Biochemistry and Human Nutrition

Module 4. Human nutrition

Lesson 27
VITAMINS

27.1 Introduction

A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound (or related set of compounds) is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and on the particular organism. (Fig. 27.1 Sources of vitamins)

• The term vitamin was derived from "vitamine," a combination word made up by Polish scientist Casimir Funk from vital and amine, meaning amine of life, because it was suggested in 1912 that the organic micronutrient food factors that prevent beriberi and perhaps other similar dietary-deficiency diseases might be chemical amines. This proved incorrect for the micronutrient class, and the word was shortened to vitamin.
• By convention, the term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids (which are needed in larger amounts than vitamins), nor does it encompass the large number of other nutrients that promote health but are otherwise required less often. Thirteen vitamins are presently universally recognized. (Table 24.1)
• Vitamins have diverse biochemical functions. Some have hormone-like functions as regulators of mineral metabolism (e.g., vitamin D), or regulators of cell and tissue growth and differentiation (e.g., some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and sometimes vitamin C).
• The largest number of vitamins (e.g., B complex vitamins) function as coenzymes, that help enzymes in metabolism. In this role, vitamins may be tightly bound to enzymes as part of prosthetic groups: For example, biotin is part of enzymes involved in making fatty acids. Vitamins may also be less tightly bound to enzyme catalysts as coenzymes, detachable molecules that function to carry chemical groups or electrons between molecules.

27.2 Classification of Vitamins

• Classically, vitamins have been divided into two groups based on their solubilities in fat solvents or in water.
• Fat-soluble vitamins include A, D, E and K, while vitamin of the B-complex and C are classified as water soluble.
• Fat soluble vitamins are found in foodstuffs in association with lipids. The fat soluble vitamins are absorbed along with dietary fats, apparently by mechanisms similar to those involve in fat absorption. Conditions favorable to fat absorption, such as adequate bile flow and good micelle formation, also favor absorption of the fat-soluble vitamins. (Fig. 27.2)
  

• Three of the four fat-soluble vitamins (vitamins A, D and E) are well stored in appreciable amounts in the animal body. Except for vitamin B12, water soluble vitamins are not well stored, and excesses are rapidly excreted.

• Fat-soluble vitamins are excreted primarily in the feces via the bile, whereas water soluble vitamins are excreted mainly in the urine.
• Water soluble vitamins are not associated with fats, and alterations in fat absorption do not affect their absorption. (Fig. 27.3)
  

• A continual dietary supply of the water soluble vitamins and vitamin K is needed to avoid deficiencies.

27.3 Water Soluble Vitamins

• B-complex vitamins and vitamin C are water-soluble vitamins that are not stored in the body and must be replaced each day.
• These vitamins are easily destroyed or washed out during food storage and preparation.
• The B-complex group is found in a variety of foods: cereal grains, meat, poultry, eggs, fish, milk, legumes and fresh vegetables.
• Citrus fruits are good sources of vitamin C.
• Use of megadoses of vitamins is not recommended. The following animations will describe about these vitamins viz. Fig. 27.4 Thiamine (B1), Fig. 27.5 Riboflavin (B2), Fig. 27.6 Niacin (B3), Fig. 27.7 Pantothenic acid (B5), Fig. 27.8 Pyridoxine (B6), Fig. 27.9 Folic acid (B9), Fig. 27.10 Cobalamin (B12), Fig. 27.11 Biotin (B7), Fig. 27.12 Ascorbic acid (C), and Fig. 27.13 Summary of Water Soluble Vitamins

27.4 Fat Soluble Vitamins

• Small amounts of vitamins A, D, E and K are needed to maintain good health.
• Foods that contain these vitamins will not lose them when cooked.
• The body does not need these every day and stores them in the liver when not used.
• Most people do not need vitamin supplements.
• Megadoses of vitamins A, D, E or K can be toxic and lead to health problems .

The following animations will describe about these vitamins viz. Fig. 27.14 Vitamins A, Fig. 27.15 Vitamins D, Fig. 27.16 Vitamins E, Fig. 27.17 Vitamins K, and Fig. 27.18 Summary of Fat Soluble Vitamins