21st Century's Calcium 500 + D3, similar to its counterpart, is a highly economical and popular supplement. It provides calcium sourced from oyster shell, which is a common form of calcium carbonate, and importantly, includes Vitamin D3 as indicated by its name. Vitamin D3 is crucial for maximizing calcium absorption and ensuring its proper integration into bone tissue. The product's main appeal lies in its exceptionally low price per serving, making it an excellent choice for budget-conscious consumers seeking foundational bone, joint, and heart health support. While the oyster shell source might raise minor concerns for some regarding heavy metals (though reputable brands typically purify), its widespread positive reviews affirm its reliability and value.