The energy value of foods is currently expressed in Kcalories
(Kcal) and in Kjoules (Kj), the conversion factor being 4.184 to
obtain Kjoules from Kcalories. This dual system emerged from a
recommendation of the International Union of Nutritional Sciences to
express the energy content of foods also in joules, since they
represent a more scientifically correct unit to describe
biological work, even though the use of calories is still
predominant.
The chief food sources of energy to the human body
are fat, carbohydrate, and protein. Fats and carbohydrates contain
carbon and hydrogen which can be oxidized to their end products, CO2 and
H2O, both in the bomb calorimeter and in the body. In addition, protein
contains nitrogen. This nitrogen together with some carbon and hydrogen
leaves the body chiefly in the form of urea.
The energy yielding food factors are:
-Carbohydrates: banana, breadfruit, jackfruit, plantain, dates, prunes, raisin
-Proteins and amino acids: nuts, dried apricot, fig
-Fats: avocado, olive, nuts
The energy value of foods can be expressed in terms of kilo calories (KCal) or megajoules (MJ).
*Kilo
Calorie: One kilo calorie is the quantity of heat required to to raise
the temperature of 1kg of water through 10C. It is one thousand times
the small calorie used in physics measurements.
*Mega Joule : One
kilo calorie equals 4.186 kilo joules. Hence thousand kilo calorie
equals 4.186×103kilo joules or 4.186 mega joules.
Most of the
fundamental work on the energy value of foods was carried out
by the pioneer scientists Rubner (in Germany) and his pupil
Atwater (in USA) at the end of the nineteenth century. Rubner measured
the heats of combustion of a number of different proteins, fats, and
carbohydrates in a bomb calorimeter and also studied the heat of
combustion of urine passed by a dog, a man, a boy, and a baby.
What is food energy value?