The energy contained within food and feed is not fully available to the body for use. When an organism consumes a meal, only a fraction of the total potential energy is successfully extracted and absorbed by the digestive system. Digestible Energy (DE) represents the portion of consumed energy that the body is able to absorb across the gut wall and into the bloodstream. This metric offers a more accurate picture of the energy value of a diet than simply measuring the total energy a food contains.
Defining Digestible Energy
Digestible Energy (DE) is defined as the total energy measured in a food minus the energy that is subsequently excreted in the feces. The starting point for this calculation is the Gross Energy (GE), which is the total chemical energy stored within a feedstuff. GE is precisely determined by completely combusting a sample in a device called a bomb calorimeter. This process measures the total heat released, which represents the maximum potential energy available.
The calculation for Digestible Energy is represented by the formula: DE = GE (feed) – GE (feces). To obtain this value, the energy content of both the ingested feed and the resulting fecal matter must be measured using the bomb calorimeter. The difference between these two values is the energy that was successfully digested and absorbed by the body.
The Components of Energy Loss
The energy lost in the feces, which is subtracted from Gross Energy to calculate DE, is composed of two primary categories. The first component is the undigested portion of the feed itself. This includes materials that resist the body’s enzymatic breakdown, such as high-fiber structural carbohydrates like cellulose and lignin. Organisms lack the necessary enzymes to break these bonds, so the energy passes through the digestive tract unabsorbed.
The second category of energy loss is known as endogenous loss, which does not strictly originate from the consumed food. This fraction includes energy-containing compounds from within the body, such as sloughed-off intestinal cells and unabsorbed digestive enzymes. The microbial population in the gut also contributes to this loss, as fecal matter contains energy from dead microbes and their metabolic byproducts.
How Digestible Energy Compares to Metabolizable Energy
While Digestible Energy represents the energy absorbed by the body, Metabolizable Energy (ME) is a refinement that measures the energy truly available for metabolism. ME is calculated by taking the DE value and subtracting additional energy losses that occur after absorption. These losses include the energy excreted in the urine and the energy lost through combustible gases, such as methane.
The energy lost in the urine comes from nitrogenous end-products of metabolism, such as urea, which still contain chemical energy. Gaseous energy loss, primarily methane, is a byproduct of microbial fermentation in the gut, especially in ruminant animals. The formula for Metabolizable Energy is expressed as: ME = DE – Energy (urine) – Energy (gases).
Metabolizable Energy is often considered a more practical and accurate measure of a food’s caloric value, as it accounts for the energy used by the body’s cells for functions like maintenance, growth, and reproduction. The difference between DE and ME is relatively small in monogastric animals, like humans, but can be substantial in ruminants due to significant methane production.