Digestibility or bioavailability?
Digestibility is the body’s ability to break food down into smaller molecules whereas bioavailability is the fraction of those molecules that are absorbed and utilised by the body. In other words, digestibility indicates the quantity of material broken down; bioavailability gives you the quality of nutrition actually delivered.
Digestibility is one of the most quoted numbers in canine nutrition. Commercial foods use it to signal “quality”, research papers rely on it to compare diets and dog owners often assume that a higher percentage simply means “better”. But the move to home feeding involves closer attention to nutrient use – and, for me at least, digestibility gives way to bioavailability.
As a measure, apparent digestibility can illuminate certain aspects of a diet while completely missing others. It is essentially a subtraction exercise: what went in versus what came out. A dog eats a fixed amount of food, excretes a smaller amount and the difference is labelled “digested”. That simplicity is part of the appeal. But this metric should be interpreted in context, particularly when diets include whole foods, because it also obscures the fact that the faecal mass includes far more than undigested bits of food. It contains endogenous losses: microbial biomass, sloughed intestinal cells, digestive secretions, water and fibre that has behaved physically rather than nutritionally. A rise in faecal bulk might reflect a change in motility or hydration, not a loss of nutrients.
In other words, digestibility “penalises” the very characteristics that many owners intentionally seek: variety, intact ingredients and minimal processing.
Fibre and the food matrix
The food matrix affects the rate and extent to which nutrients are released and absorbed. It can protect nutrients from degradation or make them more accessible. It reflects the processing that the product has undergone, including changes in physical state of the product. The structure of the food matrix can have a significant impact on physiological responses and overall health, sometimes leading to different health effects than what the individual nutrients would suggest[1].
Fibre is one of the clearest examples: it dramatically alters apparent digestibility, but not necessarily nutrient utilisation. Soluble fibres found in flax or chia mucilage promote regularity, improve stool consistency and help maintain consistent gut behaviour – all without compromising nutrient uptake. The mathematical effect is predictable: more water in the stool, more microbial mass as fermentation increases – and thus a lower apparent fat or energy digestibility.
None of this implies that fats, amino acids or essential minerals are lost or inaccessible. It is simply a reflection of how fibre shapes the physical behaviour of digesta. Insoluble fibres add bulk and speed transit; soluble fibres add viscosity and support fermentation. Both change stool output without altering the dog’s ability to absorb nutrients from the meal. Mucilage is a perfect example of this.
The difference
There is a clear reason for the difference between fresh and extruded diets. Kibble diets are typically high in processed carbohydrates (starch) due to the need for binding agents in the extrusion process. While starch is highly digestible in the small intestine, it breaks down rapidly, which can cause rapid blood-glucose spikes. The intact fibre delivered in whole-food diets, on the other hand, acts as an indigestible component that promotes healthy digestion and fuels more diverse and active gut bacteria.
Protein digestibility
Protein digestibility is often treated as a measure of protein quality. But that’s not the whole story. While digestibility captures the ease and speed with which enzymes can break down a protein, it tells us nothing about nutritional gain, what happens after absorption, whether amino acids are used for repair or immune function, or simply oxidised for energy.
Fresh, lightly cooked proteins tend to score well because their structure remains close to native form, which pancreatic enzymes recognise. Overprocessing, excessive heat or repeated cooking cycles can cause proteins to cross-link, lose solubility, or bind to carbohydrates in Maillard reactions that reduce enzymatic access. None of this is captured by gross digestibility outcomes in a meaningful way unless damage is severe.
Conversely, a modest drop in apparent protein digestibility in a fibre-rich diet does not imply poor-quality protein or inadequate amino acid supply. Fibre increases microbial biomass and those microbes incorporate nitrogen that later appears in the stool. Without correction for endogenous losses, the calculation underestimates how much protein the dog has actually absorbed and used.
Processing effects
Processing is where digestibility numbers most obviously mislead. Extrusion – high heat, pressure and mechanical shear – gelatinises starch, denatures proteins and breaks cell walls in ways that dramatically increase apparent digestibility. This is one reason commercial kibble frequently reports digestibility values in the high eighties or nineties. The lab sees highly accessible starch and simplified proteins.
But the same processing also damages heat-sensitive vitamins, reduces certain phytonutrients, promotes the formation of advanced glycation end-products, all of which force the manufacturer to rely on synthetic premixes to compensate. None of this is reflected in digestibility scores. The test doesn’t see structural or biochemical changes; it can only report how much matter disappeared between intake and output.
By contrast, home-cooked meals, with fresher matrices, more intact fibre and more fermentable material, will often produce lower apparent digestibility percentages – yet dogs eating these diets commonly show excellent stool quality, steady energy, improved coat condition and better metabolic markers.
In reality, the practical questions for most dog owners remain more straightforward:
- Are the stools well-formed and consistent?
- Does the dog maintain good condition and steady energy?
- Does the food behave predictably for this dog?
Apparent digestibility is a useful measure, but it is only one part of the picture. It cannot describe nutrient utilisation, tissue-level availability or the quality of what is absorbed. Used with context, it can help guide decisions; taken at face value, it risks oversimplifying a process far more intricate than a percentage on the bag can encompass.
Digestibility measures disappearance. Bioavailability measures usefulness.
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[1] Aguilera JM. The food matrix: implications in processing, nutrition and health. Crit Rev Food Sci Nutr. 2019;59(22):3612-3629. doi: 10.1080/10408398.2018.1502743. Epub 2018 Sep 10. PMID: 30040431.
Important Considerations:
- Always consult your veterinarian before making any significant dietary changes, particularly where there are pre-existing health conditions or dietary restrictions.
- If you are feeding commercial food, check the label for ingredients before giving more. Excessive intake of any foods can have adverse effects.
- Ensure (where possible) that you use high-quality, organic products specifically formulated for pets (or better still, human grade ingredients) to avoid any potential adverse effects.
- Introduce new foods gradually to avoid adverse effects such as gastrointestinal upset or diarrhoea.
- I provide nutritional information purely as a helpful guide. Nutritional information on ingredients is obtained from the US Department of Agriculture’s FoodData Central site (https://fdc.nal.usda.gov/index.html) and any nutritional information provided in recipes is based on an online calculator: calories and other information will vary based on brands, ingredients and other factors.
- Check nutrient levels and recommendations for your dog’s weight, age and activity. For example this nutritional guideline produced by FEDIAF.
- I am not a professional canine nutritionist but supporting research is cited.
- The recipes shared were created by me and tested in my kitchen – and tasted and approved by our doggy friends!