63. Combustion heat of nutrients, utilization of energy content of nutrients in the body

Question 1

Combustion heat of nutrients

Answer 1

• The combustion heat (calorie value) of a nutrient equals the amount of heat made when burning of 1 g of that particular nutrient.
• Heat of carbohydrates and fats equals to their physiological caloric value.
• Not in the case of proteins. These materials are not entirely combusted in the body, metabolites of them appear in the urine (eg. urea) contain further combustible energy.
• The heat content of nutrients can be measured in a bomb calorimeter.
• The amount of heat measured this way (after complete oxidation), gives the maximum amount of heat (energy) that can be yielded from the particular nutrient in the body.

Question 2

Unit of the (heat) energy

Answer 2

Unit of the (heat) energy is calorie (cal).

• 1 cal is the amount of energy required to raise the temperature of one gram water by one degree Celsius, from 14.5 °C to 15.5 °C.
• The “Calorie” used in physiology equals 1000 cal (kcal). -Calorie can be used only for measuring heat, while joule is suitable for the exact expression of all forms of heat and energy. 1 cal = 4.184 J (1 J = 0.239 cal).

Question 3

Law of Hess

Answer 3

-The law of Hess says that as long as the final products are the same, the amount of energy, released during total oxidation, will be the same independently of the intermediary steps.

Question 4

Heat equivalent of nutrients

Answer 4

• The cause of the latter is the incompleteness of combustion of proteins in the body, and final products of their degradation are, besides CO2 and H2O, urea and other nitrogen containing compounds with relatively high combustion heat.
• During metabolism of 1 g of volatile fatty acids, energy yield for acetic acid is 3.5 kcal (14.6 kJ), caloric value for propionic acid and butyric acid are 5.0 kcal (20.8 kJ)and 6.0 kcal (24.9 kJ), respectively.

Question 5

Utilization of energy content of nutrients

Answer 5

Utilization of energy derived from nutrients is accompanied by certain loss.

• Indigestible parts of the nutrients leave without being absorbed.
• After subtracting the energy, which left the body with the feces (10%-60%)
• Forms of energy, leaving the body with urine (3-5%), or methane (5%) in ruminants, are also considered to be further losses of energy.
• The amount of energy leaving the body with 1 mol of urea is 634kJ, while 1 mol of excreted uric acid contains 1925 kJ energy. - ME (metabolizable energy) = Digestible energy - energy leaving the body with the urine and gases. -NE (Net energy) = difference between metabolizable energy, and the energy emitted from the body in the form of heat. This amount of energy is made for the animal to cover energy needs of its vital processes, or its utilization (milk, egg, wool, pregnancy, growth) or (external) work.
• Different food have different energy equivalents. 70% of the metabolizable energy is utilized, if it can be used for vital processes, only 30- 40% is turned to net energy. Conversion is the worst in the case of pregnancy, it is merely 20%.

Question 6

Gross energy

Answer 6

Total energy content of nutrients consumed by the animal is termed gross energy (GE).

Question 7

Digestible energy

Answer 7

After subtracting the energy, which left the body with the feces (10%-60%) ,the remaining portion of energy is termed digestible energy(DE).

Question 8

ME (metabolizable energy)

Answer 8

• ME (metabolizable energy) = Digestible energy - energy leaving the body with the urine and gases. -In birds, Urine is excreted together with feces, we can directly measure metabolizable energy.

Question 9

NE (Net energy)

Answer 9

-NE (Net energy) = difference between metabolizable energy, and the energy emitted from the body in the form of heat. This amount of energy is made for the animal to cover energy needs of its vital processes, or its utilization (milk, egg, wool, pregnancy, growth) or (external) work.