Mono, Di and Poly Carb

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Taken literally carbohydrate means ‘hydrated carbon’ or ‘carbon + water’. The chemical formula for glucose, the most abundant sugar/carbohydrate in the blood, is C6H12O6 – so 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, or empirically Cn(H2O)n – reflecting the same proportion of hydrogen and oxygen atoms appearing in these molecules as in water.

The origin of glucose is traced back to plants which produce this sugar in the presence of sunlight (photosynthesis) and then incorporate it into larger more complex storage molecules, for example starch. The human body needs glucose to make energy and in the presence of oxygen it is completely broken down in cells as follows…

C6H12O6 + 6O2 >>> 6CO2 + 6H2O + ENERGY
(1 glucose mol.* + 6 oxygen mol. yields 6 carbon dioxide mol. + 6 water mol. + ENERGY) *mol. = molecule

As the title of this post suggests, carbohydrates are made up of one, two or more sugar molecules, hence the terms monosaccharide, disaccharide and polysaccharide. There is actually a fourth type of sugar, oligosaccharide, which is a bit bigger than a disaccharide but not as big as a polysaccharide. Mono and disaccharides can be grouped together as simple sugars/carbohydrates, while oligo and polysaccharides are complex sugars/carbohydrates. It can be rather confusing that the names sugars and saccharides are used interchangeably but they are all carbohydrates.

The following table gives some examples of simple and complex carbohydrates. Di, oligo and polysaccharides are joined together by chemical bonds. The aim of carbohydrate digestion is to cleave the bonds within di and polysaccharides, allowing the resulting monosaccharides to be absorbed through the wall of the small intestine. It is the nature of these bonds which determine whether we can or can’t digest the carbohydrate.

Table 1: Carbohydrates

Sugar Classification Examples/Sources/Characteristics
  • single molecule
  • readily absorbed through the wall of the small intestine
  • simple carbohydrate
  • Glucose - generally found in foods as a di or polysaccharide; less sweet than fructose; the most abundant sugar in the blood; used to provide energy; can be used to build proteins; can be converted to fat for storage
  • Fructose - found in honey, fruit and vegetables; metabolised in the liver
  • Galactose - found in milk and milk products; metabolised in the liver
  • 2 molecules
  • simple carbohydrate
  • Sucrose (1 glucose + 1 fructose) - aka table sugar and found in sugar cane and sugar beet
  • Lactose (1 glucose + 1 galactose) - found in milk and milk products
  • Maltose (1 glucose + 1 glucose) - a product of starch digestion and also important in beer fermentation
  • 3-9 molecules
  • complex carbohydrate
  • Raffinose - found in many plants, e.g. beans and cabbage; humans do not have the necessary enzymes to break the bonds of this compound
  • Stachyose - mainly found in beans and peas and again indigestible by human enzymes
  • 10 or more molecules
  • often hundreds of molecules
  • complex carbohydrate
  • Starches - long chains or branches made up entirely of glucose molecules; found in potatoes, grains and legumes
  • Glycogen - branched chains of glucose molecules used as a means to store moderate reserves of glucose in the body, mainly in the liver and some muscle tissue
  • Non-starches, e.g. cellulose, hemicellulose, pectins, and gums whose bonds are resistant to human enzymes; collectively known as dietary fibre, some of which is fermented in the large intestine by the gut microbiome

Are carbohydrates essential for nutrition? 
Technically, the answer is no1. In fact very low carbohydrate diets have been used to treat neurological illnesses, e.g. epilepsy for a number of years and are currently of interest in the management of type 2 diabetes2.

Reducing carbohydrate intake to around 20 – 50g per day (the equivalent of approximately 1 large potato or 2 slices of bread) and replacing this energy source with fats, will generally induce ketosis2. Ketosis is the production of ketone bodies by the liver from fats; the ketone bodies are then used as an alternative fuel source. There are expectations, over time, this process would cause raised serum cholesterol levels but this cannot be reliably predicted and the advice would be for cholesterol levels to be monitored2.

From a negative perspective, a lack of starchy vegetables and grains associated with very low carbohydrate intake will drastically reduce dietary fibre; also an absence of glycogen stores in the liver and skeletal muscle can mean there is no ‘quick access’ to stored energy. Two clear reasons why dietary recommendations include carbohydrate as part of a balanced diet with a view to promoting optimal health. The UK government’s Scientific Advisory Commission on Nutrition (SACN) recommend the dietary reference value for carbohydrate should be approximately 50% of total energy intake3.

To continue the journey with carbohydrate metabolism and in particular, dietary fibre, ‘Bulking up…’ is coming soon.


  1. Westman, E.C. (2002) Is dietary carbohydrate essential for human nutrition? The American Journal of Clinical Nutrition, [online] Volume 75 (5), pp 951-953. [viewed 5 March 2021]. Available from:
  2. Kirkpatrick, C.F., Bolick, J.P., Kris-Etherton, P.M., Sikand, G., Aspry, K.E., Soffer, D.E., Willard, K., Maki, K.C. (2019) Review of current evidence and clinical recommendations on the effects of low- carbohydrate and very-low-carbohydrate (including ketogenic) diets for the management of body weight and other cardiometabolic risk factors: A scientific statement from the National Lipid Association Nutrition and Lifestyle Task Force. Journal of Clinical Lipidology, [online] Volume 13, pp 689-711. [viewed 9 March 2021]. Available from:
  3. Scientific Advisory Commission on Nutrition (SACN). Carbohydrates and Health, 2015 [viewed 31 March 2015]. Available from:

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