Sugars, a major form of carbohydrates, are found probably in all green plants. There are three main simple sugars -sucrose, fructose, and glucose. Sucrose is a amalgamation of fructose and glucose and the body quickly breaks it down into simple sugars.

This all-inclusive review of the fundamental biochemical and physiological processes concerned in the digestion and metabolism of sugars emphasizes that the source of sugars in foods does not, in itself, affect the rate of absorption or the metabolism of sugars. Nevertheless, the forms in which sugars are ingested, and the physical and chemical properties of food, do affect the rates of absorption.

Monosaccharides such as glucose and fructose have a low molecular weight. Consequently, they cross the mucosal cell membranes in the small intestine by simple diffusion. Glucose, the major C_nH_2nO_n species entering the mucosal enterocyte, is absorbed via a carrier-mediated co-transporter system. This transporter system is sodium dependent and research shows that feeding carbohydrate-rich diets can induce it.

Disaccharides like sucrose and lactose are too bulky to cross over the mucosal membrane and have to be hydrolyzed prior to absorption. Sucrose is somewhat effortlessly hydrolyzed in weak acidic conditions at ambient temperatures. Disacchardases as sucrases, lactases etc in the margin membranes of the intestinal mucosal break down sucrose and lactose to monosaccharides.

The absorption rate of sugars is dependent on the form in which they are ingested, the collective effects of individual food patterns on gastric emptying and the physical properties of the intestinal contents. Gastric emptying is influenced by a number of factors- soluble components of a meal are emptied from the stomach before the insoluble components; the greater the volume of stomach contents, the slower the rate of emptying. 
Osmolarity plays an important role - high solute concentrations delay emptying; low gastric pH & fat emulsions inhibit emptying. As soon as sugars have been absorbed, their transport and metabolism is analogous and the dietary source differences cease to affect.

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Monosaccharides

Disaccharides

Starches

Glycogen

Cellulose

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