Table of Contents
Metabolism and Factors Affecting It:
Energy is needed for the multitude of activities performed by the body. It is also required for growth and repair of tissues. This energy is obtained from ingested food, which is first digested and absorbed, and finally metabolized.
Metabolism is the total of the physicochemical reactions which occur in the whole body. The substances undergoing changes are known as metabolites. This involves the conversion of nutrients into useful energy and the creation of a new carbon skeleton for synthesizing other vital compounds. Oxidation of nutrients like carbohydrates and other related substances forms the main source of energy. During oxidation, the energy that binds the molecules of these is liberated; part of it is utilized for immediate work and the rest is distributed to other molecules containing phosphates to form high-energy compounds. The chief form in which the energy is stored is adenosine triphosphate (ATP). This is synthesized and stored in the mitochondria of the cell and thus provides an energy reserve that is available for cellular metabolism.
There were two distinct types of metabolism, namely endogenous and exogenous. If the food molecules become a part of the body tissue it was said to go the endogenous route of metabolism. Food molecules that do not become a part of the body tissue but are oxidized for energy were said to undergo the exogenous route. These two routes of metabolism were thought to be distinct and independent of each other. The building-up phase of endogenous metabolism was also called anabolism and the breakdown of tissues was referred to as catabolism. Both these phenomena proceed simultaneously. However, in the period of growth, the anabolic reactions are more while in starvation, disease, or senescence the catabolic reactions predominate.
The metabolic rate is the rate at which energy is released in the body. A convenient method of estimating the energy requirements of the body is to measure it in terms of heat. The unit of heat used for this purpose is the calorie. A calorie (or kilocalories) may be defined as the amount of heat required to raise 1 litre of water through 1-degree centigrade.
The energy value of food is also measured in calories:
|1 gram of carbohydrate = 4 calories (16 KJ, or Kilo Joules)|
|1 gram of protein = 4 calories (17 KJ)|
|1 gram of fat = 9 calories (37 KJ)|
Basal Metabolic Rate (BMR):
It is the rate of the body’s energy expenditure under ‘basal conditions’. This means the individual is at rest, mentally and physically, has not eaten for at least 12 hours (i.e., is in the post-absorptive state), and is in a warm comfortable environment. Under these conditions, the metabolic needs of the body are at their lowest, energy being used only to sustain vital functions (for example breathing, the beating of the heart, and maintenance of normal body temperature).
The basal metabolic rate can be calculated by estimating the amount of oxygen consumed in a given time. Since individuals vary greatly in size, the BMR is expressed in calories per square metre of the body surface per hour. The body surface area is calculated from measurements of an individual’s weight and height.
Men oxidize food faster than women and therefore have a higher basal metabolic rate. For example, a male in his twenties has a BMR of about 40C per square metre of body surface per hour, whilst a woman of the same age has a BMR of about 37C per square metre of body surface per hour.
Factors Influencing Metabolism:
(1) Age- The metabolic rate of children is relatively greater than that of adults due to high rates of cellular activity and growth. The BMR decreases with increasing age.
(2) Exercise- Exercise requires energy. Strenuous physical exercise can increase metabolic rates as much as hundred times that of the BMR of an individual for a few seconds at a time.
(3) Body Temperature- An increase in body temperature increases the BMR. A decrease in body temperature results in a decrease in the metabolic rate and in oxygen consumption.
(4) Environmental Temperature- The average metabolic rate of individuals living in tropical countries is considerably lower than that of people living in cold climates.
(5) Thyroid Hormone- This hormone plays an important part in metabolism. When an excess of the hormone is secreted the basal metabolism is increased; when there is a deficiency of thyroid hormone basal metabolism is slow.
(6) Stimulation of the Sympathetic Nervous System- As in fright or acute anxiety, causes a temporary increase in the metabolic rate in order that the body may cope with an emergency.
(7) Drugs- Certain drugs, such as amphetamines, or caffeine, can increase the BMR.
Much of the food entering the blood from the digestive tract can be used by the body tissues without alteration, but some tissues require special chemicals which are not normally found in the food. To supply these, much of the absorbed food passes to the special organs where it is changed into new substances needed by the cells. This process is called intermediate metabolism. The liver can store and split fat and protein into smaller units. It is done by a number of reactions in a definite way called the metabolic pathways.