to increase after 4 weeks of compliant oral therapy despite alternate-day dosing and vitamin C supplementation. 1
Hemoglobin does not rise ≥10 g/L by 2 weeks, which strongly predicts overall treatment failure. 1
Constipation remains intolerable despite alternate-day dosing with iron bisglycinate. 1
Transferrin saturation remains <20% after an adequate oral trial. 1
Critical Pitfalls to Avoid
Do not prescribe daily dosing initially—alternate-day dosing with bisglycinate provides superior absorption and tolerability for constipation-prone patients. 2
Do not use modified-release iron preparations, as they release iron beyond the duodenum (primary absorption site) and are classified as "less suitable for prescribing." 1
Do not switch to ferrous sulfate expecting better results—bisglycinate has 3.4-fold higher bioavailability and markedly better GI tolerability. 5, 6
Do not discontinue therapy when hemoglobin normalizes—continue for an additional 3 months to restore iron stores. 1