Energy intake is the chemical energy content of ingested food over a given period. Energy loss is the chemical energy that remains in faeces and urine produced by the animal over the same period. The energy content of food and wastes is found out by burning them in a bomb calorimeter. The material to be tested is placed and burned with the aid of oxygen in a chamber surrounded by jacket of water. The heat produced is determined by the rise in temperature of the surrounding water. Table provides the caloric value of the common food stuffs estimated in bomb calorimeter and in the body.
These values are essentially average values. During oxidative degradation in animal body, carbohydrates and lipids are fully oxidised to carbon dioxide and water just as in a bomb calorimeter but proteins are not degraded because the major end-product of protein metabolism, is urea which still possess some energy. Accordingly, the value is lower in the body (4.1 kcal/g) as you can see from Table. Energy derived from one gram of fat is much more than that derived from 1 gram of protein or of carbohydrate.
Table: Fuel Content of Food Materials