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Why we use Metabolizable Energy (ME) and not Metabolic Weight (MW)

Metabolizable Energy vs Metabolic Bodyweight

There seems to be a great deal of confusion about how nutrient requirements for pets are determined. "Pet nutritionists" fall into two camps. One group chooses to define nutrient requirements using Metabolizable Energy (ME), while the other group insists that Metabolic Weight (MW) is more accurate for an individual pet. We want to discuss both approaches and explain why we use Metabolizable Energy (ME) in Pet Diet Designer for Windows.

When we talk of Metabolic Weight (MW), we are referring to the Amt./kg BW x ^.75 column for dogs and Amt./kg BW x ^.67 column for cats used by NRC in the tables of nutrient requirements in Chapter 15 of their 2006 publication, "Nutrient Requirements of Dogs and Cats". We are not referring to the metabolic weight which we use in formulas to determine dog and cat energy requirements. This is a subtle but very important point that we see many fledgling "pet nutritionists" fail to comprehend.

Dogs and cats eat to meet their energy requirements. Therefore we formulate pet food per unit of energy so that dogs and cats get the appropriate levels of nutrients when fed to meet their energy needs. NRC lists 3 columns of nutrient recommendations for each life stage for both dogs and cats. The first column lists nutrient recommendations relative to Dry Matter (DM). The second column lists nutrient recommendations relative to the Metabolizable Energy (Amt./1,000 kcal ME) in food. The third column lists nutrient recommendations relative to Metabolic Bodyweight (Dogs: Amt./kg BW x ^.75 - Cats: Amt./kg BW x ^.67). ME determines the amount of food that must be consumed to maintain BW and Body Condition Score. Requirements expressed relative to ME do not change as the energy density of a diet changes, making formulation by ME the preferred method by professionals. Designing recipes only using NRC Amt/kg BW x ^.75/.67 is incorrect.

From: Applied Veterinary Clinical Nutrition (A. Fascetti, et al. 2012)

"The NRC also uses an additional unit not used by AAFCO. The unit of amount per kilogram body weight raised to the 3/4 power (i.e., Amt./kg BW x ^.75) is also provided by the NRC, which is more analogous to the 'dosing' of medications. This third method of expressing requirements is not as commonly used by nutritionists, who generally think about the nutrient concentrations needed in foods rather than a 'dose' for a particular patient."

From: Canine and Feline Nutrition 3rd ed - L. Case, et al., (Mosby, 2011)

"Energy density (the number of calories provided by a food in a given weight or volume) is the most important factor in determining the quantity of food that a pet eats each day. A food's energy density directly affects the amount of all other essential nutrients that an animal ingests. Therefore, the most accurate way to express levels of essential nutrients in the food is in terms of ME (units per 1000 kcal of ME)."

"Because energy intake determines total food intake, it is important that diets are properly balanced, so that requirements for all other nutrients are met when energy needs are satisfied. For this reason, it is more appropriate to express levels of essential nutrients in terms of ME rather than in terms of percentage of the food's weight. Expressing nutrient content as units per 1000 kcal of ME is called nutrient density, and provides a standardized format that can be used to compare all types of foods because it accounts for both moisture and energy density differences between products. Actual nutrient intake can be readily determined from a pet's daily energy requirement, and foods of dissimilar energy content can be compared quickly based upon this method of expressing nutrient levels. The energy contributing nutrients, protein, fat, and carbohydrate, can be expressed either as g per 1000 kcal ME or as a percentage of the total ME of the diet."

From: Nutrient Requirements of Dogs and Cats, (NRC 2006)

"For kittens, the values for BW x ^.67 apply only to 800g kittens with an expected mature body weight of 4kg. To calculate the amounts for kittens of different actual or expected mature body weights, then values for 1,000 Kcal ME should be used."

"For lactating queens, the values for Amt./BW ^ 0.67 only apply to a queen weighing 4kg in peak lactation with 3-4 kittens consuming 540 kcal ME/day. If the queen does not fit these criteria, then values for 1,000 Kcal ME should be used. To calculate the requirements for late gestation only use the values for 1,000 Kcal ME."

"For adult cats, the values for Amt./BW ^ 0.67 only apply to a lean cat with 100 kcal x BW ^ 0.67. If the cat does not meet these criteria, then values for 1,000 Kcal ME should be used."

"The values that the NRC calculate for BW x ^.75 for puppies apply only to puppies weighing 5.5 kg with an expected mature body weight of 35 kg. If the puppy has a different current body weight or expected mature body weight, then values for 1,000 Kcal ME should be used."

"The values that the NRC calculate for BW x ^.75 for bitches apply only to a 22 kg bitch in peak lactation with 8 puppies and consuming 5000 kcal per day. If the bitch has a different body weight or litter size, then values for 1,000 Kcal ME should be used."

For adult maintenance dogs with unusually low energy intake then the values for BW x ^.75 are appropriate.

"To calculate the amount to feed of each nutrient, multiply the value for Amt./1000 kcal ME for each nutrient by the energy requirement and divide by 1,000. For dogs with an unusually low energy requirement use the nutrient amounts in the Amt/kg BW x ^.75 column."

So how do we know when to use the Amt/kg BW x ^.75 column? Again, the NRC gives guidance on this:

Below average requirements:

"Dogs kept in a domestic environment with little stimulus and opportunity to exercise and where the kcal factor is 95 or less when using the formula (kcal x kg BW x ^.75). Senior dogs and giant breeds where the kcal factor is 105 or less when using the formula (kcal x kg BW x ^.75)."

In our software, we will expose the kcal factors used when choosing activity level for maintenance dogs.

The current beta version uses Amt./1000 kcal ME to calculate nutrient requirements. Our next release (PetDiet365 Home) uses both Amt./1000 kcal ME and Amt/kg BW x ^.75 as follows; to calculate the amount to feed (for normally active and very active dogs) of each nutrient, we use the Amt./1000 kcal ME. For adult dogs with an unusually low energy intake, we use the Amt/kg BW x ^.75 values.

This switch of values happens automatically in Pet Diet 365 Home.

Note that the above behavior only applies to dogs at maintenance (adults). This does not apply to cats of all life stages, puppies, or lactating/gestating bitches, where the use of Amt/kg BW x ^.75/.67 column is not appropriate and is provided by NRC for "convenience" in doing calculations.

PetDiet365 Home takes all the above into consideration automatically when formulating diets.

We follow the NRC guidelines "to the letter" to ensure that the nutrient values are exact. 

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