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Biochemistry
Lec: 2 Dr. Ahmed Dalalbashi
Basal Metabolic Rate:
When a subject is at complete rest and no physical work is being carried
out energy is required for the activity of the internal organs and to
maintain the body temperature constant. This energy is called the basal
metabolic rate or Resting metabolism.
The BMR is determined experimentally when the subject is lying down at
complete physical and mental rest, wearing light clothing, in a room
comfortably worm and at least 12 h after the last meal.
Energy requirement:
There is a wide variation in the energy requirements of individuals, even
of those having the same occupations and apparently living similar kind
of lives. The energy requirements of individual are dependent on four
variables:
1. Physical activity.
2. Body size.
3. Age.
4. Climate and environment.
Some references regard sex as another variable.
There is also extra need of energy for growth in childhood and
adolescence and during pregnancy and lactation.
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Physical Activity:
About ½ of the energy expenditure of moderately active person is utilized
in maintaining the BMR or Resting metabolism. The rest is used by the
muscles in doing external work, mostly in moving the mass of the body
and partly in doing external work. The muscles also utilize energy in
maintaining the posture of the body.
Table showing different requirements of different activity of peoples:
Sedentary Moderately Active Very Active
At sleep 500 500 500
At work 900 1200 1800
Non-Occupational 1300 1300 1300
Activity ______ _______ _______
2700 Kcal 3000 Kcal 3600 Kcal
Example of sedentary occupations includes office workers, drivers,
teachers, journalists, doctors, lowers, students, and so on.
Examples of moderately active occupations include industry and railway
workers, postmen, farm workers and so on.
Examples of very active occupations include steel workers, builders and
so on.
Body Size:
A tall man weighing 100 kg obviously needs more food than a short man
weighing 50 kg.
There are many researches showing the relationship between body
requirement of energy and body weight and indicating that the big man
requires more calories than a small man.
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Age:
Age affect energy requirements into 2 ways:
As people become older they tend to work in employment which
requires a smaller expenditure of energy and they reduce their
physical exercise.
The BMR per unite of body weight decreases slowly after the early
twenties. This decline with age is largely due to a reduction in the
proportion of metabolically active tissues in the body.
Climate and Environment:
It is a common opinion that cold weather stimulates appetite and hot
weather depresses it.
Any effect of climate is mainly due to changes in physical activity, but
also the BMR is some 10% lower in tropics than in temperate climates.
Growth and body size of childhood and adolescence:
The energy intake of children of different ages must obviously allow for
satisfactory growth and physical development and for the high degree of
activity characteristic of healthy children.
The energy requirement per kg of child body weight is more than double
that of adult during the first year of life but falls slowly as the rate of
growth per unit of body weight falls with age.
Pregnancy:
During pregnancy extra energy is needed for the growth of fetus and the
placenta. Additional energy is also required for movements and activities
when the mother becomes heavier.
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Lactation:
The quantity of breast milk produced by individual women varies widely.
However an additional dietary intake of about 500 Kcal/day during
lactation is required, although it is normal for a woman to lay down about
4 kg of fat during pregnancy and this provides a reserve for use in
lactation.
Carbohydrate in diet:
Carbohydrates are a major source of energy in the human diet; each gram
of carbohydrate provides approximately 4 Kcal.
Although fat and protein can replaces carbohydrate as a source of energy,
however some carbohydrates are essential for humans.
Brain, nerve and lung tissues require glucose as their source of energy.
During hypoglycemia and when the brain deprived of glucose convulsion
may result.
Carbohydrates especially sugars are converted to triglyceride in the liver
and become available to cell for energy or stored in the adipose tissues.
Also during transamination reaction intermediary products of
carbohydrate metabolism can be converted into amino acids and used for
protein synthesis.