Foods as Nutrition Source

Foods consumed by human to continue their life in the world. The main of content in foods are carbohydrate, protein and fat. In a simple word, carbohydrate is Sugars and starches, which provide 50–70% of energy intake. Chemically they are composed of carbon, hydrogen and oxygen in the ratio Cn:H2n:On. The basic carbohydrates are the monosaccharide sugars, of which glucose, fructose and galactose are nutritionally the most important. Disaccharides are composed of two monosaccharides: nutritionally the important disaccharides are sucrose, lactose, maltose and trehalose. A number of oligosaccharides occur in foods, consisting of 3–5 monosaccharide units; in general these are not digested, and should be considered among the unavailable carbohydrates. Larger polymers of carbohydrates are known as polysaccharides or complex carbohydrates. Nutritionally two classes of polysaccharide can be distinguished:
(a) starches, polymers of glucose, either as a straight chain (amylose) or with a branched
structure (amylopectin);
(b) a variety of other polysaccharides which are collectively known as non-starch polysaccharides (NSP) and are not digested by human digestive enzymes. The carbohydrate reserve in liver and muscles is glycogen, a glucose
polymer with the same branched structure as amylopectin. The metabolic energy yield of carbohydrates is 17 kJ (4 kcal)/g.
More precisely, monosaccharides yield 15.7kJ (3.74kcal), disaccharides 16.6 kJ (3.95 kcal) and starch 17.6 kJ (4.18 kcal)/g.
glycerol is a three-carbon sugar alcohol, and is classified as a carbohydrate; it yields 18.1 kJ (4.32 kcal)/g.