Carbohydrates are compounds of carbon, hydrogen and oxygen.
Fats are compounds of fatty acids and glycerol, again built from carbon, hydrogen and oxygen, but with less oxygen than the carbohydrates. This makes them a more compact form of energy.
Proteins all contain nitrogen in addition to the carbon, hydrogen and oxygen. They form the tissues which make living things work and control the processes within the cells.
Most of our food starts off in the green leaves of plants, where the energy from sunlight is trapped. This energy is then used to join a molecule of water to a molecule of carbon dioxide to form a simple sugar. If the plant wants this energy for some other purpose it can split the simple sugar and add oxygen to the parts, to reform water and carbon dioxide. The energy is then used to keep all the living processes in the cell going.
Unfortunately for the plant, simple sugars are not easily stored in its cells, because they dissolve in water. To overcome this problem, the plant joins two simple sugars together to make a slightly larger sugar molecule, but then joins these in long chains to make starch. These starch molecules are an excellent way of storing the sun's energy and are the main source of energy in food..
Starches are a good way of storing energy, but for some purposes take up too much space. The same energy can be stored in a small space if the molecules are altered to produce oils and fats. Plants will often manufacture oils to store energy for seeds and nuts.
Starches and sugars are known as carbohydrates, because they only contain the elements carbon, hydrogen and oxygen.
Starches are also known as polysaccharides - compounds built of many sugar molecules. Non-starch polysaccharides are known collectively as dietary fibre.
Monosaccharides (single sugars) such as glucose and fructose are the simplest sugars. The more complex sugars and starches are broken down into these during digestion.
Disaccharides (two sugars) such as sucrose, maltose and lactose are formed of two simple sugar molecules linked together.
Many people cannot digest lactose (milk sugar) as they grow older.
Sugars inside fruit, or formed by the digestion of starch are released slowly from food and absorbed steadily by the blood-stream. Sugars added to food or drinks are digested much more quickly and surges of sugar entering the bloodstream can cause fluctuations in blood sugar levels that have a harmful effect on health.
Non-sugar sweeteners provide a sweet alternative to sugar, but since they are all highly processed, and not a useful part of the diet, are best avoided.
Polysaccharides are long chains of glucose molecules which are formed into granules in the storage organs of plants. They are insoluble in water and in this form are indigestible. When heated with water, the starch absorbs the water, swells and gelatinises and can then be broken down by the digestive system. If the processing takes place without water, part of the starch remains indigestible. This happens in the production of some breakfast cereals.
Dietary Fibre consists of the cellulose cell walls and the pectins and gums that cannot be digested by the human system ( although they can be digested by a cow). Dietary fibre helps the correct passage of food through the digestive system. A diet rich in fresh fruit and vegetables will provide sufficient fibre. Excess fibre can trap minerals such as calcium, iron, copper and zinc. Wheat fibre is also abrasive and in excess can scratch the lining of the digestive system.