Vitamin D was first discovered in the year 1922. The effects of vitamin D on calcium homeostasis in the human body and on bone health has since been extensively investigated. Vitamin D exists in two forms, namely ergocalciferol (vitamin D2) which is found in plants; and cholecalciferol (vitamin D3), which is available in animals. Cholecalciferol is also produced within the skin following exposure to ultraviolet-B light. Vitamin D 3 is an inactive substance that is later converted in the liver to 25 OHD3 which is a stable circulating form of vitamin D3. This is later on converted in the brain to 1, 25 (OH)2 D3.

Vitamin D has both anti-inflammatory and antioxidant effects. Recently, vitamin D has been researched in the context of cognitive disturbance, mood disorders (unipolar depression and bipolar disorder) and schizophrenia. Vitamin D deficiency is associated with poorer cognitive function. Bipolar disorder and unipolar depression are associated with lower levels of vitamin D than the general population. In addition, vitamin D is implicated in playing multiple roles in the onset of depres- sive episodes, however its replenishment did not ameliorate the depressive symptoms. Lower levels of vitamin D deficiency are noted in schizophrenia. In fact vitamin D supplementation in the prenatal period may be associated with a lower chance of developing schizophrenia in later life.

Vitamin D absorption is affected by several factors such as the molecular form of vitamin D, the presence of concurrent dietary lipids and fibre upon ingestion of vitamin D. Absorption of vitamin D via protein mediated transport depends highly on the affinity of the transporters to different molecular forms of vitamin D. Studies also suggested that 25-hydroxy chole- calciferol (25OHD3) and the dihydroxy cholecalciferol (1,25(OH)2D3) forms were absorbed more efficiently compared to cholecalciferol and ergocalciferol. Absorption of both the metabolites are unaffected by lack of biliary salts. In addition, the hydroxylated 25OHD3 form also has better retention compared to the non-hydroxylated forms.

Due to its properties as a fat-soluble vitamin, it is hypothesised that the concurrent presence of lipids enhances absorption of vitamin D when ingested together in a meal, rendering it more bio-accessible. High fibre intake, on the other hand, is postulated to reduce bioavailability of vitamin D. Bioavailability of vitamin D supplements are documented to be high in lactose capsule form, followed by oil form and finally a powder-based form. Emulsification and encapsulation of vitamin D are also thought to improve absorption of vitamin D.

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