Macronutrients-Carbohydrates

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

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

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