This is a horrible topic! It’s dry, and it involves numbers and maths, so nobody really likes it – myself included. However, there are some really important concepts here.
With more and more pet parents educating themselves on how to read pet food labels, it’s essential to discuss how nutritional information is presented. And this means discussing the terms, ‘as fed basis’, ‘dry matter basis’ and ‘energy (or caloric) basis’. In this post, I’ll explain why caloric basis is the most accurate way to compare diets, and give you an example of how misleading it can be to use either as fed or dry matter basis.
As fed basis, dry matter basis and caloric basis are the three ways that pet food manufacturers present nutritional information for products, so we need to understand what they are. The figures provided in the nutritional panel often affect which diets pet parents choose (for example, a higher protein diet, a lower phosphorus diet), so it’s essential to use the most accurate methods of comparing and selecting diets. Let’s get started with some basics:
As fed basis:
An as fed analysis is the percentage or amount of nutrient by weight in the diet, exactly as you would feed it (including the water in the product).
On an as fed basis, for example, a canned diet might have up to 80% water, and 9% protein. A dry diet, however, might have 10% water, and 35% protein. Does this mean that the dry diet is much higher in protein? No, not necessarily.
Canned and fresh diets include a lot more water than kibble or air-dried diets, so in order to compare apples with apples, we need to remove the moisture content when comparing the nutrient profiles of wet vs. dry products.
Dry matter basis:
For the example above, the canned diet actually has 45% protein on a dry matter basis, and the dry diet has 38% protein on a dry matter basis. So – as fed basis can be quite deceptive.
So, the dry matter analysis is the percentage or amount of nutrient in the diet, if you removed all of the water. This helps us to compare the nutrient content of dry and wet diets more accurately, without the big difference in moisture affecting the figures as much.
Energy or caloric basis:
The caloric analysis is the amount of nutrient per 1000 kcals of food. Some manufacturers will report this per 100 kcal basis, so always be careful to double-check the units. So, an example of this would be “15 g calcium per 1000 kcal ME (metabolisable energy)”. This means that for every 1000 kcal your pet eats, they will ingest 15 grams of calcium.
Metabolic body weight basis:
The NRC (National Research Council) provides dietary nutrient concentrations on a metabolic body weight basis, as well as on a dry matter basis and per 1000 kcal ME. This is the amount of nutrient required per kilogram of metabolic body weight. Some nutrient requirements are thought to vary directly with body weight, however there is little information on this.
As pet food manufacturers are not formulating products for individual animals, you won’t come across metabolic body weight basis anywhere. It is mainly relevant to nutritionists formulating a diet for an individual, and is used fairly infrequently.
I read online that I should compare diets on a dry matter basis. Why is this not necessarily always the most accurate method?
Let’s answer this question with an example – it’s a bit easier to understand that way.
Here are two diets for cats with kidney disease, a canned diet, and a homemade raw diet on the right. For simplicity, let’s consider only one nutrient – protein:
For the canned diet:
As fed basis = 9.8% protein
Dry matter basis = 38.7% protein
Caloric basis = 94.9 g protein per 1000 kcal ME
Energy density: 1.03 kcal/gram
For the raw diet:
As fed basis = 14.6% protein
Dry matter basis = 49.8% protein
Caloric basis = 94.5 g protein per 1000 kcal ME
Energy density = 1.54 kcal/gram
Look at the figures carefully – what you will notice is that the protein content of the diets differs a lot on both an as fed basis and a dry matter basis. The canned diet looks much lower in protein.
But now look at the energy (or caloric) basis figures. In fact, on an energy basis, the raw diet is slightly lower in protein than the canned diet.
What does this actually mean, though?
Let’s consider an imaginary cat with advanced kidney disease: Bob. Bob is a larger cat, weighing 6 kg and with ideal body condition. He needs around 345 kcal per day to meet his energy requirement and maintain his body weight.
Now I’ll work out how many grams of each diet he will need to eat per day, by dividing his caloric requirement by the kcal/gram for each diet.
Canned diet = 345 / 1.03 = 335 grams per day
Raw diet = 345 / 1.54 = 224 grams per day
So, as you can see, you would need to feed a lot more of the canned diet every day, to meet Bob’s energy requirement.
However, and this is the most important thing – if Bob eats 345 kcal of either diet, he will ingest almost exactly the same amount of protein (32.7 g per day for the canned diet vs. 32.6 g per day for the raw diet).
It is now obvious how misleading figures on an as fed basis or dry matter basis can be – the protein content of the raw diet appears more than 10% higher than the canned diet (on a dry matter basis). Why is this though? And what can you do about it, when many pet food manufacturers don’t provide information on their diets on a caloric basis?
Let’s look again at the kcal/gram for both diets – the canned diet has 1.03 kcal/gram, and the raw diet has 1.54 kcal/gram. This means that the raw diet is more energy-dense, due to higher fat, lower moisture and/or lower fibre content.
What this means is that every gram of the raw diet does contain more protein, compared with the canned diet. But because the raw diet is more energy dense (higher kcal/gram), you feed less of it per day, and therefore the overall amount of protein consumed is equivalent to the canned diet.
If you fed exactly the same amount of each diet (for example, 100 grams), then the 100 grams of raw diet would certainly contain more protein, relative to 100 grams of canned diet. But that's not how we feed dogs and cats - and we don't feed them a set amount of dry matter per day either. We feed them to meet their energy requirement, because if we don't they either lose weight or gain weight. So the point is - it's important to know how much of each nutrient your pet is ingesting, relative to the number of calories they need every day.
Even if it’s best to compare diets on an energy basis, what can I do with the as fed analysis that manufacturers commonly provide?
It is actually quite quick and easy to convert figures on an as fed basis, to energy basis. To do this you will need:
The guaranteed or typical analysis including the percentage of the nutrient you are interested in (for example, 10% protein, as fed)
The kcal per kilogram of the diet (for example, 3630 kcal/kg)
Step one:
Convert the percentage to grams per kilogram, by multiplying by 10.
For example, if the diet contains 10% protein, then it will contain 100 grams of protein per kilogram of diet.
Step two:
Divide the grams/kilogram by the kcal/kilogram, then multiply by 1000.
For the example above, divide 100 grams of protein by 3630 kcal/kilogram.
100 / 3630 x 1000 = 27.5 g protein per 1000 kcal ME.
If that’s sounding nightmarish, for a quick/rough estimate, you can divide the grams per kilogram (as fed) by 4.
For example, 100 grams of protein per kilogram of diet / 4 = 25
This is reasonably similar to the 27.5 g we calculated above.
The other thing you can do it to contact the manufacturer of the diet and ask for this information. They should be able to provide you with a typical analysis of the diet, expressed on an energy basis.
Below you can see a screenshot of the nutritional information provided by Primal Pet Foods for one of their freeze-dried diets, and you can select either as fed, dry matter basis, or caloric basis. Hopefully more and more manufacturers will start to provide analyses on an energy basis, as well as dry matter basis and as fed basis.
Practical applications
Many pet parents are trying to select non-prescription diets with:
Higher protein
Lower fat
Lower carbohydrate
Lower phosphorus
To try and improve accuracy when making these choices:
Ask the manufacturer for a typical analysis, instead of using the (less accurate) guaranteed analysis. Sometimes the guaranteed analysis is all you get, but it’s important to understand that it might not be accurate.
Use figures provided on an energy basis, wherever possible. Do the maths yourself if you are happy to. This is especially important when comparing diets that differ a lot, in terms of their energy density.
Any feedback is always welcome - please leave comments or questions below and thanks for reading!
Very good explanation, clear and concise.