Angie James, age 15, of Freeport, I11., for her question:
JUST WHAT IS METABOLISM?
Metabolism is an inclusive term for the chemical reactions by which the cells of an organism transform energy, maintain their identity and reproduce. All forms of life from the single celled algae to mammals are dependent on many hundreds of simultaneous and precisely regulated metabolic reactions to support them from conception through growth and maturity.Two metabolic processes are recognized: anabolism and catabolism.
"Catabolism" or destructive metabolism, is the breaking down of food substances to release energy and digest food. "Anabolism," or constructive metabolism, is the synthesis or building up of cells and tissues and the repair of worn out tissues.
Energy released during catabolism is used in three ways:
• To make the reactions of anabolism work.
• To heat the body.
• To allow the body's muscles and nerves to do their work.
Materials used in the process of metabolism are formed during digestion and put to work through respiration in the cells.
The rate of metabolism depends on various factors, including amount of food eaten, activity and temperature. However, at rest, at room temperature and several hours after a meal, metabolism settles down to a minimum constant rate. Doctors call this rate the "basal" metabolism.
Normal persons of the same body size, age and sex, have much the same basal metabolic rate, or rate at which the tissues burn food. The rate may be expressed as a plus or minus percentage of the normal rate. The basal metabolism provides a standard to compare metabolism under the varying conditions of health and disease.
The thyroid hormone, which is secreted into the blood by the thyroid gland, performs most of the regulation of the rate of metabolism.
By measuring the basal metabolic rate, doctors can determine whether the thyroid is operating properly. A low rate indicates "hypothyroidism," or too little thyroid hormone while a high rate, "hyperthyroidism," indicates that there is too much.
To measure basal metabolism, doctors usually use a blood test called the protein bound iodine (PBI) test. The PBI test measures the amount of iodine bound or connected to protein molecules in a blood sample.
Thyroid hormones contain iodine. These hormones, connected to proteins in the blood, contain the only protein bound iodine that circulates in the blood. Great care must be taken in administering the PBI test because extra iodine can enter the sample from a variety of sources.
In only a few cases, abnormal thyroid hormone levels do not indicate an abnormal metabolic rate. In these cases, doctors use in instrument called a metabolimeter to measure basal metabolism. A metabolimeter measures the amount of oxygen a person uses while resting. The amount of oxygen used indicates how quickly the body is using up food and producing heat and other forms of energy.