Metabolism

1. The chemical processes occurring within a living cell or organism that are necessary for the maintenance of life. In metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesized.
2. The processing of a specific substance within the living body: water metabolism; iodine metabolism.

reference  http://www.answers.com/topic/metabolism#ixzz1AVKhZjj7

The easiest way to describe what is metabolism is countless chemical process continuously inside the body that allow life and normal function. The amount of energy that the body burn depends by the metabolism.

Types of metabolism

Catabolism - breakdown of food component into simpler form, which can be used to create energy. 

( Protein - amino acid, Fats - fatty acid, Polysaccharides - monosaccharides) 

Anabolism/ Anabolic - This reaction require energy, many of its process powered by ATP. Its a process towards building/ and growth.

Metabolic rate ( total energy expenditure )

divided into three components;

• Basal metabolic rate (BMR) - is the amount of energy burned at rest and contributes 50-80 per cent of our energy used.
• Energy used during physical activity - this is the amount of energy burned during movement and physical activity; in a normally active person, this component contributes 20 per cent of daily energy use.
• Thermic effect of food - this is the energy we use to eat, digest and metabolise food. It contributes about 5-10 per cent of your energy use.

Basal metabolic rate (BMR)

BMR is the rate of metabolism that occurs when an individual is at rest in a warm environment and is in the post absorptive state, and has not eaten for at least 12 hours. Lean mass responsible for BMR. It is describe like an engine, the larger the engine are the more fuel it burn.  This is why having a large amount of lean mass is beneficial and preserving the lean mass during the process of losing weight is important. 

Energy used during physical activity 

During heavy physical training, the muscles may burn certain amount of energy. The energy used during exercise is the only form of energy expenditure that we have any control over.

Thermic effect of food 

BMR rises afterwe eat because we use energy to eat, digest and metabolise the food we’ve just eaten. It rise  soon after we start eating and peaks two to three hours later. This rise in the BMR can range between 2-3 per cent and up to 25-30 per cent, depending on the size of the meal and the types of foods eaten.