TABLE OF CONTENTS Carbohydrates are the principal source of energy in the human diet. Although they do not provide as much energy on a weight for weight basis as fats, the volume of carbohydrate we take in compensates for this. Carbohydrates come in many forms, from the simple sugars to complex carbohydrates (polysaccharides), which are made up of many linked sugar units. In simple terms we can utilise the sugars and some complex carbohydrates (primarily starch), but not others (e.g. cellulose).
The different macronutrients in food have different energy values. These are used by food manufacturers to calculate the total energy content of the food. For consistency of labelling, the figures to be used in these calculations are laid down in law in the EU. Figures have to be quoted both in kilojoules (kJ) and kilocalories (kcal).
| Component | Energy value kJ | (per gram) kcal |
|---|---|---|
| Fat | 37 | 9 |
| Carbohydrate | 17 | 4 |
| Protein | 17 | 4 |
| Polyols | 10 | 2.4 |
| Ethanol | 29 | 7 |
| Organic acids | 13 | 3 |
The building blocks of carbohydrates are the simple sugars monosaccharides which are made up of carbon, hydrogen and oxygen. There are three major monosaccharides, glucose, fructose and galactose, which are important in nutrition. For energy-generating purposes they can all be considered to be equivalent to glucose, and are metabolised, ultimately, to carbon dioxide and water. There are various mechanisms for this to happen. The detailed biochemistry behind these is beyond the scope of this publication, but there are many textbooks that deal with it...
