Calculating the energy content of dry food.

Nutrition Your Questions Answered

Hello, I’m Bill Wiadrowski from LifeWise Pet Nutrition and today’s question comes from Fiona who wants to know what the energy content of our foods are. Well, that’s really an interesting one because what Fiona is trying to do is work out a more accurate method of determining how much food to feed the dog and obviously if you know how much energy is in the food then you’ve got a better idea of being able to calculate a feeding rate.

There are a lot of drawbacks to this of course, because the actual feeding rate is then going to be determined by the efficiency of use of the nutrients – in other words, how available those nutrients are to digestion, but more importantly what the concentration of nutrients is relative to the energy level of the food. So a really simple way, well actually it’s not that simple.. but a way of calculating this is to use what we call Atwater Factors to do an analysis of the food and calculate how many grams of protein per 1000 kcal of energy is in the food.  This is then going to give us a yard stick for how much we are going to feed.

So if we took an average sort of diet and we said that we have 25% protein, 15% fat, sometimes you usually get a fibre level of the diet, but lets say assume that the diet’s got 5%, ash is sometimes quoted, we will assume 8%, so those figures are fairly generic and quite common in most of the super premium foods in the market today.  Most foods will have 10% or slightly less of moisture, so we will allow 10%. The one measurement that’s missing on the label, is going to be the Nitrogen Free Extract figure, or NFE as it’s commonly known which is fundamentally the carbohydrate content of the diet but we need to know that – because the big contributors of energy to the food are the protein, the fat and the NFE.

So, to find the NFE, we have to do a bit of a calculation.  We add up all of these things, protein, fat, fibre, ash, moisture and NFE that will come to 100%. So what we do is we add them all up (excluding the NFE) and deduct that from 100% and the answer we get for NFE in this case is 37%. NFE is always calculated by this method. It is a calculation based on the presence of all the other components of the food. Once we know that, we’ve got our three major contributors to energy; protein, fat and NFE. What we can do now is we can use Atwater factors to calculate the actual energy level of the diet.

Atwater factors:  Protein = 3.5   Fat = 8.5   NFE = 3.5

So what we do to work this out is that we multiply the number of grams of protein in our food by 3.5, so in this case it’s a 25% protein, so that means that there is 250g of protein in every kilogram so if we multiply 250g x 3.5, we get the answer of 875 kcal/kg.

The Atwater factor for fat is 8.5, so if we are to multiply the 150g (because 15% of 1000g is 150g) 150g x 8.5 = 1,275 kcal/kg of this food.  And for the NFE content of the food, we said it was 37%, which is 370g of NFE in a kilogram of food, multiply that by our Atwater factor of 3.5, and we get the answer which is 1,295 kcal/kg.  If we add those up, we can see it adds up to 3,445kcal/kg food.

So we now have a standardised method, we can compare most foods to see what the energy level is.  Now, Atwater factors are not the be-all and end-all. Certainly, a lot of this is determined by the availability of nutrients in the food or put another way the quality of the ingredients that are used in the manufacturing of the food but as a rule of thumb it works reasonably well. Atwater factors would rarely give you a energy level that’s higher than what’s actually in the food, but the actual energy level of the food could well be lower by virtue of poor quality ingredients.

Once we know that, what we do then is we want to calculate how many grams of protein are in 1,000kcal of energy. So that means that we’ve got 250g of protein and we divide that by 3.445 (because we’ve got 3.445 lots of 1,000kcal in 1 kilogram of food). If we do that, we end up with an answer that says that we have 72.5g (protein) /1,000kcal (energy) in the food. So what does that tells us?  Well, not a lot on its own, but this is a very useful method to be able to be able to compare one food to another.

When we look at the National Research Ccouncil recommendations from the Academy of Sciences they say that anything below 90g of protein per 1,000kcal is the medium range, below 50g/1,000kcal is low and above 90g/1,000kcal obviously is high.

What we find in today’s market, we find a lot of foods that give us Atwater figures that are way over 100, and what that actually tells us is that a lot of the protein in the food is being used as an energy source. That’s placing a burden on the body, that’s a stressor, and that is one of the recipes for renal issues going forward – too much protein in the food means that there is a heavy load on the kidneys and we don’t want our animals suffering from kidney damage, so it’s useful to work out this. For an average, healthy, active adult, we would expect the actual figure here to be down around about the 60 mark. Lower down is better, particularly if you’ve got a high efficiency food with high quality ingredients you can actually get down below 60 and you will have a very, very healthy animal because that is reducing the load on the body in terms of digestive efficiency and the bodily functions that accompany that. But, as we go up, we get into this higher stress type areas and for an adult eating over 90g protein/1000kcal energy that’s not good at all.

Puppy foods we would expect to see that up around about the 80-90g protein/1,000kcal, provided that once again protein sources are good and digestibility factors are high. More than that is not going to achieve any substantial gain in the wellbeing of the animal, it’s more than likely going to make the animal fat because you have an excess of protein and excesses of protein are generally stored in the body as fat.

So there you go Fiona, I hope that helps you a little bit, you can sit down and do your calculations using these Atwater factors and it gives you a system to be able to classify the effectiveness or efficiency factors of food.  It’s not the be all and end all by a long shot, but it is a far more accurate way of looking at these things. There are other factors that you would want to bring into consideration when you are actually assessing a diet like the ingredients used and the different ingredients that are there and the general spread or source of those ingredients, but in general terms, as far as energy density is concerned, this is not a bad starting point.

So there you go Fiona, hope it answers your question, and if you’ve got any more please send them in, but until next time, I’ll see you later.


Note: The energy content of our foods are listed on the back of our printed 2.5kg bags.

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