To simplify BMR
- The energy required by an individual during physical, emotional and digestive rest.
- It is the minimum energy required to sustain vital functions like working of heart, brain, circulation, respiration, ion transport and maintenance of cellular integrity
1) Person should be awake but at complete physical and mental rest
2) Person should be without food for 12-18 hours- Post absorptive state. This is to avoid the effect of digestion and absorption, the SDA of food stuffs and to prevent any chances of starvation.
3) He should be in recumbent or reclining position in bed.
4) There should be normal conditions of environment- temperature, pressure and humidity. The temperature should be between 200– 250 C. Under above conditions, energy expended by the individual is to maintain respiration, circulations, muscle tone, functions of viscera like kidney, heart, liver and brain and for the maintenance of body temperature.
Energy Expenditure in basal metabolism– Usually 50- 70% of the daily expenditure in sedentary individuals, BMR can be responsible for burning 70% of the total calories but the figure varies due to different factors. The factors that affect BMR are as follows-
Factors affecting BMR
1) Age- BMR decreases with advancing age. In children it is high due to more surface area.
2) Sex-Males have more muscle mass and lower body weight. Thus they have high BMR. Females have lower muscle mass more body fat thus have lower BMR.
3) Hereditary factors– Some people are born with faster metabolism and some with slower metabolism
4) Body Surface area- Surface area depends on weight and height. Greater the surface area, higher is the BMR and vice versa. Tall, thin people have a higher BMR than short persons. Since much of the basal metabolism is for the maintenance of body temperature and the heat loss is proportional to body surface area. Thus BMR is proportional to surface area.
5) Environmental conditions- Temperature outside body also affects BMR. Exposure to cold causes an increase in BMR so as to create the extra need for heat for the maintenance of body temperature. A short exposure to heat has little or no effect on BMR but upon prolonged exposure to high temperature there is compensatory heat loss, this results in increase in BMR. In colder regions of the world, BMR is higher and in tropical regions the BMR is lower.
6) Body Temperature-For every 0.5 degree rise in body temperature there is 7% increase in BMR. With the rise in temperature, there is increase in the rate of chemical reactions causing increased BMR.
7) Exercise- Physical exercise not only influences body weight by burning calories. It also helps to raise BMR by building extra lean tissues. Lean tissue is metabolically more demanding than fat tissue. The increase in BMR due to exercise is also due to increased cardiac output.
8) Drugs- Caffeine, Benzedine, alcohol, epinephrine and nicotine increase BMR. Reverse is seen with certain anesthetics.
9) Pregnancy- The BMR of pregnant mother rises after 6 months of gestation. BMR of mother is a sum total of her own BMR as in the non pregnant state and of that of fetal metabolism.
10) Racial variations- BMR varies with different racial groups. Higher values of > 33 % above normal have been reported in Eskimos.
11) Barometric pressure- Moderate decrease in pressure causes no effect on BMR, but a fall of pressure to half an atmosphere as occurs in mountain climbing increases BMR.
12) State of nutrition- BMR is lowered in states of starvation, malnutrition and wasting diseases.
13) Hormones-Thyroid hormone increases BMR. In thyrotoxicosis BMR rises 50-100% above normal, while in Myxoedema, BMR falls 35-45% below normal. Adrenaline, catecholamines, growth hormone, all of them increase BMR, Male sex hormone increases BMR to 10% or more. Anterior pituitary through its effect on TSH also affects BMR.
Short term factors affecting BMR
Illnesses such as fever, high level of stress hormones in the body and either an increase or decrease in environmental temperature result in an increase in BMR. Fasting, starvation or malnutrition all result in a lowering of BMR.
Measurement of BMR
BMR can be determined by the following methods-
1) Open Circuit system- In this system both O2 consumption and CO2 output are measured. Although it is very accurate but due to high degree of skills involved, it is less commonly used.
2) Closed circuit method-In clinical practice, the BMR is estimated by measuring O2 consumption of the patient for 2-6 minutes period under basal conditions. The O2 consumption is measured under closed circuit system. The apparatus commonly used is Benedict’s Roth metabolism apparatus. The test is usually run for 6 minutes and the volume of O2 consumed in that period is measured and corrected to standard conditions of temperature and pressure.
The average O2 consumption for 6 minutes is multiplied by 10 to convert into hourly basis and then multiplied by 4.825oC. The heat production is represented by each litre of O2 consumed. This gives the heat production in C/hour. Since the BMR is expressed as C/Sq.met/Hour, the energy output per hour has to be divided by the surface area of the individual.
Calculation of surface area of an individual
The surface area of an individual adult is about 1.8 square meter.
1) Du- Bois Surface area formula- A simple formula for calculating the surface area is as follows
BSA = (W 0.425 x H 0.725) x 0.007184
where the weight is in kilograms and the height is in centimeters and body surface area is in square meters.
2) By using normograms– Most conveniently the surface area is calculated from the normograms if the height and weight are known. Height is measured in feet/cm ; weight is measured in pounds/kg and surface area is calculated in square meters.
3) Harris–Benedict equations
BMR calculation for men (metric)
BMR = 66.47 + (13.75 x weight in kg ) + ( 5.003 x height in cm ) – ( 6.755 x age in years )
BMR calculation for women (metric)
BMR = 655.1 + (9.563 x weight in kg ) + ( 1.850 x height in cm ) – ( 4.676 x age in years )
4) Read’s formula
BMR = 0.75 (PR +0.74 x PP)
PR is pulse rate
PP is pulse pressure.
The result is expressed as percentage of the normal and is corrected within a range of ± 10%
Normal range of BMR
A healthy adult male has a BMR of about 40C/sq.m/hour and adult female has about 37 C/Sq.m/hour
Example of calculation of BMR
1) The normal BMR for an individual of the patient’s age and sex is obtained from the standard tables.
2) The patient’s actual rate is expressed as + or – % of the normal.
3) In a male aged 35 years with height-170 cm , weight- 70 kg and O2 consumption 1.2 liters in a 6 minutes time, the BMR can be calculated as follows-
O2 consumption/hour = 1.2 x10 = 12 liters
- or 12 x 4.825 = 58 Kcal/hour
Surface area from normogram = 1.8 sq.m.
Therefore BMR= 58/1.8 = 32 C/Sq.m/hour
The normal BMR for this patient by reference to standard table is 39.5 C/Sq.m/hour.
Hence the patient is having surface area of
= – 18.98 % below normal.
Significance of BMR
1) Diagnostic Aid
It is used for the diagnosis of various pathological conditions specially assessing the thyroid functions. The pathological variations in BMR are as follows-
a) Fever- Infections and febrile diseases elevate BMR, usually in proportion to increase in body temperature.
b) Diseases- BMR is increased in-
- Some types of anemia
- Cardiac failure
- Perforation of ear drum(False increase)
- Cushing syndrome
BMR is deceased in-
- Addison’s disease
2) Calculation of Caloric requirement- It is essential in the calculation of caloric requirement of an individual for prescribing a diet of adequate calorific value and planning nutrition.
3) To know the effect of food and drugs- BMR is calculated to know the effect of special food stuffs and drugs.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!