Vitamin D deficiency: Definitions Vitamin D adequacy is traditionally assessed using circulating serum 25-hydroxyvitamin D (25OHD) levels. Concentrations below 25 nmol/L are associated with rickets and histologically evident osteomalacia indicating severe deficiency. Levels between 25 and 50 nmol/L are indicative of moderate deficiency and have been associated with mild elevations in parathyroid hormone(PTH) levels.9 However, there is a growing body of literature to suggest that levels below 75 nmol/L are inadequate. For example, healthy individuals on no vitamin D supplementation who live and work in sun-rich environments (i.e. lifeguards) have 25OHD levels between 100 and 158 nmol/L.10 Furthermore, vitamin D supplementation reduces fracture rates and improves bone mineral density (BMD) when serum 25OHD levels are > 100 nmol/L.11;12 And finally, 25OHD levels between 75 and 100 nmol/L result in maximal suppression of PTH and maximize calcium absorption from the gastrointestinal tract.13;14 This evidence supports the notion that the ideal target serum 25OHD level approximates 75 nmol/L or higher.