Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds. Proteins include structural proteins, enzymes, nucleoproteins, proteins that transport oxygen, muscle protein, and many other types of proteins that have specific functions (Guyton, 1986). Proteins differ from one another in their nutritive value and maybe classified into three groups (Pruthi, 1999):
Fats are organic compounds made up of carbon, hydrogen, and oxygen. Fats are the most concentrated source of energy in foods, and they belong to a group of substances called lipids (Food and Nutrition Board, 2005). Dietary fat consists primarily (98 %) of triacylglycerol, which is composed of one glycerol molecule esterified with three fatty acid molecules, and smaller amounts of phospholipids and sterols (Food and Nutrition Board, 2005). Fatty acids are hydrocarbon chains that contain a methyl (CH3-) and a carboxyl (-COOH) end (Food and Nutrition Board, 2005). The fatty acids vary in carbon chain length, and may be saturated or unsaturated depending on their proportions (Food and Nutrition Board, 2005). Sources of fat include cooking fats and oils, butter, margarine, fried foods, animal products etc (Pruthi, 1999).
Minerals are found as a constituent of the mineral matter of the bones, as structural constituents of the soft tissues and as constituents of physiologically active substances e.g. Iron in haemoglobin. The important minerals are calcium, phosphorous and iron.
Vitamins are nutrients and biomolecules required in small amounts (Lieberman &. Bruning 1990. Pruthi 1999.). The vitamins include: Vitamin A (retinal), Vitamin B group (B1-thiamine, B2-riboflavin, B6-pyridoxine, B12-cobalamin), Vitamin C (ascorbic acid), Vitamin D (calciferol), Vitamin E (tocopherol) and Vitamin K, biotin, folate, pantothenic acid and Niacin (Lieberman &. Bruning 1990).
Carbohydrates and fats serve as the main sources of