BMR, definition, Factors affecting BMR, Measurement of BMR, Measurement of Body Surface Area, the clinical significance of BMR

1. Namrata Chhabra
MHPE, MD, CMCL FAIMER FELLOW
REGIONAL INSTITUTION,
Principal-in charge, Professor& Head, Department of Biochemistry,
SSR Medical College, Mauritius

2. Learning
objectives
• To conceptualize:
• Definition of BMR
• Factors affecting BMR
• Measurement of BMR
• Significance of BMR
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3. BMR-
definition
• 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.
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4. Basal metabolic rate
• The rate of energy
consumption under basal
conditions per unit time (one
hour) per square meter of
surface area is known as “Basal
metabolic rate”.
• A constant ratio of
carbohydrates, lipids and
proteins are metabolized under
such basal conditions.
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5. Basal
Conditions
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 20- 25
degrees centigrade.
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6. Energy expenditure under
basal conditions
• Varies between 50- 70% of the
daily expenditure in sedentary
individuals.
• BMR can be responsible for
burning 70% of the total calories;
however, it is affected by several
factors.
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7. Factors affecting BMR
1) Age
• BMR decreases with advancing
age.
• In children it is high due to more
surface area.
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8. Factors affecting BMR
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.
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9. Factors affecting BMR
3) Hereditary factors
Some people are born with faster
metabolism and some with slower
metabolism
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10. Factors affecting BMR
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.
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11. Factors affecting BMR
5) Environmental conditions
Exposure to cold causes an increase in BMR
in order 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.
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12. Factors affecting BMR
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.
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13. Factors affecting 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.
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14. Factors affecting BMR
8) Drugs
• Caffeine,
• Benzidine,
• Alcohol,
• Epinephrine and
• Nicotine increase BMR.
Reverse is seen with certain
anesthetics.
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15. Factors affecting BMR
9) Pregnancy
The BMR of pregnant mother rises
after 6 months of gestation.
BMR of mother is a sum of her
own BMR as in the non pregnant
state and of that of fetal
metabolism.
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16. Factors affecting BMR
10) Racial variations
• BMR varies with different racial
groups.
• Higher values of > 33 % above
normal have been reported in
Eskimos.
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17. Factors affecting BMR
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.
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18. Factors affecting BMR
12) State of nutrition
BMR is lowered in states of :
• starvation,
• malnutrition and
• wasting diseases.
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19. Factors affecting BMR
13) Hormones
• Thyroid hormone increases BMR.
• In thyrotoxicosis BMR rises 50-100%
above normal, while in Myxedema,
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.
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20. Short-term factors affecting BMR
• high level of stress hormones in
the body and
• Illnesses such as fever,
• 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
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21. Measurement of BMR
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.
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22. Measurement of BMR
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.
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23. Measurement
of BMR
Calculation
• The average O2 consumption for 6
minutes is multiplied by 10 to convert into
hourly basis and then multiplied by 4.825-
degree C.
• The heat production is represented by
each liter 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.
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24. Calculation of
surface area
• The surface area of an individual adult
is about 1.8 square meter.
• i) Du- Bois Surface area formula- A
simple formula for calculating the
surface area is as follows
BSA =
Body surface area is in square meters.
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0.007184 x height (m)0.725 x weight (kg)0.425

25. Calculation of surface area
ii) 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.
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26. Calculation of BMR
2) 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 )
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27. Calculation of
surface area
3) 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%
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28. 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
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29. 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 :
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30. Example of
calculation of
BMR
O2 consumption/hour = 1.2 x10 = 12 liters
or 12 x 4.825 = 58 Kcal/hour
Surface area from normograms = 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
39.5- 32 x 100/ 39.5
= - 18.98 % below normal.
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31. Significance of
BMR
• 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.
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32. Significance of
BMR
b) Diseases- BMR is increased in:
• Leukemias
• Polycythemia
• Some types of anemia
• Cardiac failure
• Hypertension
• Dyspnea
• Perforation of ear drum(false increase)
• Hyperthyroidism
• Cushing syndrome
• Acromegaly
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33. Significance of
BMR
• BMR is deceased in-
• Addison's disease
• Hypothyroidism
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.
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34. Summary
• The basal metabolic rate (BMR) is the rate of
energy expenditure of a person at 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.
• BMR can be responsible for burning 70% of
the total calories; however, it is affected by
several factors.
• Estimation of BMR helps in the diagnosis of
diverse diseases, calculation of energy
requirement and to know the effectof food
and drugs.
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35. Thank you
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