Lithothamnium, a genus of red algae, is used as a natural source of calcium. It is primarily composed of calcium carbonate, along with trace minerals like magnesium and iron. The rationale for its use in supporting or treating calcium deficiency is based on its high natural calcium content. Several in vitro analyses confirm that Lithothamnium-derived supplements can provide significant quantities of bioavailable calcium, and it is approved as a calcium source in certain food products and supplements.

However, while there is scientific validation that Lithothamnium is a rich source of calcium and that its calcium is bioavailable, robust clinical evidence directly demonstrating its effectiveness in treating or preventing calcium deficiency in humans is limited. Most studies focus on its composition, absorption rates, and safety profile rather than clinical outcomes like improved bone mineral density or reduction in deficiency symptoms. Some small-scale human studies and animal studies suggest Lithothamnium calcium may be as bioavailable as calcium carbonate from other sources, but these are not large or long-term clinical trials.

In summary, the use of Lithothamnium for calcium deficiency is justified by its chemical composition and some bioavailability studies, but high-quality clinical evidence is lacking; thus, the evidence rating is moderate.