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In this article we will discuss about the Basal Metabolism:- 1. Definition of Basal Metabolism 2. Conditions for Measurement of Basal Metabolism 3. Factors Influencing 4. Measurement 5. Significance.
Contents:
- Definition of Basal Metabolism
- Conditions for Measurement of Basal Metabolism
- Factors Influencing Basal Metabolic Rate
- Measurement of Basal Metabolic Rate
- Significance of Basal Metabolic Rate
1. Definition of Basal Metabolism:
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The total heat produced or the energy spent by the body under conditions to perform minimum possible work is known as basal metabolism. The lowest level of energy production consonant with life is the basal metabolic rate (BMR).
2. Conditions for Measurement of Basal Metabolism:
a. Post-absorptive State:
The patient should not take anything by mouth for the past 12 hours.
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b. Complete mental and physical relaxation.
c. Patient should be awake.
d. Recumbent position during the test.
e. Environmental temperature should be between 20°C-25°C.
3. Factors Influencing Basal Metabolic Rate:
a. Surface Area:
The BMR is directly related to the surface area of the subject. Larger the surface area greater would be the BMR.
b. Age:
The BMR is inversely proportional to age. Children have larger BMR than adults.
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c. Sex:
Males have higher BMR than females. The BMR of females declines more rapidly between the ages of 5 and 17 than that of males.
d. Climate:
The BMR is lower in warm climates.
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e. Racial Variations:
The BMR of oriental women living in the USA is 10 per cent below the standard BMR of American women of the same age.
f. Habit:
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Persons accustomed to heavy exercise or hard physical work have a higher BMR than those involved in sedentary work.
g. State of Nutrition:
The BMR is decreased in starvation and undernourishment.
h. Pregnancy:
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The BMR is not changed during pregnancy. The higher values of BMR in late pregnancy is due to the BMR of the foetus.
i. Disease:
The BMR is increased in infectious and febrile diseases. The increase is usually proportional to the rise of the temperature. The BMR is also increased in increased activity of cells and, therefore, it increases in leukemia, cardiac failure, hypertension, polycythemia, dyspnea, and some types of anemia.
j. Effects of Hormones:
The BMR is increased in hyperthyroidism and decreased in hypothyroidism. In adrenal insufficiency (Addison’s disease), the BMR is subnormal.
k. Drugs:
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Caffeine and benzidine increase the BMR.
l. Emotion:
The BMR is increased in emotional stress.
m. Diet:
The BMR of strict vegetarians is 11% lower than that of meat eaters.
4. Measurement of Basal Metabolic Rate:
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a. The BMR can easily be measured either by the apparatus of Benedict and Roth (closed circuit device) or by Douglas bag method (open circuit device). Benedict-Roth’s apparatus is abundantly used in hospitals and laboratories.
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b. This method provides sufficient accuracy only by measuring the oxygen consumption of the patient for two 6-minute periods under basal conditions. The average of two such readings is multiplied by 10 to obtain the hourly consumption of oxygen and then multiplied by 4.825 kcal, the heat produced by one litre of oxygen. This gives the total heat produced in kcal/ hour.
c. This above value is divided by the surface area of the subject to get the BMR in C/hr/sq.m.
The simple classical formula of Du Bois is applied which is as follows:
A = H0.725 x W0.425 x 71.84,
where A = surface area in cm2.
H = height in cm.
W = weight in kg.
The above value obtained is in sq. cm. This should be divided by 10,000 to get the surface area in sq. metre.
A BMR between – 15% and + 20% is considered normal. The BMR may exceed + 50% to + 75% in hyperthyroidism. In hypothyroidism, the BMR may be – 30% to – 60%. The normal BMR is 39.5 C/m2/hr. The BMR increases as much as 600%- 800% over basal during exercise.
5. Significance of Basal Metabolic Rate:
a. The determination of BMR is the principal guide for diagnosis and treatment of thyroid disorders.
b. The BMR – 10% of the normal develops moderate hyperthyroidism. In severe hypothyroidism, the BMR may be decreased to 40 to 50 per cent below normal.
c. It aids to know the BMR to have an idea of the total amount of food or calorie required to maintain body weight or calorie.
d. The BMR is below normal in starvation, under nutrition, hypothalamic disorders, Addison’s disease, and lipoid nephrosis.
e. The BMR is above normal in fever, diabetes insipidus, leukemia and polycythemia.
