Basal
metabolic rate or BMR is the amount of daily energy expended by humans
and other animals at rest. It is the minimum calorific requirement
needed to sustain life in a resting individual. The rest is defined as
existing in a neutrally temperate environment while in the
post-absorptive state. It is
the number of calories that is burned if one stayed in bed all day,
when the subject is at complete mental and physical rest, but not
sleeping. The release and
use of energy in this state is sufficient only for the functioning of
the vital organs, the heart, lungs, nervous system, kidneys, liver,
intestine, sex organs, muscles, and skin. Normally calories are
being burned by body processes such as respiration, circulation of
blood, and maintenance of body temperature.
Photo credit: extremeslimquick.com
BMR
is an important factor in determining overall metabolic rate and how
many calories one needs to maintain, lose, or gain weight. The BMR values are measured in Kcal or Kj per square meter of body surface per hour. The normal adult BMR value for a healthy male is 40 Kcal (168 Kj)/sq m/hr and in healthy female it is 37 Kcal (155Kj)/sq m/hr.
BMR is influenced by a combination of genetic and environmental factors.
The Factors Influencing BMR Values Are
1.
Age: The BMR decreases with age. In childhood the BMR is high and in
old age it tends to fall. After about 20 years it drops by 2% per
decade.
2.
Sex: BMR is higher in males than in females. Men have greater muscle
mass and lower body fat percentage and so higher basal metabolic rate.
3.
Weight: The heavier one is, the higher will be the BMR. Metabolic rate
in obese people are higher than the metabolic rate in slim.
4.
Genetics: People different in the rate of metabolism and it is
determined by the genetics. Some people have faster metabolism and
BMR and some have slower BMR.
5.
Diet: In foods that contains excess protein the BMR value increases due
to the specific dynamic action of the food protein. Starvation and
severe reduction in calorie intake can reduce BMR, and so low calorie
weight loss diets tend to drop the BMR.
6.
Body Surface area: The increase in the body surface area tend to
increase the BMR. Body surface area is also a reflection of a persons
height and weight. Tall and thin people have higher BMR.
7.
Body Fat Percentage: The higher the body fat, the lesser the BMR and so
increase in the body fat decreases the BMR. Men tend to have lesser
body fat and so higher BMR.
8. Climate: Colder climate increases the BMR and vice versa.
9.
Body Temperature: Every increase in internal body temperature the BMR
increases. It is because the chemical reactions in the body increases
with higher temperature.
10.
Endocrine Factors: The BMR is related to thyroid activity and thyroxin
produced by the thyroid gland speeds up metabolic activity and increases
the BMR. In thyrotoxicosis there is elevated BMR and in hypothyroidism
the BMR falls.
11.
Exercise: Physical exertion burns more calories and thus increases BMR.
Exercise also helps to build lean body tissues, which is metabolically
more demanding that body fat tissues.
12. Pregnancy: In later stages of pregnancy the BMR increases as the maternal BMR also includes that of the fetus.
Photo credit: vrp.com
Physiology
The
primary organ which is responsible for regulating metabolism is the
hypothalamus. It is situated on the diencephalon and forms the floor and
part of the lateral walls of the third ventricle of the cerebrum. The
chief functions of the hypothalamus are
1.
Control and integration of activities of the autonomic nervous
system(ANS). The ANS regulates contraction of smooth muscle and cardiac
muscle, along with secretions of endocrine organs such as thyroid gland.
It regulates the visceral activities such as heart rate, movement of
food through the gastrointestinal tract, contraction of urinary bladder
etc.
2. Production and regulation of rage and aggression.
3. Regulation of body temperature.
4. Regulation of food intake through the 2 centers, which is the feeding center and thirst center.
All these functions sustain the body process and maintain the BMR.
Photo credit: actabit.com
Calculating BMR
The basal metabolic rate is usually expressed in terms of daily rates of energy expenditure. We
need to calculate how many calories that is burned in a day, which is
the total daily energy expenditure (TDEE). Total energy expenditure is
the total number of calories that the body expends in 24 hours. The caloric maintenance values can be derived using age, sex, height, weight, body surface area, activity level, and the oxygen and carbon dioxide measures taken from calorimetry. There are many formulas for determining the BMR taking into consideration the various factors and varying in accuracies.
Photo credit: nature.com
The Harris-Benedict formula
This
is a calorie formula taken using the factors of height, weight, age,
and sex to determine the BMR. It is more accurate than determining
calorie needs based on body weight alone. The only variable that it does
not take into account is the lean body mass. So this equation will be
very accurate in all individuals except extremely muscular or extremely
fat.
For men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) - (6.8 X age in years)
For women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)
Note:
1 inch = 2.54 cm.
1 kilogram = 2.2 lbs.
