One of the aspects of pet food labels that was not impacted by the new approach to AAFCO’s (the Association of American Feed Control Officials) Pet Food Label Modernization is the determination of calories in pet foods and treats, but perhaps it should have been.
Pet foods and treats in the U.S. must display metabolizable energy (ME) on the product label. Calculating calories in pet food is done using an approach well over 100 years old. Max Rubner and W.O. Atwater are the scientists credited for developing the foundations for ME equations that are still used today. In the late 19th and early 20th centuries, new methods, such as respiration calorimetry, allowed great advances in nutrition science, leading to the establishment of the energy equivalents for macronutrients and development of equations to predict the calories from foods based on their content of protein, fat and carbohydrates.
Does one size fit all?
Pet food products today contain diverse ingredients that affect their ME contribution mainly through digestibility and fiber interactions. Ingredients used in pet foods to deliver carbohydrates, proteins and fats vary widely in their digestibility — typically >95% at the high end to <70% on the low end — therefore impacting their caloric value. Fiber content can range from low to high, with various sources impacting calorie contribution differently. Processing methods also influence digestibility and energy contribution. Additionally, dogs and cats metabolize foods differently. This diversity in ingredients, processing and species differences makes a “one-size-fits-all” equation an imprecise and inaccurate predictor of ME for all pet foods.
Known as the “Atwater factors,” the estimates of 4 kcal/g for protein and carbohydrate and 9 kcal/g for fat provide good prediction of calories for homemade pet meals, milk substitutes, enteral nutrition products and commercial pet foods that use highly digestible, low-fiber ingredients, such as boneless meats, organs, fish, refined starches and milk products. The accuracy of the Atwater calorie estimates can be further improved to predict ME of these highly digestible or purified ingredient-type pet foods by applying known energy digestibility, fiber content and urine energy loss factors.
Biology vs. equations
The gold standard for determination of ME of pet foods is conducting feeding studies with dogs or cats. Measurement of pet food ME by biological testing is a time-consuming and expensive method, but it is the only way today to accurately assess calorie content in certain types of pet foods. Decades ago, AAFCO adopted the “Modified Atwater” equation to calculate ME in pet food. This equation uses factors of 3.5 kcal/g for both protein and carbohydrates, and 8.5 kcal/g for fats. These values were selected to account for the generally lower digestibility of protein, fat and carbohydrate nutrients found in ingredients commonly used in dog and cat food formulations. In the years since the industry started using the Modified Atwater equation, nutrition researchers have identified issues with this simplistic calorie prediction model.
In recent years, researchers have critically evaluated the basis for use of Atwater and Modified Atwater factors for predicting ME in pet foods. Reviews of Atwater’s original methods and data reveal significant weaknesses and questions. Of concern, is that AAFCO’s Modified Atwater equation consistently shows the greatest deviations and bias compared to other proposed equations. Numerous studies since the early 2000s confirm that Modified Atwater underestimates ME, particularly for highly digestible pet foods. The higher the actual ME, the greater the underestimation. Average underestimates of around 5% for dry foods and around 11% for wet foods have been reported, with one wet dog food underestimated by 58% using Modified Atwater. Nutrition scientists have developed vastly improved calculation methods to replace the Modified Atwater approach. Future advancements, including artificial intelligence, could enable even more accurate and precise ME predictions based on specific ingredients, content, processing methods and other parameters.
Calorie counting and pet health
Providing accurate ME content of pet foods is not just a desire to be scientifically correct; it has downstream effects that ultimately could impact pet health. An accurate calculation of calories in the food is essential because pets are fed to meet pets’ calorie needs, and thus all other nutrients in the food must be adjusted accordingly. Since feeding recommendations rely on a pet’s daily calorie needs, underestimating a pet food’s true ME is problematic because it results in mistakenly recommending larger food portions with consumption of excess calories.
Individual pets may vary in their response to excess calories. Factors such as age, breed and activity level can influence weight gain; however, excess calorie consumption contributes significantly to overweight and obesity conditions in pets, which are currently at epidemic proportions. It is known that current ME equations may not be equally accurate for all types of pet foods, and there is a need to help pets battle the bulge. Is it time to rewrite the pet food metabolizable energy equation?
Pet weight management: Feeding for health, longevity
www.PetfoodIndustry.com/15705770
Briefly: Top 5 takeaways
- Pet food products today contain diverse ingredients that affect their metabolizable energy (ME) contribution mainly through digestibility and fiber interactions.
- A diversity in ingredients, processing and species differences makes a “one-size-fits-all” equation an imprecise and inaccurate predictor of ME for all pet foods.
- Measurement of pet food ME by biological testing is a time-consuming and expensive method, but it is the only way today to accurately assess calorie content in certain types of pet foods.
- Numerous studies since the early 2000s confirm that Modified Atwater underestimates ME, particularly for highly digestible pet foods.
- An accurate calculation of calories in the food is essential because pets are fed to meet pets’ calorie needs, and thus all other nutrients in the food must be adjusted accordingly.