The fat soluble vitamins are vitamin A, D, E and K. The lipid solubility of these vitamins has implications for their absorption, transport and storage, when compared to their water soluble counterparts. Generally, other lipids are required for the correct absorption of the fat soluble vitamins, and very low fat diets can impair their transport into enterocytes. As with other lipids soluble substances, the fat soluble vitamins are absorbed following emulsification in biles salts and packaged into chylomicrons for transport to the subclavian vein, where they enter circulation. Because they are retained in tissues, particularly adipose tissue, plasma levels can be maintained despite sporadic intakes. However, this retention also increases the likelihood of toxicity if high intakes are maintained for long periods. The exception to this appears to vitamin E, which can be taken in very large intakes for long periods without causing any noticeable side effects. Of the fat soluble vitamins, vitamin D is not a true vitamin, as it can be produced in the skin by the action of ultra violet light on cholesterol. It is therefore technically a steroid hormone.
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