Thousands of chemical reactions and conversions take place continuously in body cells to keep the body alive and healthy and to generate energy. Metabolism is the collective term for all these chemical processes. The raw materials for metabolic processes are obtained from nutrients in food, which are broken down into simple molecules during digestion. These molecules are either recycled and built up into new complex molecules that can be used to repair or make new cells (anabolism) or are further broken down to release energy (catabolism).
In anabolic processes, body cells are built up and repaired, or complex substances are constructed out of simpler ones. In catabolic processes, complex molecules are broken down into simple molecules, such as glucose and amino acids, and these simple molecules are broken down to supply the cells with energy and materials for renewing cell structures.
The amount of energy a person uses for essential functions, such as maintaining body heat, breathing, and heart rate, is called the basal metabolic rate (BMR). BMR decreases naturally with age, but it is raised in the short term by factors such as illness, pregnancy, breast-feeding, and menstruation. All forms of exercise increase the body’s use of energy above the BMR.
Every living body cell depends on essential nutrients in food. Carbohydrates, proteins, and fats are converted into glucose, amino acids, and fatty acids respectively during digestion. These molecules enter the lymphatic system and the bloodstream and are converted into a usable form (metabolized) in the liver and in all body cells. Glucose is used to produce energy, as are fatty acids when glucose is in short supply. Amino acids are used to build the complex proteins needed to make and repair cells.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.