4.3 Carbohydrates

Dietary carbohydrates, containing carbon, hydrogen and oxygen, are an essential source of energy, required by the cells to make ATP. Carbohydrates also play a part in body structure and function. For example, the individual cells of the body are joined together with a ‘cement” made of carbohydrate. Cartilage has a high carbohydrate component in its structure, which contains sulphur and the substances carried on blood cells that identify the blood groups are of a carbohydrate character. Many of these structural or functional forms of carbohydrate also contain phosphorus, nitrogen or sulphur as well as carbon, hydrogen and oxygen.

There are no specific forms of carbohydrate that are essential in our diet and if we have an adequate supply of quality carbohydrate, its exact form does not matter. Some forms of carbohydrate we digest and absorb and other types (bound to fibres) pass through the digestive tract relatively unchanged.

The principal forms of carbohydrate are:

• Sugars
• Polysaccharides

The sugars are simple substances, freely soluble in water, whilst the polysaccharides have a more complex structure.

Simple Sugars

The main simple sugars you will come across in foods are:

• Sucrose – a common sugar obtained from beet or sugar cane and can be found in different forms in foods, e.g. granulated, icing, castor, demerara, liquid etc.
• Glucose and fructose – simpler sugars in structure compared to sucrose. Formed when sucrose breaks down, e.g. during digestion, they occur freely in many natural foods such as vegetables and fruit. Fructose can be described as fruit sugar; high fructose corn syrup or fruit juice concentrates on food labels.
• Mannose – less common sugar that occurs in natural foods.
• Lactose – milk sugar (a major component of milk) that is broken down in the intestine to glucose and a sugar called galactose.
• Trehalose and raffinose – special sugars that are characteristic of particular foods; trehalose found in certain plant and animal sources including shrimp; raffinose occurs in plants and whole grains including Brussel sprouts, cabbage and broccoli.

You will notice that all these sugars, and more besides, have the suffix “-ose”. This can help you to identify sugars listed on food labels.

Polysaccharides

The polysaccharides are more complex substances that are built up from simple sugars. In many cases polysaccharides can be broken down again in the body to yield those same component sugars. Significant polysaccharides include:

• Starch – exists in various forms but which is always built up entirely from glucose and from plants, which use starch as an energy store whilst living. You will probably be familiar with the expression ‘starchy foods’ including items like potatoes and bread. In the intestines, starch is broken down entirely to glucose and absorbed into the blood.
• Inulin – a less common polysaccharide found in sources like Jerusalem artichoke and chicory. It’s comprised from fructose and yields simple fructose sugars that are absorbed into the blood stream when it is digested in the intestines. This type of polysaccharide is also classed as a ‘prebiotic’, which supports the growth of population of healthy gut bacteria. We will discuss this further in Module 7.
• Glycogen – like starch as it’s comprised entirely from glucose, but is actually an energy store found exclusively in animals. The human body forms glycogen from any excess glucose and lays it down as a temporary carbohydrate (energy) store in our liver and muscles. Therefore, when we eat liver or meat we consume glycogen, which is digested to glucose again in the intestines.
• Cellulose – a plant polysaccharide and major component of plant fibre comprised entirely from glucose; it is quite different from any other plant polysaccharide as our bodies cannot break it down (although cows and horses can) and passes through our digestive system relatively unchanged.