Eggshell is primarily composed of calcium carbonate, along with small amounts of protein and other minerals. Its use in supporting the nails body system is based mainly on its high calcium content. Calcium is an essential mineral for the growth and maintenance of healthy nails, as it plays a role in keratinization, the process by which nail cells are formed and hardened. Some studies have suggested that calcium supplementation may help strengthen brittle nails, although the evidence is not robust and often confounded by other factors such as overall nutritional status.

Eggshell powder has been used as a dietary calcium supplement, and a few studies have demonstrated its bioavailability. However, the direct impact of eggshell on nail health has not been widely studied in controlled clinical trials. Most recommendations for its use in promoting nail strength are extrapolated from its general benefits for bone health and structural tissues, given the shared requirements for calcium. There are no well-established traditional practices specifically linking eggshell to nail health, but its use as a calcium source is longstanding in some cultures.

In summary, while there is a scientific rationale for eggshell as a calcium source that may indirectly benefit nail health, direct evidence supporting its targeted use for the nails body system is limited, warranting a modest evidence rating.