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Carbohydrates are the sugars of the molecular scale. Carbohydrates are so-called because they contain carbon atoms linked with hydrogen and oxygen in the same 2:1 proportion as water: -C(H2O)n-; effectively photosynthesis uses energy to add carbon from carbon dioxide to water molecules, with a side product of oxygen. Carbohydrates include: cellulose, glucose, lactose, pectin, starch and sugar. Whereas animals are mainly made up of protein, plants consist of mainly carbohydrates.

At its simplest, carbohydrates are single sugar molecules, or monosaccharides like dextrose or glucose. These can then double up to become two sugar molecules or disaccharides - lactose and sucrose, where sucrose is the main component of cane sugar. Larger sugars with many single sugars in a chain are called polysaccharides and include the more complex carbohydrates - cellulose and starch.

Carbohydrates in Human Nutrition

After solar energy is captured by plants through photosynthesis, this energy is stored as carbohydrates. In effect, carbohydrates are the molecule that life uses to store energy over the long term, so are the majority of the solid matter in plants. Animals are mainly made from protein, but use carbohydrates as their main fuel to power all their activities. For example, 55 - 65% of energy in the European diet comes from carbohydrates, with the rest from fats and proteins. In England, 16% of energy derives from added sugars which is significantly above public health guidelines of no more than 11%[1].

However, carbohydrates are not essential, because the Inuit live on a predominantly protein-based diet from fish and meat, together with the fats from these foods. But carbohydrates are a more efficient fuel for human activities than protein and fat, plus these also contain other vital nutrients that are lost when metabolised; therefore, a diet based on carbohydrates for fuel, then a small amount of proteins and fatsfor vital nutrients is a better structure for a human diet.

The process by which humans access the energy stored in carbohydrates is through digestion, or in science terms hydrolysis. This essentially breaks the links between the single sugars by inserting a water molecule into the sugar chain. This insertion is assisted by heat, by acidic conditions or by digestive enzymes, a type of active protein. Not all carbohydrates can be digested by humans, for example cellulose is indigestible, but has an important role as fibre or roughage helping food to travel through the gut. As mentioned earlier, cooking (heating) starts the process of breaking down carbohydrates, so does make foods more digestible in the human digestive system.

In humans, a small amount of glucose is in the blood for short term energy. When the level falls, the glucose is topped up by converting glycogen (a carbohydrate stored in the liver). which is regulated by insulin. Faulty control over this mechanism is the disease, diabetes.


  1. Public Health England (2014) New National Diet and Nutrition Survey shows UK population is eating too much sugar, saturated fat and salt, 14 May 2014, accessed 14 June 2016 [1]

Further Reading

  • Davidson, A. (1999) The Oxford Companion to Food, Oxford University Press. ISBN 0192115790.
  • Stobart, T. (1981) The Cook's Encyclopedia: ingredients & processes, Harper & Row. ISBN 0060141271.