Basal Metabolic Rate: Factors & Measurement | Metabolism | Human | Biology

In this article we will discuss about:- 1. Definition of Basal Metabolic Rate 2. Factors Affecting Basal Metabolic Rate 3. Changes 4. Measurement.

Definition of Basal Metabolic Rate:

Energy, in terms of heat, produced as a by-product of total cellular metabolism is essential for the maintenance of life of the organism. Although the amount of energy, required for any individual varies directly with the degree of activity and environmental condition, but the rate of energy production in an individual by its overall cellular metabolism is more or less constant under some standard conditions known as basal metabolism and the rate of its energy production at basal condition per hour and per sq. meter of body surface is known as basal metabolic rate (B.M.R.).

The basal conditions are as follows:

i. The person should be in awake but at complete rest, both physical and mental.

ii. The person should remain in normal condition of environment, i.e., at normal temperature, pressure and humidity.

iii. The person should be without food atleast for 12-18 hours, i.e., in the post absorptive state.

Thus the B.M.R. may be defined as the amount of heat given out by a subject who, though awake, is lying in a state of maximum physical and mental rest under comfortable conditions of temperature, pressure and humidity, 12-18 hours (post-absorptive) after meal.

Is usually expressed as the heat production per sq. metre of body surface per hour. In adult male normal B.M.R. is about 40 calories per sq. metre of body surface per hour and in the adult female about 37 calories. The surface area of an average adult is about 1.8 sq. metres, and can be calculated from the following formula by DuBois

S = 0.0071 × W^0.425 × H^0.725

When W = body weight in Kgm, H = height in cm, and S = surface area in sq. metres.

The rate of metabolism at basal conditions has been formed to vary in different individuals and therefore the B.M.R. varies with different factor.

Factors Affecting Basal Metabolic Rate (Fig. 10.3):

i. Age:

The B.M.R. of children is much high­er than the adults. Roughly speaking it is inversely proportional to the age. In other words, with advancing age B.M.R. gradu­ally falls. This is due to the fact that chil­dren possess a greater surface area in pro­portion to their body weight. [But in newly born baby it is low-about 25 calories per sq. metre of body surface per hour. In pre­mature infants it is still lower.]

ii. Sex:

The B.M.R. of the males is slightly higher than the females (Fig. 10.2).

iii. Surface Area:

The B.M.R. is directly pro­portional to the surface area of the subject. Larger the surface area greater will be the heat loss, and equally higher will be the heat production (i.e., metabolic rate.) Otherwise temperature balance will be upset. For this reason B.M.R. depends upon the surface area.

iv. Climate:

In colder climates the B.M.R. is high and in tropical climates the B.M.R. is proportionally low.

v. Habit:

Trained athletes and manual workers have a slightly higher B.M.R. than persons leading a sedentary life.

vi. Diet:

Prolonged under-nutrition lowers the metabolic rate.

vii. Hormones:

Circulating levels of hormones secreted by adrenal medulla, adrenal cortex, thyroid and the anterior pituitary increase B.M.R. One mgm of thyroxine increases B.M.R. by about 1,000 calories. In thyrotoxicosis, B.M.R. may increase by 50-100% above normal, but R.Q. remains unaltered since both O₂ consumption and CO₂ production increase proportionately in such cases.

In myxoedema B.M.R. is diminished to 30% or even 45% below normal. Anterior pituitary through its thyroid-stimulating hormone (TSH) affects B.M.R. Growth hormone (STH) secreted by this gland also causes B.M.R about 20% rise. Adrenaline (epinephrine) and noradrenaline (norepinephrine) increase B.M.R. by about 20% of the resting value. Male sex hormones cause a 10% increase in B.M.R., female sex hormones increase it insignificantly.

viii. Barometric Pressure:

Moderate reduction of atmospheric pressure does not affect the B.M.R.; but a fall of pressure to half an atmosphere (viz., O₂ tension- 75 mm of Hg) -as occurs in mountain climbing—increases B.M.R., but increased pressure of oxygen does not raise B.M.R.

ix. Pregnancy:

The B.M.R. of the pregnant mother, after six months of gestation, rises. It is seen that the B.M.R. of the mother is the sum-total of her own metabolism as in her non-pregnant state and combined with that of the foetus. Hence, pregnancy exerts no specific effect upon B.M.R.

x. Body Temperature:

The B.M.R. increases by about 12% with the rise of 1°C., (such as in fever). This is due to the fact that increased temperature stimulates the chemical processes of the body and thereby increases B.M.R.

xi. Drugs.

Some drugs like caffeine, benzedrine, etc., increase the basal metabolic rate. The reverse is observed by anaesthetics.

xii. Racial Variation:

Natives of Yucatan were found to have their higher metabolic rate than the Americans, whereas South-Eastern Asians were found to have much lower rate. Thus, there may be significant devia­tions in the metabolism according to race.

In addition to the above factors, the metabolic rate of an individual may be influenced by the following:

(a) Muscular exercise-light exercise, as sitting and standing, etc., increases metabolism by 30%-40%. Moderate exercise, as walking, by 50%-60%, and severe exercise (hard work), by 100% of the B.M.R.

(b) Mental exercise causes a slight rise. Solving mathematical problems increases by 3-4% only. But it should be remembered that the basal oxygen consumption of brain is high and amounts to about 10% of the total O₂ requirement of entire body, and

(c) Strong emotions increase metabolism by 5-10%.

(d) Sleep reduces it by 10-13%.

Changes of Basal Metabolic Rate:

Changes of B.M.R. are found in the following pathological conditions:

i. Conditions Increasing B.M.R.:

(a) Hyperthyroidism (may rise up to 100%),

(b) Fever,

(c) Cardiorenal disease with dyspnoea (25-50%),

(d) Leukaemia (21-80%), and

(e) Polycythaemia (10-40%), etc.

ii. Conditions Reducing B.M.R:

(a) Starvation and under-nutrition

(b) Hypothyroidism

(c) Addison’s disease, and

(d) Lipid nephrosis, etc.

Measurement of Basal Metabolic Rate:

The apparatus most commonly employed clinically for the determination of B.M.R. is Benedict- Roth appara­tus/although Tissot method and Douglas bag method are also sometimes employed.

Read’s Formula:

This formula gives a rough estimate of B.M.R. and is often and at the bedside.

It is as follows:

B.M.R. = 0.75 (P.R. + 0.74 × P.P) – 72

Where P.R. = pulse rate, P.P. = pulse pressure.

The result comes out as the percentage of the normal and is correct within a range of ± 10%, viz., if above 10% the B.M.R. is higher, if below 10% it is lower than normal.