Unit 9
Fat-Soluble Vitamins: A, D, E, and K

Although each fat-soluble vitamin—A, D, E, and K—has distinct physiological functions in the body, all share the following characteristics:

• they are present in fatty or oily portions of foods; bile and digestive enzymes are required to release them.
• once released, they form aggregates (micelles) with long-chain dietary fatty acids for absorption. Any condition that interferes with fat absorption will also reduce absorption of the fat-soluble vitamins.
• after absorption into the intestinal mucosal cells, the fat-soluble vitamins are incorporated into chylomicrons, which travel through the lymphatic system into the general blood supply. The fat-soluble vitamins are then taken up by the liver where they are stored or changed into biologically active forms to be transported by special protein carriers to various body sites.
• since the fat-soluble vitamins are insoluble in water, they cannot be excreted in the urine. Only small losses may occur through the bile, and thence to the feces. Most of these vitamins remain in the liver and adipose tissue, and the body can draw on the storage when dietary intakes are low. Hence, deficiency symptoms may take a long time to develop. When intake is high, stores can accumulate to toxic levels.
• they are generally more resistant to loss during the cooking and processing of foods. They are fairly stable to heat, and are not lost in cooking liquid. The most susceptible to destruction is vitamin E.

In this unit, we provide a brief overview of each of the fat-soluble vitamins, A, D, E, and K: the functions, deficiency and toxicity symptoms, and major food sources. Where appropriate, we also discuss their therapeutic uses and chemical stability.

This unit consists of the four sections listed below.

• Section 9.1  Vitamin A
• Section 9.2  Vitamin D
• Section 9.3  Vitamin E
• Section 9.4  Vitamin K