Leo Sher, M.D.

Vitamin D, also called calciferol is a lipid soluble vitamin which exists in two major forms: vitamin D2 (ergocalciferol, which is largely ingested) and vitamin D3 (cholecalciferol, which is synthesized in human body). Both forms are inactive form which are converted into active form by two enzymatic hydroxylation reactions, first in liver forming 25-hydroxyvitamin D mediated by 25-hydroxylase and second in kidney mediated by 1-alpha-hydroxylase forming the final activated product calcitriol (1,25-dihydroxyvitamin D).
 
Vitamin D functions as a hormone. It stimulates intestinal calcium absorption and is important in maintaining adequate phosphate levels for bone mineralization and bone growth. Vitamin D is also involved in the regulation of cell growth proliferation and apoptosis (programmed cell death), regulation of the immune system and other biological functions. Alone or in combination with calcium, vitamin D has been shown to reduce the risk of fractures in elderly men and postmenopausal women. Vitamin D deficiency may lead to rickets in infants and osteomalacia (softening of the bones) in adults.

Epidemiological, clinical and animal studies suggest that low vitamin D status may be associated with neurological and psychiatric conditions, including multiple sclerosis, Parkinson’s disease, dementia, autism, depression, schizophrenia and other psychotic disorders. It has been proposed that vitamin D may be involved in the pathophysiology of seasonal affective disorder. Possibly, vitamin D influences particular neurotransmitters and cortical function.