Factors Affecting Energy Expenditure and Requirements:
There are several factors that affect the energy requirements of an individual. They are:
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1. Basal Metabolic Rate (BMR)
2. Muscle Activity
3. Specific Dynamic Action of Food
4. Mental Effort
5. Growth
6. Climate
Basal Metabolic Rate:
The amount of energy required to maintain the body temperature, to carry out various cellular processes and the work of digestion and metabolism of food under specific conditions is known as Basal Metabolic Rate (BMR). It includes the activities of the various organs such as brain, heart, liver, kidney and lungs, the secretory activities of the glands, the peristaltic movements of the GIT, oxidation’s occurring in resting tissues and the maintenance of muscle tone and body temperature. About 1/3rd of this energy is used for the activities of the organs and the remaining 2/3rd is used for the tissue oxidation processes in the resting period.
BMR can also be measured by the indirect calorimeter. The amount of oxygen consumed is measured in a given length of time usually 6-8 mins and then calculations are made. Several important factors may lead to faulty results if the BMR is not measured under controlled conditions. Therefore, standard conditions have been set up for the test and these are known as the “Basal Conditions“.
i. The patients should be in the post absorptive state (12-16 hrs after the last meal) to eliminate the influence of food. Therefore, the test in usually performed in morning before breakfast.
ii. The patient should be reclining and relaxed but awake as sleep decreases the rate by 10% while movements may increase the rate of oxidation. Therefore, 1/2 hour or 1 hour rest before the test is necessary.
iii. The patient should be free from emotional upsets or fear of the test.
iv. The patient should be in a comfortable room environment about 21°-24° Celsius.
iv. The patient should be a febrile because with each degree Fahrenheit rise in body temperature, the BMR is increased by 7%.
Muscle Activity:
Next to the basal energy requirements, physical activity accounts for the largest energy expenditure. In fact, for some persons who are vigorously active, the energy needs for activity are much more than for basal metabolism. Physical activities can be broadly classified as – Sedentary, light, moderate, vigorous and strenuous. Sedentary work, which includes office work, book keeping, typing, teaching, etc, needs less energy than more active and strenuous activities like nursing or gardening. A still greater amount of energy is required by those individuals who do hard manual labor such as laborers, shifting freight, coolies, etc.
The energy expenditure for many activities has been measured in adults and children and the data serves as a guide in setting standards for various groups of people. A wide range of activities has been classified in five groups as shown below table:
The calorie expenditure listed for each category include the basal metabolism. It represents for adults of average body size. The lower figure for each category would apply to women and the higher figure to men. It must be mentioned here that these figures vary from one individual to another because of their body size and also because of the difference in the intensity of effort applied.
Specific Dynamic Action of Food:
This is also known as calorigenic Effect of food. The ingestion of food results in an increase in heat production known as the calorigenic effect of food. This is related to the digestion and absorption of food, which in turn stimulates cellular metabolism. Proteins when eaten alone increase the metabolic rate by 30%, whereas carbohydrates and fats produce a much smaller increase. On the basis of a mixed diet usually eaten, the calorigenic effect of food is approximately 10% of the total energy requirement.
Mental Effort:
The nervous system is continuously active and it’s energy requirement is about 20% of the basal rate. But beyond this, intense mental effort in problem solving or writing examination does not add appreciably to the calorie requirement.
Growth:
The building of new tissue represents a storage of energy in one form or another. Each gram of protein in body tissue represents 4 Kcal. When growth is rapid, as during infancy and childhood, the energy allowance is higher. Similarly in pregnancy and lactation, the energy needs are increased for the building of new tissues and the production of milk.
Climate:
Under normal conditions the temperature of the body is controlled by the amount of blood brought to the skin. Vasodilation of blood vessels occurs when the environmental temperature is high and vasoconstriction occurs when the temperature is low. In a cold climate, when the surrounding temperature is low, most of heat is lost by radiation and convection. But in a hot climate, when the environmental temperature is high, the body heat is lost chiefly through evaporation. It is a well known fact that more heat is lost by evaporation when the air is dry than when it is humid.
During cold weather, excessive heat losses from the body are avoided by the use of suitable clothing and heating of the home or place of work. The subcutaneous fat serves to keep heat in the body rather than allowing it to be dissipated through the skin.