Blue-green algae, particularly species such as Arthrospira (commonly called spirulina) and Aphanizomenon flos-aquae, have been marketed as dietary supplements, sometimes for their mineral content, including calcium. Scientific analyses confirm that certain blue-green algae are relatively rich in calcium compared to some other plant-based foods. For example, spirulina may contain approximately 120 mg of calcium per 100 grams of dry weight. However, recommended daily servings (often a few grams) contribute only modestly to overall calcium intake.

There is limited scientific research directly evaluating the effectiveness of blue-green algae supplementation in treating or preventing calcium deficiency in humans. Most available studies focus on the nutritional profile of the algae rather than clinical outcomes. While blue-green algae can be a supplemental source of calcium, they are not considered a primary or significant treatment for calcium deficiency, especially compared to conventional dietary sources (such as dairy or fortified foods) or calcium supplements. The calcium bioavailability from blue-green algae also has not been well-established in human studies.

In summary, there is some scientific rationale for blue-green algae as a minor calcium source, but current evidence supporting its use for treating or preventing calcium deficiency is weak, and it is not a standard or widely recommended intervention for this purpose.