After digestion and absorption, nutrients are carried by the blood to the cells of the body. Within the cells, nutrients are changed into energy through a complex process called metabolism. During aerobic metabolism, nutrients are combined with oxygen within each cell. This process is known as oxida-tion. Oxidation ultimately reduces carbohydrates to carbon dioxide and water; Proteins are reduced to carbon dioxide, water, and nitrogen. Anaerobic me-tabolism reduces fats without the use of oxygen. The complete oxidation ofcarbohydrates, proteins, and fats is commonly called the Krebs cycle.
As nutrients are oxidized, energy is released. When this released energy is used to build new substances from simpler ones, the process is called anabolism. An example of anabolism is the formation of new body tissues. When released energy is used to reduce substances to simpler ones, the process is called catabolism. This building up (anabolism) and breaking down (catabolism) ofsubstances is a continuous process (metabolism) within the body and requires a continuous supply of nutrients.
Metabolism is governed primarily by the hormones secreted by the thyroid gland. These secretions are triiodothyronine (T3) and thyroxine (T4). When the thyroid gland secretes too much of these hormones, a condition known as hyperthyroidism may result. In such a case, the body metabolizes its food too quickly, and weight is lost. When too little T4 and T3 are secreted, the condition called hypothyroidism may occur. In this case, the body metabolizes food too slowly, and the patient tends to become sluggish and accumulates fat.