Two forms: vitamin D2, vitamin D3
Sources:
Activation of vitamin D
biologically inert and must undergo two hydrocylation steps for activation
Physiological roles:
serum concentration of 25 (OH)D → assess the vitamin D status
abetalipoproteinemia
poor absorption of dietary fat
Friedreich type of spinocerebellar ataxia
defects in hepatic alpha-tocopherol transferase