energy

Key points

• Energy is needed by the body to stay alive, grow, keep warm and be active.
• Energy is provided by food and drink. It comes from the fat, carbohydrate and protein that they contain. Food and drinks provide different amounts of energy.
• Different people need different amounts of energy.
• Some activities use more energy than others.
• To maintain body weight, it is necessary to expend as much energy as is derived from food; to lose weight, energy must exceed intake.

What is energy?
Energy is the power used to do work or to produce heat or light. Energy cannot be created or destroyed but it can be changed from one form to another. For example, when a piece of coal burns, the energy locked up in the chemicals in the coal is converted to heat and light.

Where does energy come from?
Energy comes from the sun and is called solar energy. Living plants are able to convert solar energy to chemical energy by a process called photosynthesis. This chemical energy is used to make other substances such as carbohydrate, fat and protein, all of which provide energy. Animals cannot use solar energy directly but can use the chemical energy contained in food (either plants or other animals). They can oxidise carbohydrate, protein, fat to produce energy, carbon dioxide and water. The energy is needed:

• To maintain body functions - to breathe, to keep the heart beating, to keep the body warm and all the other functions that keep the body alive
• For active movement - muscle contraction
• For growth and repair which require new tissues to be made.

When energy is being used, some heat is nearly always generated.

How is energy measured?
Energy can be measured in either joules or calories. A joule (J) can be defined as the energy used when 1 kilogram (kg) is moved 1 metre (m) by the force of 1 Newton (N). A calorie (cal) can be defined as the energy needed to raise the temperature of 1 gram of water from 14.5 to 15.5ºC. In practice, both units are used, just as different units are used to measure liquids, e.g. pints and litres.

One calorie is equivalent to 4.184 joules.

Energy in the diet
The average daily energy intake in the UK is 9720 kJ (2313 kcal) for men and 6870 kJ (1632 kcal) for women. The energy in the diet is provided by carbohydrate, protein, fat and alcohol. The amount of energy made available to the body by each of these varies. A gram of carbohydrate (starch or sugar) provides 16kJ (3.75 kcal), protein provides 17kJ (4 kcal) per gram, fat provides 37kJ (9 kcal) per gram and alcohol provides 29kJ (7 kcal) per gram.

Fat is the most concentrated source of energy - weight for weight it provides just over twice as much as either protein or carbohydrate. Alcohol provides almost as much energy as fat. The energy content of a food or drink depends on how many grams of carbohydrate, fat, protein and/or alcohol are present. For some people alcoholic drinks form a large part of their energy intake. This can be harmful to health since a high alcohol consumption is a risk factor for several diseases.

© British Nutrition Foundation