Food digestibility and nutrient bioavailability are at the heart of nutrition. Digestibility is a measure of how much nutrition a food provides in a given volume. It indicates how much of the food is absorbed by the gut (intestines) into the bloodstream. It is the difference between what your cat eats and what your cat excretes.
Nutrient bioavailability is the proportion of the absorbed nutrients that are carried to target tissues and are available for use by the body. Because a highly digestible food provides a higher proportion of absorbed nutrients than a less digestible food, digestibility provides an important 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.
To understand which foods will provide the highest digestibility to our cats, we need to understand a little bit about their physiology.
We seek convenient food with our busy lifestyles. And most of us do not intuitively understand what cats need nutritionally to support their best health – they seem like little aliens. So we ask our vets. And despite the fact that 80% of cats over the age of three years have periodontal disease, many of our vets still tell us that for dental health, cats need kibble, and that to mimic their "natural" pattern of eating many small meals (being hunters of small mammals), we should leave the food out. We didn't know any better, and we trusted our vet to know what’s best for Bella.
Or we free-feed kibble to our kitties, because that’s what and how our parents fed our cats when we were growing up, and those kitties lived long, healthy lives. So 80% of us go to the store and buy a bag of kibble, come home, and pour it into a dish we keep full for Boots, and Socks, and Tigger. And yet we keep Boots and Socks and Tigger indoors exclusively, where our childhood friends were usually indoor-outdoor cats, supplementing their dry diet with healthy, freshly hunted prey. (If your kitty brought you gifts, that most certainly meant he was eating them, as research indicates that kitty only returns one in four hunted animals to present as gifts to his family.)
…except cats evolved eating high protein, moisture-rich food in the desert. Kibble:
This discrepancy between cats' needs and the dry food format has profound implications for their health, as discussed in our article Cat Food vs Cat health.
Many assume the influence of living among humans would have had an impact on what cats have the ability to eat and use for sources of energy and nutrition, as appears to the be case for dogs. Nope. Not so. Not at all.
Despite their proximity to humans for at least 10,000 years, cats retain their unique anatomic, physiologic, metabolic, and behavioral adaptations consistent with eating a strictly carnivorous diet. That is to say that cats, to this very day, remain obligate carnivores – and not just obligate carnivores, but hypercarnivores. By their genetic makeup, cats must eat the tissue of other animals in order to thrive.
If cats were in charge of the pet food industry, food instructions would read “remove mouse or rabbit from freezer, thaw and feed.” That idea makes many of us laugh and say, "My cat would never eat that!" That is only because they weren't raised eating properly (for a cat!) and usually have to learn to eat biologically appropriate fresh raw foods, having been imprinted to commercial food options, that use chemicals and flavoring to keep them eating. For cats, a highly digestible food is moisture rich, low carb, and made with fresh, high quality animal proteins.
The Digestibility of Cat Foods
Merriam-Webster defines digestibility as “the percentage of foodstuff taken into the digestive tract that is absorbed by the body.” Simply put, it is the difference between how much food your cat eats and how much is excreted in stool. What they can’t digest, they excrete.
Example: your cat eats one 5.5 ounce can of food a day. Your cat excretes 0.8 ounces of stool. That means your cat digested 4.7 ounces of the food she ate. 4.7 ounces of food absorbed divided by 5.5 ounces of food eaten = 85%. This food is 85% digestible. Yes, water is a factor, but not one we have to worry about unless comparing canned and kibble. The point is the difference between what they eat and what comes out is what is meant by “digestibility.”
Example Stool Comparison
On the left:
Food: High protein (meat by-products), low carbohydrate canned with guar gum, food coloring and "natural" flavors
Amount: 4.5 ounces per day.
Frequency of stool: Daily
On the right:
Food: Homemade raw food made with EZcomplete fur Cats using beef as the boneless muscle meat.
Amount: 4.5 ounces per day
Frequency of stool: Every-other-day
Why does this matter? What does this illustrate?
Let’s look at protein, being so important to a cat.
To determine the digestibility of protein (or any individual nutrient), the amount of the individual nutrient in the stool is determined. So if that 5.5 ounce can of food contains 19 grams of protein, and kitty excretes 3 grams of protein in the stool, kitty used, kitty absorbed, kitty metabolized 16 grams of protein. 16 grams of protein absorbed divided by the 19 grams of protein eaten = 84%. The protein in that food is 84% digestible. But different protein sources have differing digestibility due to their biological values, and different foods have varying ingredients that make the protein more or less accessible to the body.
Example: Canned foods 1 and 2 both have 12% protein as listed on the guaranteed analysis. They both have the same moisture content of 72%. This means each 5.5 ounce can of food has about 19 grams of protein.
Food 1 has a digestibility of 90%
Food 2 has a digestibility of 80%
Food 1: 19 grams x 90% = 17 grams of protein absorbed by the body
Food 2: 19 grams x 80% = 15 grams of protein absorbed by the body
Even though each can of cat food is labeled as having 12% protein and 72% moisture, and we expect they will provide the same amount of protein to our cat, they don’t. Food 1 provides more protein than Food 2.
If we presume both canned foods have the same protein as the main ingredient, how can there be such a big difference in digestibility? Digestibility is impacted by many factors apart from protein sources, though protein quality is an important factor in digestibility of the food, and bioavailability of the nutrients. Differences arise from protein source, protein quality, the macronutrient content, other ingredient differences of the foods: the presence (or lack) of fiber, gums or thickeners, particle sizes, processing techniques, the temperatures achieved in processing – and, of course, the age and health of the animal eating the food.
What Is a “High Quality” Protein?
First, we need to understand that not all proteins are created equal, especially when feeding an obligate carnivore. As Mark Peterson, DVM (renowned small animal endocrinologist) says, “The biological value of a protein is a measure of that protein's ability to supply amino acids (especially the 11 essential amino acids required by cats) and to supply these amino acids in the proper proportions. It is well-established that animal proteins (e.g., meat, meat by-products) have higher biological values than vegetable proteins (e.g., corn gluten meal, soybean meal, soy protein isolate).”
High quality protein is animal-based. High quality protein provides the 11 essential amino acids in proper proportions. The highest quality proteins come from fresh, raw human grade meats, not cooked meats, and not pet grade meats, which include diseased and downed animals in both canned and kibble cat foods.
So how do we know which foods are highly digestible and provide the most nutrition to our cats?
Kerr at al 2012. Apparent total tract energy and macronutrient digestibility and fecal fermentative end-product concentrations of domestic cats fed extruded raw, beef-based and cooked beef-based diets. J Anim Sci 2012, 90:515-522
Hamper et al 2015. Apparent nutrient digestibility of two raw diets in domestic kittens, J Feline Med Surg 2015 Sep 23
When discussing cat food, “high quality food” starts by being a food that is appropriate given the cat’s physiologic and metabolic needs. This is called “species appropriate” or “biologically appropriate” food. It refers to food that best resembles their natural diet, the evolutionary diet of the cat. This will place the least stress on their GI system, provide the most nutrition, and thus support their best long-term health. That diet is
The importance of feeding a cat a highly digestible, species appropriate diet cannot be ignored nor overstated, not only for its nutritional aspect, but also for the strain a highly processed diet puts on the digestive system of the cat, particularly on the pancreas, in trying to digest a food that is void of proper nutrients and enzymes, ultimately resulting in inflammation. Many of the top diseases our cats suffer are directly related to their diets and can easily be avoided by providing them with whole food-based nutrition.