Properties Of Fat Soluble Vitamins

The fat-soluble vitamins are A, D, E and K. Because these oil-based nutrients are readily stored in your tissues, toxicities of fat-soluble vitamins --- particularly A and D --- are more likely to occur than with water-soluble vitamins, such as vitamin C or the B complex vitamins. Likewise, the therapeutic window, which is the difference between effectiveness and toxicity, is much narrower for fat-soluble vitamins.

Vitamin A

Vitamin A, or retinol, is available in several different forms in foods and supplements. Animal sources of vitamin A, such as liver or eggs, contain preformed vitamin A, either as retinol or a retinyl ester, such as retinyl palmitate. Plant sources contain pro-vitamin A, usually in the form of carotenoids, such as beta-carotene. According to a 2011 review in "Nutrients," your body converts the various forms of vitamin A to a usable form. Vitamin A is necessary for good eye health, normal tissue healing, healthy skin and normal chromosomal function. It also serves as an antioxidant in your tissues by neutralizing free radicals, particularly the singlet oxygen radical.

Vitamin D

Vitamin D, or calciferol, is often called the "sunshine vitamin," because your skin produces a biologically active form of the vitamin, called cholecalciferol, when you are exposed to ultraviolet light. Vitamin D is best known for its effects on calcium metabolism and bone growth. Without enough vitamin D, your bones do not form well, which causes a condition called rickets in children and contributes to osteoporosis in adults. Following the discovery that most of your cells and tissues possess vitamin D receptors, scientists have linked this nutrient to several other important processes, such as normal immune function.

Vitamin E

Vitamin E actually consists of two families of compounds with varying levels of biological activity. The tocopherols consist of eight slightly different molecules, only four of which have demonstrated activity in the human body. Alpha-tocopherol is the most common and most active of these compounds, and it is the only form of vitamin E stored in your tissues, according to the Linus Pauling Institute. The tocotrienols are four additional vitamin E-like molecules whose functions are not yet fully understood, but they appear to alter lipid metabolism in your cells. Vitamin E functions primarily as an antioxidant, particularly within your cell membranes and in other lipid-rich environments.

Vitamin K

Vitamin K occurs in three forms, two of which are found in nature. Phylloquinone is the form of vitamin K found in foods, menaquinone is manufactured by bacteria in your large intestine, and menadione is a synthetic form. Vitamin K is required for the enzymatic completion of several proteins in your blood called clotting factors. Clotting factors are responsible for helping to form a thrombus, or clot, following any injury to your blood vessels, thus preventing excessive hemorrhage. Warfarin, a medication used to prevent clotting, interferes with the recycling of vitamin K in your liver, thereby preventing the formation of normal clotting factors.

Considerations

In the United States, the Institute of Medicine establishes dietary reference intakes for vitamins, minerals and other nutrients. These guidelines are generally intended to inform you about the minimum daily requirements for preventing deficiency syndromes. Since your body stores all of the fat-soluble vitamins to some extent, the IOM has also established daily upper limits to help you avoid the side effects of excessive consumption of these nutrients.