Dear Doctor: Comparing Nutrients from one Good to Another
Q I read with interest in the February issue your lead article about the Guaranteed Analysis on dog food labels — and came away frustrated that dog food labels will not be like people food labels for the foreseeable future, making it next to impossible to compare nutrient levels in two different dog foods. Is there any way to figure it out, maybe with a formula or something?
Dear Mr. Beaudoin,
A It is indeed vexing that the Guaranteed Analysis on dog foods cannot be used as they’re written to compare one food to another. It allows you only to compare by the weight of the food, not by the serving size of food a dog actually eats. And the weight of a serving is going to differ depending on such things as whether the food is canned (which will make it weigh more because canned food has more water than dry) or contains a lot of fiber (which also weighs a lot but doesn’t say anything about nutrient content).
For instance, if a dry food has a minimum protein of 27%, that just means that for every 100 pounds of the food, at least 27 pounds are protein. A canned food will have much less protein per 100 pounds because the water in the food weighs a lot. Consider that canned food is about 75 percent water by weight; dry food, only about 10 percent. But that doesn’t mean a dog will take in less protein if he eats the wet food instead of the dry — perhaps the serving size is larger.
This doesn’t even begin to get at calorie levels, which are all over the map. For instance, to get the same level of protein from two different foods, a dog might need to consume twice as many calories of one of the foods. The only true comparison is nutrient level per a standard number of calories, not nutrient level per weight of the food.
The good news in all of this is that if the label has a statement from the Association of American Feed Control Officials, or AAFCO, saying the food provides “complete and balanced nutrition,” and your pet is healthy, it is highly likely that your dog’s nutrition status will be okay. Still, you should be able to compare for your own edification.
One way is to use Tufts’ handy dandy nutrient calculator, which can be found at http://vetnutrition.tufts.edu/2017/08/nutrient_converter/. All you will need are the number of calories per kilogram of food (listed on the label) and the way in which the nutrient you want to compare is listed — whether as a percentage or as milligrams per kilogram or as International Units per kilogram (all listed on the label). Plug in those numbers and voila, you’ll get the amount of the nutrient per 100 calories of the food.
If you don’t have access to the Internet or want to go it on your own, here are the steps you’ll need to take, at least for when ingredients are listed as percentages:
1. Select a nutrient you wish to compare in two dog foods, like protein.
Diet A: Adult Dry Dog Food Protein (Min): 27%
Diet B: Adult Canned Dog Food Protein (Min): 8%
2. Find the calorie densities by weight, also listed on the label (as kilocalories per kilogram):
Diet A: Adult Dry Dog Food 3,606 kcal/kg
Diet B: Adult Canned Dog Food 1,198 kcal/kg
3. Get your calculator, or a sharpened pencil. Divide the percentage by the number of kilocalories per kilogram and then multiply by 1,000.
Diet A: Adult Dry Dog Food 27 divided by 3,606 = 0.00748752 x 1,000 = 7.49 grams of protein per 100 calories of food
Diet B: Adult Canned Dog Food 8 divided by 1,198 = 0.006678 x 1,000 = 6.68 grams of protein per 100 calories of food
The two are pretty close, with the dry food containing 0.81 more grams per 100 calories — about a 3-gram difference in a 400-calorie meal.