Carbohydrate

Biochemical Classification

Biochemically, carbohydrates can be classified by their chain length. The 4 main groups are mono-, di-, oligo- and polysaccharides.

Monosaccharides

As the name suggests monosaccharides are made up of one unit (or sugar). Examples of these are glucose, fructose and galactose.

Glucose - The significance of glucose to nutrition is tremendous. It links with other sugars to form disaccharides and is the unit from which polysaccharides are almost exclusively made.

Fructose - Fructose is the sweetest of the sugars. It is found abundantly in fruits, honey and saps

Galactose - Galactose seldom occurs free in nature. It binds with glucose to form the sugar present in milk (lactose).

Disaccharides

When two monosaccharides are joined together they are called a disaccharide. This is in fact how we get what is know as table sugar, Table sugar is a disaccharide of glucose and fructose. The main carbohydrate found in milk is lactose. This is a disaccharide of glucose and galactose

Oligosaccharides

Oligosaccharides are made up of 3-9 units. The units are linked by what are known as b-2-1 links. These links are similar to those found in fibre and cannot be digested by human enzymes.

Oligosaccharides are present in various plant foods including, bananas, wheat, garlic, onions, rye, chicory,

They are sometimes used in the food industry to sweeten low calorie foods and to improve the texture of reduced-fat foods.

There has been recent interest in oligosaccharides and human health. It has been suggested that oligosaccharides may be used as a prebiotic.

What is a Prebiotic?

A Prebiotic is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one of a limited number of bacteria in the colon that can improve host health.

e.g. Bifidogenic effect - increases the bifidobacteria counts in the colon. These have an "antibiotic-like" effect.

Polysaccharides

Polysaccharides can be made up of hundreds or thousands of linked monosaccharides. For example starches found in plant foods are made up of hundreds of linked glucose molecules.

There are in fact 2 forms of starch. Amylose is a straight chain of repeating glucose molecules whereas amylopectin is a branched chain. This physical difference is important in how foods containing these starches affect your blood sugar levels (see section on glycaemic index)

Dextrins

You may have heard of or seen on food labels "dextrins". Through digestion or food processing, starches can be partially broken down to smaller chains called dextrins.

Fibre

Unlike other polysaccharides that have units linked by a1-4 or a1-6 bonds, the units in fibre are linked by what are known as b1-4 links. Human enzymes cannot break these links and therefore they go through the stomach and intestine, largely undigested. Bacteria secrete enzymes which can cleave these links. Fibre is also known as non-starch polysaccharides.

From a nutritional point of view fibre is often grouped on the basis of its solubility in water.

Fibre has been linked with many aspects of human health including weight control, blood cholesterol levels, constipation, diarrhoea, cancer, diverticulosis and blood glucose control and diabetes.

The biochemical classification is where we get the classification of carbohydrates as "simple carbohydrates" or "complex carbohydrates". Mono- and di-saccharides are grouped as "simple" and polysaccharides as "complex".  One might guess that the simple molecules (table sugar, fruit, dairy foods) would be absorbed more rapidly than the larger ones (breads, cereals, starchy vegetables). Research over the last 2 decades has found that this assumption is not entirely the case. Digestion and absorption do not occur at the same rates for all carbohydrates within a biochemical grouping. A newer system of classification that looks at how the body actual handles different carbohydrates has been devised. This is known as the Glycaemic Index.