Energy is the capacity to do work. Energy must be supplied regularly to meet the needs of the body’s survival. The body needs energy for maintaining body temperature, metabolic activity, supporting growth, for physical work, to maintain constant body weight and good health.
The energy yielding food factors are
(i) carbohydrates (ii) fats and(iii) proteins. Within the body, these are oxidised in the cells. The process is one of continuous utilization of O2 and production of CO2 H2O and heat.
The energy value of foods can be expressed in terms of kilocalories(KCal) or megajoules(MJ).The International Union of Nutritional Sciences has suggested the use of megajoule as the energy unit in place of
Kcal. These units are defined as follows:
Kilocalorie: One kilogram calorie is the quantity of heat required to raise the temperature of 1 kg of water through 1° C.
Joule: A joule is defined as the energy required to move 1kg mass by 1 metre by a force of 1 Newton acting on it.
Newton: One Newton is the force needed to accelerate 1 kg mass by less than a second.
1 Kcal = 4.184 KJ
1000Kcal = 4.184 Megajoule(MJ)
1 MJ = 240Kcal
The energy in various foods is measured by calorimetry. Calorimetry is the measurement of heat loss.The energy value of foods is determined using the instrument called Bomb calorimeter.
When samples of carbohydrate, fat, protein are burned, the amount of heat produced is always the same for each of these nutrients. The average gross energy value of carbohydrates,fats and proteins determined with bomb calorimeter is as follows:
1g of Carbohydrate = 4.1 kcal
1g of fat = 9.45 kcal
1g of protein = 5.65 kcal
In the utilization of carbohydrates, fats and proteins in the body a certain percentage of these nutrients is lost in digestion and the nitrogen of protein is excreted in urine as urea which still contains some energy value.The average losses in digestion in human subjects have been estimated to be 2.0% for carbohydrates,5.0% for fats and 8.0% for proteins. The loss of energy in urea is estimated to be 1.2kcal per gram of protein oxidised. The physiological energy values of foods calculated from the gross energy values after allowing for the losses in digestion and metabolism are as follows: Carbohydrates 4.0;fats 9.0 and proteins 4.0.
These values are known as ‘Atwater Bryant factors’ or physiological fuel values.
The coefficient of digestibility is used to express the proportion of an ingested nutrient that ultimately becomes available to the body cells. The coefficient of digestibility for carbohydrate, fat and protein are 0.98,
0.95 and 0.92 respectively. It is observed that carbohydrate and fat are metabolized almost completely, whereas protein metabolism is incomplete due to the presence of nitrogen.
The physiological fuel value, Co- efficient of digestibility and digestibility percent of carbohydrate, fat and proteins is presented in table 9.4.