Carbohydrates have many roles in the body:

  • They are involved in energy metabolism.
  • They make up the muscle energy store.
  • They help form the ground substance of connective tissue in the body as well as many more different substances, e.g. lubricants for the joints, transport molecules, structural chemicals.

Carbohydrates contain carbon, hydrogen and oxygen. Although carbohydrate is a good source of energy, body stores don’t actually last very long. The body stores carbohydrates in the form of Glycogen to use as a short-term energy supply.

Carbohydrates are the body’s main source of energy however during fasting, or the consumption of very low carbohydrate diets, glycogen stores will be used up more quickly, on average they will be depleted within 24 hours if not replaced. Once glycogen stores are used up the body will start to break down proteins and fats for energy instead. This is not sustainable in the long term as it results in a condition called ‘ketosis’ characterised by a sickly-sweet ‘pear-drops’ smell on the breath. This condition only happens in cases where carbohydrate intakes are minimal. 

Foods high in carbohydrates include breads, pasta, beans, potatoes, bran, rice and cereals. Carbohydrates are the most common source of energy in living organisms. Proteins and fat are necessary building components for body tissue and cells can also be a source of energy for most organisms.

 The carbohydrate groups are different not only in where they are found in foods, but also in how the body digests and uses them. For dietary purposes, carbohydrates can be classified as simple (monosaccharides and disaccharides) or complex (oligosaccharides and polysaccharides).

Simple Carbohydrates 

Simple sugars are just that, they contain either one (monosaccharides) or two (disaccharide) sugar molecules, e.g. glucose or fructose.  These are broken down quickly causing a swift increase in blood sugar levels for a faster but shorter release of energy. 

They are found in some fruits, sugar and sugary snacks and confectionary and refined foods. Useful for periods when he body needs to call upon energy reserves at short notice for physical activity or intense mental activity. All sugars are broken down into glucose which is the main fuel for the brain, neurons and red blood cells. The body produces insulin to reduce the blood sugar level after ingestion of carbohydrates. 

Whilst the body needs a source of quickly available energy simple carbohydrates do not give sustained energy levels causing the blood sugar level to drop down once this has been used up. A diet high in refined simple carbohydrates can lead to imbalances in blood sugar levels and insulin resistance as the body has to work hard to try to reduce the blood sugar levels back down to normal to maintain homeostasis. 

Lactose, the milk sugar responsible for lactose intolerance, is made from the monosaccharides glucose and galactose. Lactose is a sugar unique to milk. Many people have difficulty digesting lactose so it is fermented into the lower gut, causing flatulence and possibly diarrhoea. This condition is known as lactose intolerance and is caused by absence of the enzyme lactase which breaks down lactose. Those suffering from lactose intolerance may buy preparations containing the enzyme lactase which digests (breaks down) the lactose in milk products.

Complex Carbohydrates 

What used to be known as starches are examples of oligosaccharides or polysaccharides, which means ‘many sugars’, because their structure is made from thousands of single sugars called monosaccharides, the structure of starches is very complex. 

Many polysaccharides can be digested in the body. Because these are long links, the body takes a long time to digest these starchy foods, starting first in the mouth, then in the lower parts of the digestive tract. The enzymes that break down carbohydrates into simpler sugars are called ‘amylases’.  

Polysaccharides in foods are broken down into smaller disaccharides, (meaning ‘two’ or ‘double’ sugars), and then down to single sugars or monosaccharides. Household sugar, sucrose, is a disaccharide whose basic units are made up of two monosaccharides (glucose and fructose) linked together. 

During the digestive process, complex carbohydrates are, through a series of different steps, broken down into the simpler glucose units. This digestive process can take a couple of hours more than is required to digest the simple sugars of a refined sugary snack. This is significant – as starches release sugars more slowly into the blood, they are ‘slow’ or ‘sustained release’ sugars. These longer chains sugars are broken down in the body more resulting in longer more sustained energy levels 

This has less of an impact on the blood sugar helping to maintain it at a more balanced level resulting in fewer energy dips caused by low blood sugar. It also means the pancreas doesn’t need to produce as much insulin to maintain homeostasis so insulin resistance is reduced. 

Starchy foods that are high in fibre such as those that retain their outer layer (e.g. potato skin, wholemeal products) contain complex carbohydrates. The quantity of fibre rich foods in the diet should be increased especially in those foods that have not been stripped of a high proportion of their nutrients or dietary fibre through processing. Examples include: 

  • wholemeal bread
  • brown rice
  • pulses (peas, beans and lentils),
  • muesli, breakfast cereals made from whole grains (but without added sugar)
  • jacket potatoes and wholemeal pasta.

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