Functional Vitamin D is required for absorption of calcium from the intestine and for regulating calcium and phosphorus deposition to ensure healthy bone density. More recently it has been found that vitamin D is also essential for maturation of neuronal stem cells. Activation of vitamin D is a multi-enzyme process, which requires a contribution of several co-factors including iron, vitamin B2 and vitamin B12. We have used urinary phosphoric acid as a marker of functional vitamin D deficiency, and have compared various urinary metabolic markers with phosphoric acid levels to follow the essential elements in vitamin D activation. Levels of phosphoric acid were increased in each of iron, vitamin B2 and vitamin B12 deficiency, with the greatest increase seen in low functional vitamin B12.