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 then through the feces. In excess of need, most fat-soluble vitamins remain in the liver and adipose tissue; the body draws on these stores 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 than water-soluble vitamins to loss during the cooking and processing of foods. They are fairly stable to heat, and are not lost in cooking liquid. The fat-soluble vitamin 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, including their functions, deficiency and toxicity symptoms, and major food sources. Where appropriate, we also discuss their therapeutic uses and chemical stability.

This unit consists of four sections:

9.1—Vitamin A
9.2—Vitamin D
9.3—Vitamin E
9.4—Vitamin K

Objectives

After completing this unit you should be able to do the following:

1. After completing the section on vitamin A, you should be able to
   1. identify its naturally occurring and biologically active forms.
   2. identify its functions in the body.
   3. describe the symptoms of deficiency and the prevalence of deficiency in the Canadian population.
   4. discuss its therapeutic uses.
   5. list the symptoms of toxicity.
   6. list some of its major food sources.
   7. discuss its chemical stability in relation to food preparation practices and other factors.
2. After completing the section on vitamin D, you should be able to
   1. describe the biosynthesis of its active form.
   2. identify its functions in the body.
   3. describe the prevalence of deficiency and the associated symptoms.
   4. describe the Canadian Cancer Society’s current recommendation for vitamin D intake.
   5. identify its dietary and non-dietary sources.
3. After completing the section on vitamin E, you should be able to
   1. identify its function in the body as an antioxidant.
   2. describe its deficiency symptoms.
   3. discuss the claims of benefits for the use of supplements and the possible toxicity symptoms.
   4. list some of its major food sources.
   5. discuss its chemical stability to food preparation practices and other factors.
4. After completing the section on vitamin K, you should be able to
   1. identify its functions in the body.
   2. describe the prevalence of deficiency and its associated symptom.
   3. describe the toxicity symptoms.
   4. list some of its major food sources.