The most common sources of energy for laboratory animals are fats and carbohydrates. Energy is also derived from protein, but to a lesser extent. The unit of energy is expressed in terms of heat units (calories). A calorie is defined as the amount of heat required to raise one gram of water 1°C. The unit of energy commonly used in expressing the energy content of food is kcal, or kilocalories.

There are four ways of specifying energy:

Gross energy (GE) is the energy of complete combustion measured as determined using bomb calorimetry.

Digestible energy (DE) is equal to the GE minus the energy remaining in fecal matter.

Metabolizable energy (ME) equals the GE minus the energy lost in fecal matter, urine, and combustible gases. We report ME with two procedures. Physiological fuel values are an estimate of ME and are calculated by assigning 4 kcal/gm of ME for protein, 4 kcal/gm for carbohydrates, and 9 kcal/gm for fat in the diet. A value is also given for ME based upon actual analyzed values for each species.

Net energy (NE) is the energy remaining for production after deducting from GE energy lost in the feces, urine, combustible gases, and body heat losses.