Throughout our lives, our bodies never stop using energy. When we are asleep we use less energy than when we are awake; when we move around we use more energy than when we are sitting still; and we use most energy of all when we are engaged in some form of strenuous physical activity, like hard physical work or a competitive sport. We fuel our bodies by eating foods that contain energy, primarily in the form of carbohydrates, and we measure the energy value of our food in calories.
Carbohydrates, which supply the human body with energy, are a product of photosynthesis carried out by live plants. Plants contain chlorophyll, which uses the sun’s energy to carry out a chemical process whereby carbon dioxide and water are transformed into oxygen, which is released back into the atmosphere via pores in the plant’s leaves, and carbohydrates, which the plant stores.
All carbohydrates are composed of carbon and hydrogen, but according to the exact molecular formula and the way the atoms are arranged, they can take the form of sugars, starches or cellulose. Some types of plant store more starches; some store more sugars. Cellulose is a fibrous substance, and this is what the plant builds its cell walls out of. It takes many forms, some more digestible than others – the cellulose in vegetables is a good source of fibre in the human diet, but we do not eat wood, cotton and paper which also contain cellulose. Fibre is necessary to the human body but is not high in energy.
The primary form of sugar is glucose, which is easily converted into energy by the human body. In chemical terms glucose is a monosaccharide, or single sugar; galactose, which is a sugar found in milk and yogurt, and fructose which is found in honey, are also monosaccharides. Glucose can join together with each of the other monosaccharides to form a disaccharide, a double sugar. Glucose and fructose combine to form sucrose; glucose and galactose form lactose; and two glucose molecules joined together become maltose.
When live plants store large quantities of excess glucose, they store it as chains of glucose molecules, or polymers. Glucose polymers are known as starches. They are also polysaccharides because they contain multiple sugar molecules. Unlike sugars, starches do not dissolve easily in water, so they are a more stable means of storage. Foods like potatoes and rice have a lot of starch stored in them.
How our Bodies Turn Food into Energy
The chemical process by which the human body – or any live organism – deals with the food it has consumed, is called metabolism. Metabolism is essentially a series of chemical reactions. These chemical reactions convert food into energy, which can then be burned up by physical activity (catabolism), or can be used to build or repair cells within the body (anabolism). Our bodies contain enzymes, and it is these enzymes that bring about the chemical reactions and regulate the process of metabolism.
In order to metabolise carbohydrates into usable energy, our bodies must break them down. This is done by acids during the digestive process. Depending on the complexity of the carbohydrate, it can take longer to break down. Glucose sugar gives us a quick burst of energy because it is not a complex form of carbohydrate.
Not all the carbohydrates we eat will be needed for immediate use. Some will be converted into proteins and fats. Excess glucose can be stored in the muscle tissue as glycogen, a polysaccharide, which is a reserve store of energy that can be drawn on to give the body stamina during periods of prolonged activity. Or it can be stored as fatty acids, which may become body fat.
Measuring the Energy We Use
The amount of energy that our bodies get from a particular food can be measured in calories or joules, and is usually expressed as kilocalories or kilojoules. Carbohydrates, fats and proteins are our main energy sources, and in order to stay healthy and maintain our energy levels, we need to provide our bodies with the appropriate quantities of these nutrients.
If we starve ourselves of the right food, we will become thin and unable to fight off infections because there will not be sufficient energy available for anabolism (building and repairing cells), and we will feel tired because catabolism will be inhibited. Equally, we should not overload our bodies with food that we do not need. The first law of thermodynamics states that energy cannot be destroyed, so our bodies have to do something with all the food that we consume; and if it cannot be turned into energy and used, it will end up as body fat.