Macronutrients: Carbohydrates (AQA GCSE Food Preparation & Nutrition)
Revision Note
Written by: Emma Archbold
Reviewed by: Lára Marie McIvor
Macronutrients: carbohydrates
Carbohydrates are a collection of biological molecules that include simple sugars, starches, and fibre
The main function of carbohydrates in the body is to act as an energy source
Carbohydrates contain glucose, which is released during digestion and used in respiration
Glucose is the preferred energy source for body cells, so consuming carbohydrates reduces the need to break down and use other energy sources, like protein
Sources of carbohydrate
Carbohydrates are present in foods that are high in sugar, starch or fibre, e.g.
fruit
cake
bread
potatoes
vegetables
Carbohydrate deficiency and excess
Carbohydrates are essential for energy release in the body, and a diet too high or low in carbohydrate can cause problems
A diet that is deficient in carbohydrates can lead to problems such as:
symptoms of low blood sugar, e.g. fatigue, hunger and dizziness
needing to break down protein in the muscles to release glucose for respiration
This can lead to muscle wastage, even in people who consume protein in their diet
a lack of dietary fibre that increases the risk of bowel issues like constipation, and also increases the risk of bowel cancer
A diet that contains an excess of carbohydrates can also cause problems:
Consuming a diet high in sugar can lead to weight gain and obesity
Being obese increases the risk of heart disease
Sugar can cause blood sugar levels to fluctuate, leading to conditions such as type 2 diabetes
Tooth decay is linked to a diet high in sugar
Consuming too much fibre can reduce the ability of the digestive system to absorb iron and calcium, resulting in mineral deficiencies
Dietary reference values
Dietary reference values state that 50 % of the energy supplied to the body should come from carbohydrate sources
Most of this 50 % should be supplied by starchy foods, with no more than 5 % coming from free sugars, e.g. sugars found in refined sugar and fruit juice
Dietary reference values for carbohydrates table
Type of carbohydrate | Dietary reference value |
---|---|
Free sugars, e.g. table sugar, fruit juice | <5 % of daily energy intake |
Starches and sugar in milk and whole fruits | 45 % of daily energy intake |
Dietary fibre | 30 g per day for adults |
Forms of carbohydrate
Sugars
Sugars, or simple carbohydrates, can be in the form of monosaccharides and disaccharides
Monosaccharides are single sugar molecules
These sugars do not need to be broken down in the digestive system and are absorbed quickly into the blood
Examples include:
glucose
fructose
Disaccharides are double sugar molecules that are made up of two monosaccharides joined together
Double sugar molecules must be broken down into monosaccharides during digestion before being absorbed into the blood
Examples include:
sucrose, found in caster, granulated, demerara and icing sugar
lactose, found in milk
maltose, found in cereals
Monosaccharides and disaccharides diagram
Sugars can be absorbed easily into the blood and so can cause blood sugar levels to spike
While this can be useful in some situations, e.g. during sport, repeatedly fluctuating blood sugar levels can lead to health problems
Sources of dietary sugar include:
fruit and fruit juices
honey
refined sugar products, e.g. granulated sugar and golden syrup
dairy products, e.g. milk and yoghurt
cereals
confectionary, e.g. biscuits and chocolate
jam
soft drinks
sauces, e.g. ketchup
soups
Starches
Starches are known as polysaccharides because they are made up of long chains of glucose joined together by chemical bonds
These chemical bonds must be broken down during digestion and therefore it takes more time to release the glucose into the blood
Starch is described as a slow-release carbohydrate for this reason
Polysaccharide diagram
Starches exclusively come from plant products because only plants store their sugar as starch; examples include:
root vegetables, e.g. potatoes and sweet potatoes
bread
pasta
cereals
lentils and beans
Dietary fibre
Dietary fibre is a polysaccharide found in the cell walls of plants
This polysaccharide is known as cellulose
Fibre cannot be digested by the human digestive system, meaning that it provides benefits such as:
slowing digestion and increasing feelings of fullness, so reducing snacking between meals
absorbing water and aiding the movement of digested food through the intestines; this reduces constipation
reducing the risk of bowel cancer
Foods that are high in dietary fibre includes:
wholegrain foods such as bran cereal and wholegrain bread
fruits and vegetables
brown rice
oats
nuts
legumes, e.g. beans and lentils
formulatehealth, via Wikimedia Commons
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