Understanding Energy
by Diederick Labuschagne
Energy is an important element of the diet but, fundamentally it is not a nutrient in the true sense. It is rather a collective element derived from various nutrients that maintains all biological processes and creates the capacity to work.
There is one and only one energy source that can fuel any process or activity in the body, Adenosine Tri Phosphate (ATP) which operates at cell level. Regardless of the source of energy consumed, be it carbohydrates, lipids (fat) or fibre, it has to be converted to ATP before it can be utilized. Very little ATP is stored in the body cells and readily available. It needs to be replenished on an ongoing basis. There are three energy pathways that keep this system intact. These are complex micro biological processes that all operate simultaneously at different modes and efficiencies. The predominant pathway will depend on the kind and intensity of exercise executed at the time. However, for now we will focus on the macro elements of energy.
The basic measurement of energy is calories or Joules. One calorie is equivalent to 4,184 Joules. The energy content of food stuffs and feed is normally expressed as kilocalories (kcal) or mega joules (MJ) per kilogram.
Energy can be measured at various stages of digestion and metabolism. Many models and systems have been developed to partition energy at different stages. The diagram below depicts the basic partitioning of energy.
Gross Energy.
GE is the total heat generated from the complete combustion of all nutrients in a bomb calorimeter. It is the maximum amount of energy that can be derived from a nutrient and is also referred to as IE (intake energy), i.e. the total energy that the animal consumes. The availability of GE to the animal depends on the digestibility of the various nutrients.
Digestible Energy.
DE is the energy available to the animal after the food has been digested and the waste products has been excreted in the faeces. DE is then GE less energy lost in the faeces.
Metabolizable Energy.
ME is the energy available to the animal after the metabolic processes have been completed. The waste products of these processes are excreted via urine and gas release. ME is then DE less the energy lost in urine and gas. The first available ME will always be used to sustain vital basic functions (heat increment, basal metabolism and voluntary activity).
Net Energy.
NE is the energy available to the animal for other functions such as reproduction, lactation, growth and work, after the vital basic functions have been fuelled.
There is one and only one energy source that can fuel any process or activity in the body, Adenosine Tri Phosphate (ATP) which operates at cell level. Regardless of the source of energy consumed, be it carbohydrates, lipids (fat) or fibre, it has to be converted to ATP before it can be utilized. Very little ATP is stored in the body cells and readily available. It needs to be replenished on an ongoing basis. There are three energy pathways that keep this system intact. These are complex micro biological processes that all operate simultaneously at different modes and efficiencies. The predominant pathway will depend on the kind and intensity of exercise executed at the time. However, for now we will focus on the macro elements of energy.
The basic measurement of energy is calories or Joules. One calorie is equivalent to 4,184 Joules. The energy content of food stuffs and feed is normally expressed as kilocalories (kcal) or mega joules (MJ) per kilogram.
Energy can be measured at various stages of digestion and metabolism. Many models and systems have been developed to partition energy at different stages. The diagram below depicts the basic partitioning of energy.
Gross Energy.
GE is the total heat generated from the complete combustion of all nutrients in a bomb calorimeter. It is the maximum amount of energy that can be derived from a nutrient and is also referred to as IE (intake energy), i.e. the total energy that the animal consumes. The availability of GE to the animal depends on the digestibility of the various nutrients.
Digestible Energy.
DE is the energy available to the animal after the food has been digested and the waste products has been excreted in the faeces. DE is then GE less energy lost in the faeces.
Metabolizable Energy.
ME is the energy available to the animal after the metabolic processes have been completed. The waste products of these processes are excreted via urine and gas release. ME is then DE less the energy lost in urine and gas. The first available ME will always be used to sustain vital basic functions (heat increment, basal metabolism and voluntary activity).
Net Energy.
NE is the energy available to the animal for other functions such as reproduction, lactation, growth and work, after the vital basic functions have been fuelled.