NutriStack
Calcium and Spirulina, timing-sensitive.
When spirulina (taken for its iron and mineral content) is co-ingested with a calcium supplement, the calcium reduces absorption of spirulina's non-heme iron. Human absorption studies show calcium can cut non-heme iron uptake by roughly 40 to 60 percent at common supplemental doses (maximal inhibition around 300 mg calcium per dose), with the effect occurring at the initial mucosal-uptake step. Note that typical spirulina doses (3 to 5 g) supply only about 1 to 1.5 mg of iron, so this matters most for people deliberately using spirulina to support iron intake.
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At a glance
- Pair type
- Timing Sensitive
- Evidence (highest tier)
- Moderate
- Source citations
- 3 sources
- Stack Score effect
- −5 to your Stack Score (per scored timing-sensitive row).
- Scope
- Supplement × Supplement
- Last verified
- May 30, 2026
Timing Sensitive · Moderate evidence
Timing Sensitive
What is happening. When spirulina (taken for its iron and mineral content) is co-ingested with a calcium supplement, the calcium reduces absorption of spirulina's non-heme iron. Human absorption studies show calcium can cut non-heme iron uptake by roughly 40 to 60 percent at common supplemental doses (maximal inhibition around 300 mg calcium per dose), with the effect occurring at the initial mucosal-uptake step. Note that typical spirulina doses (3 to 5 g) supply only about 1 to 1.5 mg of iron, so this matters most for people deliberately using spirulina to support iron intake.
Mechanism. Spirulina contains non-heme iron and other divalent minerals. Calcium competitively inhibits non-heme iron uptake at the intestinal enterocyte, partly via the shared divalent metal transporter (DMT1) and partly by forming poorly soluble complexes in the gut lumen. Taken together at the same time, calcium can blunt absorption of the iron and trace minerals that spirulina is often used to provide.
Recommendation. Separate the two by at least 2 hours (3 to 4 hours is ideal). A practical pattern is spirulina with a vitamin C source earlier in the day and calcium with an evening meal. If spirulina is being used specifically as an iron source (for example in vegetarians or those with low iron stores), avoid pairing it with high-dose calcium (over about 300 to 500 mg) in the same sitting.
Minimum separation. 2 hours (3 to 4 hours preferred)
Sources (3)
- Hallberg L et al. Calcium and iron absorption: mechanism of action and nutritional importance. Eur J Clin Nutr. 1992. PMID 1600930
- Lonnerdal B. Calcium and iron absorption: mechanisms and public health relevance. International Journal for Vitamin and Nutrition Research, 2010.
- Controlled human absorption studies and nutrition reviews on the inhibitory effect of dietary and supplemental calcium on non-heme iron uptake, together with compositional data on the iron and mineral profile of Arthrospira (spirulina).
Stack Score
How this pair moves the number.
Effect on the composite score
If both Calcium and Spirulina are in the same stack, this pair applies −5 to your Stack Score (per scored timing-sensitive row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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