Worldwide medical and scientific journal for animal health professionals

Issue number 24.3 Nutrition

Calculating the energy content of pet food

Published 10/03/2021

Written by Richard Butterwick

Also available in Français , Deutsch , Italiano and Español

Why is it important to know the energy content of pet foods? Because if the energy content of the food(s) and the quantity of food(s) consumed is known, it is possible to determine how much energy a pet is being fed.

Calculating the energy content of pet food

Introduction

Why is it important to know the energy content of pet foods? Because if the energy content of the food(s) and the quantity of food(s) consumed is known, it is possible to determine how much energy a pet is being fed. This in turn will enable the clinician to provide more precise feeding recommendations to an owner, advice which is particularly important for clients who are in the process of changing their pet’s diet or are feeding multiple diets. Owners may not appreciate that feeding guides can vary between pet food manufacturers, and that different manufacturers may have different approaches to estimating a food’s energy content.

Knowing the energy content of a pet food also means that different diets can be accurately compared. By normalizing to a fixed energy content it is possible to compare nutrient content, or other factors such as cost, across a range of quite different foods, e.g., one can accurately evaluate the difference between a wet and a dry diet. Some pet food manufacturers provide information on the energy content of their diets, but mandatory legislation varies from country to country and there is no standard method as to how the energy content should be calculated. The approach outlined in Table 1 provides a consistent way to estimate the energy content across a range of foods for both dogs and cats.

 
Table 1. Factors influencing the amount of energy in food.
Forms of energy in food
Food energy is expressed in units of kilocalories (kcal) or kilojoules (kJ), where 1 kcal is equivalent to 4.182 kJ. Energy in food is usually considered at 3 different levels.
  • Gross energy (GE) ; this is the total (thermic) energy in the food released by complete oxidation. However, although a food may have a high GE content, it may be indigestible and therefore unavailable to the animal.
  • Digestible energy (DE) ; this is the amount of energy which is digested and absorbed by the animal, and is equivalent to the GE minus fecal losses. However not all of the DE is available to the animal; some is lost in the urine as energy is metabolized by tissues and cells.
  • Metabolizable energy (ME) ; this is the food energy that is utilized by the tissues, and is calculated from DE minus urinary energy losses. This is the most meaningful measure of food energy as it represents energy that is truly available to the animal to use.

 

Foods differ in the amount of energy they contain, and this is primarily a function of the amount of moisture, protein, fat and carbohydrate present, and (in the case of the macronutrients) their respective digestibilities. Typically dry foods are more energy-dense per unit weight than wet foods; the energy content of wet foods can vary from 70-130 kcal/100 g of food, compared with dry foods that can vary from 280-480 kcal /100 g of food.

Measurement of metabolizable energy (ME) content of pet food

The gold standard for measuring ME is to perform a feeding (digestibility) study, but this is time-consuming and expensive to conduct, and many pet food manufacturers do not have the capability or resources to conduct such studies. Predictive equations are an alternative method to feeding studies and can provide a good estimate of a diet’s ME content. There are a number of different predictive equations, but they all use a similar approach based on the amount of protein, fat and carbohydrate in the food, and employ coefficients (and in some cases other factors) to adjust for the digestibility of the food. 

Interpreting nutritional information on pet food labels

Some, but not all, of this information may be provided on the pet food label. It is important to check that nutrient values are expressed in % units, or g/100g of food. It is also essential to verify that the given values represent typical or average values for the nutrient in question; sometimes different values (such as guaranteed minimum or maximum levels) are declared and these should not be used. If it is not clear on the label as to what the values refer to, it is advisable to check with the manufacturer.

The following nutritional values must be known to use a predictive equation; as noted above, values must be expressed as percentages or as g/100g. Also note that the carbohydrate (“Nitrogen Free Extract” or NFE) fraction may not be measured directly but can be calculated by difference (Table 2).

  • Moisture
  • Protein
  • Fat
  • Carbohydrate* (NFE)
  • Ash **
  • Crude Fiber

*if not provided this can be calculated from the other nutrients.
** sometimes referred to as “inorganic matter”.

 

Table 2. Calculation of energy content 1
Step 1: calculate carbohydrate (NFE) content of the food Carbohydrate (g/100g)) = 100 – (Moisture + Protein + Fat + Ash + CF)
Step 2: calculate GE content GE (kcal/100g) = (5.7 x protein) + (9.4 x fat) + (4.1 x [NFE + CF])

Step 3: calculate the percentage digestibility

(note there are different equations for cat and dog foods)

Cat ; % energy digestibility = 87,9 – [(0,88 x CF x 100)/(100 – % moisture)]

Dog ; % energy digestibility = 91,2 – [(1,43 x CF x 100)/(100 – % moisture)]

Step 4: calculate DE content DE = GE (from step 2) x % energy digestibility (from step 3)/100

Step 5: calculate ME content

(note there are different equations for cat and dog foods)

Cat; ME (kcal/100g) = DE (from step 4) – (0.77 x Protein)

Dog; ME (kcal/100g) = DE (from step 4) – (1.04 x Protein)

Key: GE = gross energy - DE = digestible energy - ME = metabolizable energy - CF = crude fiber
- NFE = nitrogen free extract

 

Royal Canin is pleased to announce the launch of an app that will assist calculation of the estimated metabolizable energy density of dog or cat food from its approximate analysis. Download the app using the relevant link below.

References

  1. Nutrient requirements of dogs and cats (2006); National Research Council, National Academies Press, Washington DC.
Richard Butterwick

Richard Butterwick

Richard Butterwick, Global Nutrition Advisor, WALTHAM Centre for Pet Nutrition, Waltham, UK Read more

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