Energy Partition
The intention of consuming food is so our bodies are provided with energy. Without a source of energy, organisms wouldn't be able to function and would cease to exist. This is why it is important to provide ourselves and our animals with enough sustainable energy to keep our bodies functioning.
Energy can be measured in two units; calories and Joules. A single calorie is equivalent to the amount of heat required to raise the temperature of 1g of water by 1 degrees celcius (Abdullah et al, 2014). 1000 calories is known as a kilocalorie, and 1 kilocalories is equivalent to 4184 Joules (Ashley, 2015).
The body of any organism will be using energy for all of its needs. Cells need energy to multiply, enzymes need energy to react, bodies need energy to function. However, the efficiency of energy use within the body is not perfect. Bodies lose energy through different forms, regardless of if the energy could have been potentially useful elsewhere in the body.
The majority of energy loss is from heat; endothermic (warm blooded) animals are constantly producing heat, and as such, energy is being lost. This makes it essential that organisms need to replace their loss of energy with regular consumption and digestion of feeds in order to survive.
The energy requirement of an animal will depend on their species, life stage, and purpose. For example, a newborn puppy, still drinking from their lactating mother, would on average receive 565kJ/100g daily (Waltham Centre, 1999). An adult Siberian Husky that lived in outdoor kennels (and would require to produce more heat), would need to receive an average of 5021 kJ per day (Finke, 1991). From this we can see that clear differences are made between the requirements of energy for an animal; even ones of the same species.
The process of energy loss begins with the Gross Energy; this is the physical volume of energy you should receive through the food that has been consumed (Holden, 2015).
Energy is lossed through digestion of the food, as well as through losing heat through faeces. This stage is known as Digestable Energy (Evonik, 2015).
When the food is digested, urine and gas leave the body, which loses energy as well, and more energy is lost as the digested food is converted into Metabolisable Energy. This metabolisable energy can then be changed into any other energy needed within the body; the most common being ATP (Calvert, 2002).
What energy hasn't been converted into energy then dissipates from the body through heat loss from the body constantly incrementing heat. What energy remains intact within the body becomes Net Energy, which is energy that can be used at a cellular level. This will be split for the body to use energy for most tasks depending on the lifestyle of the species, including general body maintenance, muscle gain, reproduction, and milk production.
Figure 22 shows a diagram to explain the process of energy partition visually.
Figure 22. An Energy Flow Diagram showing how energy is lost throughout the body (Oregon State, 2003).