Bone density is a measure of the mineral content packed into a given volume of bone, principally calcium and phosphorus in the form of hydroxyapatite crystals. High density gives bone its hardness and resistance to fracture, while low density characterises osteopenia and osteoporosis. It is assessed clinically with dual-energy X-ray absorptiometry and expressed as T-scores or Z-scores. Dietary calcium, vitamin D, and adequate protein support peak bone mass, which is built during youth and maintained through weight-bearing exercise. Food manufacturers fortify products such as milk, plant-based drinks, and cereals with calcium and vitamin D to support population bone health.