Vitamin D in the UK: an urgent call to redefine the threshold for deficiency.

ACT NOW Vitamin D is a multidisciplinary collective tackling persistent public health concerns in the UK, including widespread vitamin D deficiency, nutritional rickets in children, and osteomalacia across the lifespan. National Diet and Nutrition Survey data show significant declines in vitamin D intake - especially among lower intake percentiles - and rising rates of severe vitamin D deficiency (serum 25-hydroxyvitamin D <25 nmol/L) among people aged 4-64 years. Contributing factors include reduced meat, fish and seafood consumption, increasing obesity, and growing high-risk ethnic minority population groups. However, the current UK biochemical definition of vitamin D deficiency (<25 nmol/L), as defined by the Scientific Advisory Committee on Nutrition (SACN 2016), may underestimate the broader scale of population risk and obscure the clinical burden described above. This position statement therefore makes the case for UK policymakers to adopt a higher threshold for vitamin D deficiency than the current <25 nmol/L recommended by SACN 2016. Raising the threshold to <50 nmol/L would bring the UK into closer alignment with international and clinical practice frameworks and may enable a broader and preventive approach to surveillance of national vitamin D status. Such reclassification would also provide a stronger evidential basis for policy review aimed at improving population-level "Vitamin D Health".