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March 2017
Dietetics
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CARBOHYDRATES

    CARBOHYDRATES

What they are

Carbohydrates are chemical substances made up of carbon, hydrogen and oxygen and they can also be called polyhydroxy aldehydes and ketones.

What they do 

Carbohydrates have a dual function; plastic and energetic. They have a plastic function in as much as they play a part in creating essential structures for living organisms, and an energetic function because they provide organisms with energy to carry out different functions.

Requirements

Since the human body is able to synthesise carbohydrates from other nutrients, carbohydrates cannot be considered ‘essential’ nutrients. Having said this, it is still necessary to maintain a certain level of glycaemia in the body and this level depends on the needs of the central nervous system and red blood cells. The recommended daily amount of carbohydrates is around 55-60% of the  total amount of calories we consume. However, the total amount of simple sugars should not be more than 10-12% of our total calorie intake because consuming too many simple sugars provides unnecessary energy. Foods that contain complex carbohydrates, on the other hand, provide energy at a slower rate, with respect to simple carbohydrates, as well as providing some fundamental nutrients that are needed for a healthy, balanced diet. This factor is also relevant when energy intake must be limited to small portions, as is necessary for those who follow a sedentary lifestyle, for example.

Chemical make up and sources of carbohydrates

As mentioned, carbohydrates are chemical substances made up of carbon, hydrogen and oxygen and they can also be called polyhydroxy aldehydes and ketones.  

In relation to their complexity, carbohydrates are classified as:

Monosaccharides

These contain 3-9 carbon atoms and they are the most simple carbohydrate structures. Glucose, fructose and galactose are the most biologically important monosaccharides. Glucose is usually not present naturally and is only found in small quantities if it is, for example in fruit and vegetables. Fructose is present in fruit and honey.
Disaccharides These can be considered as a union of 2 monosaccharide molecules connected by glycosidic bonds. The disaccharides that are important for our bodies are sucrose, lactose and maltose. Sucrose is made of glucose and fructose and is extracted to make table sugar. Lactose is contained in milk and is formed by glucose and galactose. Maltose (glucose and glucose) comes from the fermentation (and digestion) of starch.
Oligosaccharides The term oligosaccharide is generally used for compounds made up of 3-10 monosaccharides. They contain sugars, such as raffinose, stachyose and verbascose, which we cannot digest and which is made up of galactose, glucose and fructose and is found in pulses. (The production of gas following the fermentation of these sugars in the large intestine explains the flatulence that some people experience after consuming pulses.)
Polysaccharides The term polysaccharides is generally used for compounds formed by more than 10 monosaccharides.
Starch This is the energy reserve of the plant world. The main sources of starch are staple foods (bread, pasta, rice) and potatoes. It is present in the form of a semi-crystalline granular structure and when foods are cooked this structure is altered (the process of gelatinisation), making starch digestible. Cooling down foods, which leads on to phenomena of partial recrystallisation of starch, reduces its digestibility slightly.
Glycogen This comes from animal based foods, however, in animal based foods its nutritional significance is almost meaningless as there is only a small amount of it present: after the animal dies, glycogen is turned into lactic acid due to anoxia.

  

Major carbohydrates Main food sources Digestibility Products following digestion
Monosaccharides
Glucose Fruit and honey Very good Glucose
Fructose Fruit and honey Very good Fructose
Disaccharides
Sucrose Sugar cane and beet Very good Glucose and Fructose
Lactose Milk and dairy products Incomplete in adults Glucose and Fructose
Polysaccharides
Starch and dextrins Cereals, root vegetables, pulses, etc. Very good Glucose
Glycogen Meat and fish Very good Glucose
Inulin Jerusalem artichoke and onion Partial Fructose
Mannans Pulses Very low Mannose
Pentoses Fruit and gums Very low Pentose
Cellulose Plant leavesand stems, outer shell of seeds (bran) wholegrain cereals, pulses, fruit Partially digestible by bacteria in the large intestine Glucose
Pectin Fruit, carrots, potatoes, sweets Partially digestible by bacteria in the large intestine Galactose, Arabinose

CARBOHYDRATES

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