Digestibility: Why All Calories Aren’t The Same

Wouldn’t it be great if you could eat a fat slice of cheesecake and only absorb a fraction of the calories listed on the side of the box?  That’s what happens with kibble (dry pet food) when your pet eats it.  Humans putting down a fat slice of cheesecake and only getting a fraction of the calories and nutrients could be considered good.  But with what’s intended to be a nutritional meal for your pet, it’s is not good at all.  There’s a way to determine how many calories and nutrients you’ll actually absorb from food.  It’s a measure called Digestibility, not talked about much, but it’s highly important when determining how much to feed your pet.

What is it?  The more technical overview is that a highly digestible food provides a higher proportion of absorbed nutrients than a less digestible food; digestibility provides one measure of a food’s nutritional value and quality. In general, as the quality of ingredients in the food increases so will the food’s digestibility and nutrient bioavailability.  It’s the difference between what is eaten and what is excreted.

That’s digestibility.  Said another way, it’s basically the better the food’s quality and more species-specific (all ingredients are digestible for, say, a cat), greater amounts of nutrients are absorbed into the body when eaten.  Since kibble has a fair amount of indigestible (and arguably harmful) ingredients, those calories and nutrients are just pooped out, resulting in little or no absorption.

When you buy food, pet or human, the nutritional information you see on the label is determined in a lab.  Those values  are measured and calculated through a chemical analysis of the food.  The measured lab calories and nutrient values are published on the food container with the assumption of 100%, or ideal, digestibility.  However the digestive system of a dog (or cat or human) isn’t able to extract the full amount that’s measured.  The lower the quality of food, and less species-specific, the less is absorbed.

If you were fed a cow’s diet, even though the portion given has 100 calories and various nutrients measured in a lab, you’d digest and absorb very little of the nutritional value of the meal, despite what a lab may show as being present.  That’s what’s meant by a bioavailable, species specific diet;  A cow’s diet isn’t ideal for a human.  Like the starches found in dog and cat kibble in high percentages aren’t bioavailable to dogs and cats.

If a laboratory determines a food item has 100 calories, it doesn’t mean all 100 calories will be absorbed when the food is ingested.  This is also true for the nutrients in the food: something less than 100% will be absorbed, proportional to the quality of the food and how species-specific it is.

This is one reason why the calorie requirement estimators you’ll find online tend to recommend too many calories. Kibble is the most inefficient (low digestibility) food that a dog or cat can eat, with some kibble brands having as much as 60% of the food (fillers, preservatives, starches) that are indigestible.  It can still be listed as 100 calories per cup, but only 40% will be extracted.  That’s why all calories, at least as listed, aren’t the same.  If your dog needs 100 calories of food a day, and one cup of kibble equals 100 calories, a low digestibility rating will mean you need to feed more, or even much more, than a cup of food to get those 100 calories.

As a rule of thumb, kibble or dry dog foods with digestibility values of 75% or less (which can go well below 50%) will be of poor quality, those with values between 75 and 82% are classified as moderate in quality, and foods with digestibility values that are higher than 82% are of high quality.  Don’t bother looking, you won’t find digestibility listed anywhere on a bag of kibble.  It’s not to the pet food manufacturer’s benefit to list digestibility, but they’ll recommend you feed twice as much food to make up for the low digestibility.

Depending on the formulation, raw food ranges from the upper 80s to mid 90s in digestibility.  You can’t create a completely digestible food, and wouldn’t ever want a 100% digestible food either.  Things like fiber and ash that can’t be digested help with evacuation — everyone needs to poop at some point. If you’ve wondered why people that feed raw, their pets poop so much less than kibble feeders, this is it — the body extracts the maximum amount of calories and nutrients from the food efficiently, not leaving much unused or indigestible material behind.

Here’s an example and how digestibility plays a role.

Dog Food A (kibble/dry food) contains 30 percent crude protein and is 75% digestible.

Dog Food B (raw) contains 50 percent crude protein and is 93% digestible.

Food A: 30g protein/100g diet x 0.75 = 22.5g protein absorbed

Food B: 50g protein/100g diet x 0.93 = 46.5g protein absorbed

That’s a HUGE difference in protein that’s absorbed, more than double.  It also means you can gauge from the label how much food to feed your pet.  Imagine this for the other nutrients that are ingested.  The point here is that when you feed raw you actually can feed less food but get more nutrition.

As mentioned, the bulk of calorie estimators you’ll find online tend to recommend too many calories, because they’re based on a kibble diet, the most popular form and most commonly used pet food in the USA.  They have to over-estimate the nutrition you feed your dog so your pet will get the calories and nutrient they need.  You’ll usually see that you should feed your pet 30-40 calories for every pound your dog/cat weighs (you should use a target weight, not necessarily their actual weight*).  With Fetching Foods it’s 15cals/lb for an adult dog or cat.  (For a senior dog, that will need to be reduced by 20% and for a puppy, the calories need to double to between 30 and 40 cals/lb.)  NOTE: your milage may vary with other raw formulations.

These estimators are the starting point to determining how many calories your pet needs is to gauge the quality of your food.  Most raw will be closer to a 1:1 listed calories to calories absorbed.  Then you need to realize the feeding estimators don’t take into account many factors, they are based on averages, so you’ll need to use your judgement.

Some variables that can change the 15cals/lb are your pet being spayed/neutered or not, how active the dog is, how many treats they get throughout the day, are they a puppy/kitten or a senior.   Some dogs and cats have a naturally higher or lower metabolism than average.  Start with 15cals/lb for Fetching Foods to get into the ballpark.  Then you’ll need to watch their weight and fine tune the food amount up or down.

Don’t rely on the estimations exclusively.  Most pets will continue to eat until they get fat — they won’t tell you that they’re eating too much.  If you don’t watch their body shape closely, you’ll soon have an overweight pet.  That has many unhealthy consequences, including joint problems, diabetes, and more.  The same can be said for feeding too little food.  As mentioned, use the estimator as a starting point.  Adjust the food amount up or down while monitoring your pet’s weight ensuring it stays in the healthy range.

Fetching Foods Calorie Chart for Adult Dogs/Cats

* Target weight: this is the ideal weight of your dog or cat.  If the target weight is 20lbs but your pet weighs 25lbs, feed at the 20lb quantity.  If you have a puppy, base it on what your pup needs to weigh at the end of a one or two week period.  This can change so fast with puppies.  You really have to monitor their growth.  A chihuahua is going to grow at a different rate than a great dane, and not chart will capture that, but having a target weight by week can help point you in the right direction for how much to feed.