Iron forms: a reference table
| Iron form | Elemental iron | Typical absorption | GI tolerability | Notes |
|---|---|---|---|---|
| Ferrous sulfate (standard) | 20% of tablet weight | 15–25% | Poor — constipation, nausea, dark stools | Most prescribed; lowest cost |
| Ferrous bisglycinate (chelated) | 20% of capsule weight | 20–30% | Good — minimal GI effects | Best-tolerated therapeutic form; preferred in clinical practice |
| Ferric pyrophosphate (slow-release) | Variable | 10–20% | Excellent | Used in IV and food fortification; lower oral bioavailability |
| Heme iron (from animal sources) | N/A (structural) | 25–35% | Excellent | Absorbed via separate haem transporter — independent of enhancers/inhibitors |
| Spirulina (non-heme food matrix) | 4–8 mg per 5 g | ~20–28% (estimated) | Excellent — no known GI side effects from iron specifically | Food-matrix form; lower phytate competition than legumes |
How much iron does spirulina actually provide?
Spirulina contains approximately 0.8–1.6 mg iron per gram — or 4–8 mg iron per 5 g serving. This varies by production batch, growing conditions, and processing method.
At an estimated 20–28% absorption rate (non-heme food-matrix iron, with low phytate competition), a 5 g dose provides approximately 0.8–2.2 mg absorbed iron per dose. With vitamin C pairing, this can increase to approximately 1.6–4.4 mg absorbed iron — a clinically meaningful contribution.
Compare this to:
- Ferrous sulfate 325 mg tablet: provides 65 mg elemental iron, ~10–16 mg absorbed (at 15–25% absorption)
- Ferrous bisglycinate 27 mg elemental iron capsule: ~5–8 mg absorbed (at 20–30% absorption)
- Spirulina 10 g with vitamin C: ~3–8 mg absorbed iron
When spirulina iron is sufficient
Spirulina is the right iron source for:
- Maintenance in iron-adequate individuals:People with borderline-low dietary iron intake who want to maintain adequacy without supplementing. The food-matrix form and lack of GI side effects make it practical for daily long-term use.
- Vegetarians and vegans: Iron deficiency risk is roughly 2× higher in vegetarians; spirulina is among the most bioavailable plant iron sources with the lowest phytate competition.
- People who cannot tolerate standard supplements:Ferrous sulfate causes constipation and nausea in 30–40% of users at therapeutic doses. Spirulina iron provides lower doses with no known GI iron effects.
- Athletes with mild sports anaemia:Exercise-induced iron losses are modest; maintenance rather than repletion is usually needed. Spirulina addresses the iron need while also providing the anti-inflammatory phycocyanin that reduces hepcidin-mediated iron restriction.
- Pregnant women with mild deficiency (confirmed safe product):Food-matrix iron is preferred during pregnancy; spirulina provides iron alongside folate, B vitamins, and protein. Requires batch-tested product with low heavy metals.
When dedicated iron supplementation is needed
Spirulina is not sufficient for:
- Confirmed iron deficiency anaemia (Hb below 12 g/dL in women, 13 g/dL in men): Therapeutic repletion requires 100–200 mg elemental iron per day — achievable only with dedicated iron supplements. Spirulina cannot replicate these doses.
- Severely depleted ferritin (below 12 ng/mL):At this level, tissue iron depletion is significant and takes months to correct even with therapeutic supplementation. Spirulina alone would take years.
- Post-surgical iron deficiency:Hospital protocols typically require IV iron or high-dose oral supplementation for rapid repletion.
The phycocyanin-hepcidin advantage
Spirulina offers a unique mechanism that no iron supplement provides: phycocyanin reduces CRP and systemic inflammation, which in turn reduces hepcidin production.
Hepcidin is the hormone that blocks intestinal iron absorption and traps iron in macrophages during inflammation — the “functional iron deficiency” of chronic disease. By reducing inflammation, spirulina can improve iron availability even in people whose measured iron intake is adequate but whose absorption is suppressed by elevated hepcidin.
This mechanism is particularly relevant for:
- Athletes (exercise acutely raises hepcidin for 3–6 hours)
- People with chronic inflammatory conditions
- Vegans with consistently elevated CRP from dietary patterns
The combination approach
For iron-deficient people, the most evidence-aligned approach is a combination:
- Therapeutic phase (first 3 months):Ferrous bisglycinate 25–36 mg elemental iron/day for repletion. Spirulina runs alongside for phycocyanin and overall nutritional support.
- Maintenance phase (ongoing):Reduce or eliminate the iron supplement; continue spirulina 10 g/day with vitamin C for maintenance iron. Recheck ferritin every 6 months.
This approach uses the appropriate tool for each phase — therapeutic supplement for rapid repletion, food-matrix spirulina for maintenance.
Iron absorption optimisation with spirulina
Whether using spirulina alone or alongside supplements:
- Always pair spirulina with 200–500 mg vitamin C — doubles to triples non-heme iron absorption
- Take at least 1 hour after coffee or tea (tannins reduce absorption 60–70%)
- Take at least 1–2 hours away from dairy or high-calcium foods (calcium reduces absorption 30–50%)
- If also taking a calcium supplement, separate by 2+ hours