Wednesday, February 11, 2009

The false comfort of numbers

Obesity is epidemic among our pets, and unfortunately, the calorie-counting strategy indeed works best for pets as well as their owners. Even if every bag of pet food or every batch of homemade food carried the exact calorie count, however, we couldn’t predict whether an individual animal is going to maintain, gain or lose weight on it without a lot more information. We need to recognize that there is variability in both an individual’s daily calorie needs as well as what a food provides.

For instance, we can calculate, based on your pet’s weight, what he or she probably needs to maintain weight. We do it by starting with the “resting energy requirement” or RER (there are a variety of more or less accurate equations out there for doing so). That is the number of calories it takes to sustain the weight of a pet who is doing nothing but laying around , breathing and metabolizing stuff. For pets with active daily lives, we tack on an activity factor that takes into account their level of activity, life stage, tendency for obesity, etc. So that’s a level of variability that we can guess at. But it turns out that the RER can vary by 50% on either side of our calculated value, so our guesses are often rendered useless.

A new paper from the nutrition department at Florida’s veterinary school shows that we also can’t put 100% faith in the numbers we get from bags or cans of food. Each pet food sold in the U.S. is required by states to list a guaranteed analysis so that we can get a sense of the protein, fat, fiber and moisture contents of the food. Hill and colleagues gathered data from state agriculture departments which are charged with chemically analyzing these foods to see if the guaranteed analysis is accurate.

In these guaranteed analyses, expensive nutrients are expressed in terms of minimum guaranteed amounts, while less expensive nutrients are expressed as maximums. In this study, guaranteed minimum protein and fat contents were off by – on average – 1.5% and 1% respectively. That means that if you have a dog food with a guaranteed minimum protein content of 22%, it is most likely to contain closer to 22.3% protein – not all that significant. The less expensive nutrients were off by 0.7%, 4%, and 0.5% for crude fiber, moisture and ash, respectively. For instance, a water content of 70% could actually be closer to 67.2%. It’s important to remember that these were averages – some diets erred on the wrong side of that minimum and maximum stated level, but this was not deemed to be a consistent or dangerous deviation.
BUT – the calorie content could vary more widely.

As a very general rule, dry foods contain about 4 kcal per gram of food, and canned foods contain about 1 kcal/gram of food. However, if you look at the actual nutrient analyses on these foods as opposed to the stated guaranteed analysis, those differences (especially because of overages in the minimum fat content) could account for as much as a 23% error in the calculated calorie count for the food. The example used in the paper is this: Let’s say that we think a canned food contains 1 kcal/gram, but that the nutrient analysis actually showed a difference from the guaranteed analysis of about 0.23 kcal/gram of food above calculations, giving a difference of 23% in the calculated calorie count vs the actual calorie count. The potential difference for dry foods was smaller –closer to 6%.

This means that the guaranteed analysis causes the manufacturer to OVERestimate the amount of food necessary to carry a certain number of calories. So here we have a level of uncertainty about how many calories our pets are getting in addition to how many an individual actually requires.
Bottom line? Learn to do a “body condition score” (BCS) on your pet. You can find these score sheets on some of the manufacturers websites. I have posted one made available to veterinarians from Purina on my website –
Chart for dogs: http://www.susanwynn.com/BCS__dog.php
Chart for cats: http://www.susanwynn.com/BCS__cat.php

If you are feeding to an ideal body condition score rather than worrying about caloric requirements and caloric density of different foods, you can almost not go wrong.

Reference:

Hill RC, Choate CJ, Scott KC, Molenberghs G. Comparison of the guaranteed analysis with the measured nutrient composition of commercial pet foods. JAVMA 2009; 234(3):347

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