Physiological energy value of Foods In the bomb calorimeter, carbohydrates and fats are completely oxidized to Co2 and water. Protein is oxidized to Co2, water and nitrogen. Another important error in the use of bomb calorimeter for determining the calorific value of foods of vegetable origin is that the fibre present in foods is burnt and yields energy, while it is not utilized by human beings. But, in the utilization of carbohydrates, fats and proteins in the body, a certain percentage of the above nutrients is lost in digestion and the nitrogen in proteins is excreted mainly as urea which contains some energy value.

1. Determination of Energy Value –
Physiological Fuel Value
R.Subha, M.Sc., M.Phil., D.C.T.,
Assistant Professor of Home Science,
V.V.Vanniaperumal College for Women,
Virudhunagar.

2. JOULES AND CALORIES
• The conversion factors for joules and calories
are: 1 kJ = 0.239 kcal; and 1 kcal = 4.184 kJ..

3. Physiological Fuel
• Based on the composition of carbohydrate,
fat and protein the heat of combustion of the
feed sample can be worked out using
appropriate factors. From the gross chemical
composition of the feed samples the amount
of energy yielding nutrients namely
carbohydrate, fat and protein are estimated.

4. FLOW OF ENERGY THROUGH THE BODY - A BRIEF OVERVIEW
• Ingested energy (IE) or gross energy (GE) Food that is ingested contains energy - the maximum
amount being reflected in the heat that is measured after complete combustion to carbon dioxide
(CO2) and water in a bomb calorimeter.
• Faecal energy (FE) and gaseous energy (GaE) Incomplete digestion of food in the small intestine, in
some cases accompanied by fermentation of unabsorbed carbohydrate in the colon, results in losses
of energy as faecal energy (FE) and so-called gaseous energy (GaE) in the form of combustible gases
(e.g. hydrogen and methane). Short-chain (volatile) fatty acids are also formed in the process, some
of which are absorbed and available as energy.
• Urinary energy (UE) Most of the energy that is absorbed is available to human metabolism, but
some is lost as urinary energy (UE), mainly in the form of nitrogenous waste compounds derived
from incomplete catabolism of protein.
• Surface energy [SE] A small amount of energy is also lost from the body surface.
• Metabolizable energy [ME] The energy that remains after accounting for the important losses is
known as “metabolizable energy”.
• Dietary-induced thermogenesis (DIT) Not all metabolizable energy is available for the production of
ATP. Some energy is utilized during the metabolic processes associated with digestion, absorption
and intermediary metabolism of food and can be measured as heat production; this is referred to as
dietary-induced thermogenesis (DIT), or thermic effect of food, and varies with the type of food
ingested. This can be considered an obligatory energy expenditure and, theoretically, it can be
related to the energy factors assigned to foods.
• Net Metabolizable Energy (NME) When the energy lost to microbial fermentation and obligatory
thermogenesis are subtracted from ME, the result is an expression of the energy content of food,
which is referred to as net metabolizable energy (NME).

5. • Some energy is also lost as the heat produced by
metabolic processes associated with other forms of
thermogenesis, such as the effects of cold, hormones,
certain drugs, bioactive compounds and stimulants.
• In none of these cases is the amount of heat produced
dependent on the type of food ingested alone;
consequently, these energy losses have generally not
been taken into consideration when assigning energy
factors to foods.
• Net energy (NE) The energy that remains after
subtracting these heat losses from NME is referred to
as net energy for maintenance (NE), which is the
energy that can be used by the human to support basal
metabolism, physical activity and the energy needed
for growth, pregnancy and lactation.

6. Overview of food energy flow through the body for maintenance of energy
balance

7. • Amount of energy available to the body from
a given amount of nutrient is called as
Physiological Fuel Value.
• Bomb caloriemeter – CHO, Fat and Protein -
oxidised completely.
• Human body – process of digestion not
proceed 100% - efficiency with which nutrient
is digested must be taken into account.
• Coefficient of digestibility – express the
proportion of ingested nutrient which
becomes available to the cells of the body.

8. Calculation - Physiological Fuel
Value/ Atwater Bryant Factors
CHO Fat Protein
Heat of combustion 4.15 9.45 5.65
Energy from
combustion of
Nitrogen
unavailable to the
body
- - 1.30
Net heat of
combustion
4.15 9.45 4.35
Coeffficient of
digestibility
0.98 0.95 0.92
Physiological Fuel
Value (Kcal)
4.00 9.00 4.00
Physiological Fuel
Value (KJ)
17.00 38.00 17.00

10. Reference
• Srilakshmi. B, 2018. Nutrition Science, 6th
edition. New Age International Pvt Ltd, New
Delhi.
• https://www.biologydiscussion.com/energy/m
etabolism-energy/how-to-measure-calorific-
value-of-food/17321