These vitamins are metabolized along with fat in the body and require fat for absorption in the gut. The fat-soluble vitamins may be stored in large amounts, and this gives them the potential to cause toxicity if consumed in high amounts. Deficiencies are rare in adults but may be seen in children. Megadosing of fat soluable vitamins, except where indicated by a medical professional, is potentially dangerous and should be avoided. Two of the fat-soluble vitamins, A and D, have hormone like actions, causing specific cells to increase or decrease the expression of certain genes.
Several compounds have vitamin A activity and they are referred to as retinoids.
They function with certain carotenoids to protect against the development of certain cancers and to enhance immune function. They may also have antioxidant activity. Deficiency Vitamin A deficiency causes night blindness , a condition in which vision is impaired in dim light. Dryness and ulceration of the eyes, skin eruptions and dryness, abnormal cells of the mucous membranes, urinary stones, and impaired taste and smell also characterize the deficiency. Many children in developing countries have irreversible blindness resulting from vitamin A deficiency.