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Metabolizable energy (ME)
Just as understanding the nutrient requirements of a nursing animal is vitally important to the growth and health of that animal, so is understanding the basic energy requirements in terms of calories. To a large extent, they go hand in hand, as the nutritional components determine what level of energy is present in the formula being fed.
But first, there are a few terms that need some definition. In the pet food industry there are different types of 'Energy' that are quoted and discussed.
GE - The gross energy (GE) in a food is defined as the total chemical energy measured from complete combustion of the food in a bomb calorimeter. However, digestible energy (DE) and metabolizable energy (ME) are the more typical terms used in canine and feline nutrition.
DE - Digestible energy (DE) refers to GE minus energy lost in feces.
ME - Metabolizable energy (ME) refers to DE minus energy lost in urine plus energy lost as gaseous products of digestion. However, because methane production is negligible in dogs and cats, ME is usually defined as DE minus energy lost in urine. So many dog and cat food manufacturers simply refer to ME as the quoted energy content of a food product.
The most accurate determination of the DE or ME content of food is obtained through animal feeding studies. The Association of American Feed Control Officials (AAFCO) has published accepted protocols for the determination of ME of dog and cat foods. To determine DE, it is sufficient to know the GE consumed and to collect feces and calculate fecal energy losses. To determine ME one must collect urine as well as feces or calculate urine energy losses through knowledge of urinary nitrogen loss.
Because animal studies are labor intensive, predictive equations are used extensively for calculation of ME values. The AAFCO recommends a predictive equation based primarily on fixed energy values and digestibility coefficients for dietary components (crude protein, crude fat, and carbohydrate) for estimating the ME content of dog and cat foods. The problem is that there are a variety of predictive equations that are used somewhat interchangeably. So it is important to understand the assumptions for each method.
Which method to use?
This depends on who you are and what food is being tested. Each method is described below, with an accompanying table displaying the range of results.
Atwater System. At the beginning of the twentieth century, William Olin Atwater devised a system, through an extensive system of tests and experiments, to predict ME (as measured in kcals/gram) by assigning a factor of 4 to percentage of proteins and percentage of carbohydrates, and a factor of 9 to percentage of fats which are more energy dense.
AAFCO-Modified Atwater System. AAFCO has adopted a slightly lower set of predictive values under the assumption that pet food is only about 85% as digestible as human grade food. As such, they assign a factor of 3.5 to percentage of proteins and percentage of carbohydrates, and a factor of 8.5 to percentage of fats.
NOTE: The National Research Council (NRC, 1985) (USA)has advised that the AAFCO average values may result in underestimation of the ME content of low-fiber, low-connective-tissue-containing meat and animal by-product foods. Milk products would certainly fall in this area since the standard Atwater System factors are more likely to yield a better estimate of ME in powdered milk replacers.
Doing the math
The following table shows the math of starting with GE (and associated factorials), moving across to DE (after adjusting for estimated energy loss in feces), and finally to ME (after adjusting for estimated energy loss in urine), and showing both the Atwater and AAFCO factorials to arrive at an estimate for ME. In most cases, WildAgain's website, the Wildlife Formula Calculator and accompanying spreadsheets will display both calculations for ME, although the standard Atwater calculation appears to be the preferred system to estimate ME in the powdered milk replacers.
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A closer look at a few of the products where multiple test values are available between time periods, can reveal how the ME/kcal values may have changed over time. Then if those changes are significant, either increase or decrease, the reader may want to focus on the most recent profile of the product. As shown, the ME kcal values have varied for several of the products.
References and further reading (not intended as an exhaustive list)
NRC (USA) (2006) Nutrient requirements of dogs and cats/Ad Hoc Committee on Dog and Cat Nutrition. Natl Acad Press, Washington, DC.
AAFCO (2012) Association of American Feed Control Officials. Oxford, IN: Official Publication.
Atwater WO (1902) Principles of nutrition and nutritive value of food. USDA Farmers' Bull. No. 142.
Kienzle E (2002) Further developments in the prediction of metabolizable energy (ME) in pet food. J Nutr 132: 1796S–1798S.
Laflamme DP (2001) Determining metabolizable energy content in commercial pet foods. J Anim Physiol Anim Nutr (Berl) 85: 222–230
The Bomb Calorimeter (1952). Instruction Booklet, no. 49. London: Baird & Tatlock.
Perrin DR. (1958) The calorific values of milk of different species. J Dairy Res. 1958;25:215–220
Carlos Castrillo; Marta Hervera; Maria Dolores Baucells (2009).. Methods for predicting the energy value of pet foods. R. Bras. Zootec. vol.38 no.spe Viçosa July 2009. Departamento de Producción Animal y Ciencia de los Alimentos - Universidad de Zaragoza. Miguel Servet, 177 - 50013 - Zaragoza, Spain. Grup de Recerca en Nutrició, Maneig i Bienestar Animal - Universitat Atònoma de Barcelona - 08193 - Cerdanyola del Vallès, Spain.
Cailin R. Heinze, VMD, MS, Lisa M. Freeman, DVM, PhD, Camilia R. Martin, MD, MS, Michael L. Power, PhD, and Andrea J. Fascetti, VMD, PhD (2015). Comparison of the nutrient composition of commercial dog milk replacers with that of dog milk. J Am Vet Med Assoc. 2014 Jun 15; 244(12): 1413–1422. doi: 10.2460/javma.244.12.1413. PMCID: PMC4458845. NIHMSID: NIHMS657127. PMID: 24871064
Jean A. Hall , Lynda D. Melendez, Dennis E. Jewell. (2013) Using Gross Energy Improves Metabolizable Energy Predictive Equations for Pet Foods Whereas Undigested Protein and Fiber Content Predict Stool Quality.
The information included on this website for the Proximate Analysis components (Proteins, fats, carbohydrates, fiber and moisture content) is extremely narrow in its scope and nature. It is limited to certain charts and graphs displaying content values (% of total) of various powdered milk replacers as tested by an independent chemical lab. Extremely brief overview information is provided as to the primary nutritional and medical benefits of each component, as well as, in some cases, a limited discussion of issues that may arise from concentration levels in the body that may be considered deficit or toxic. Entire textbooks on these primary dietary components are written for the medical and veterinary professions, in addition to the internet providing ready access to both scholarly and popular literature. Some of those references are included above.
The data values presented above only represent the test values for the presence and concentration of the component conducted according to standard chemical testing methods in a controlled laboratory setting. Any point test value is accompanied by a measurement uncertainty range as indicated in the charts. The concentration values are in no measure an indication of how much of the component may be provided to an animal in reconstituted formula or its bioavailability (its degree of digestibility, absorption, or ultimate utilization). Additionally, no testing was performed as to the source of the component in the product (such as inorganic salts) or the grade of any added supplements containing the component.
What the data can do is inform the reader as to (1) concentration levels in a product as most recently tested (2) changes over time and between lots, and (3) comparisons of relative concentration levels between products. It is merely data that may serve as a starting point when deciding on a milk replacer product(s) and a recipe, or information to consider if certain medical symptoms appear that could be a result of absence or excess of a specific component in the formula. The reader is encouraged to consult veterinary or nutritional professionals prior to providing additional supplementation of any component.