Evidence Breakdown

 

Understanding Digestibility vs Bioavailability

 

Digestibility describes how much of a nutrient is absorbed from the gastrointestinal tract, while bioavailability refers to how effectively the body can utilize that absorbed nutrient for physiological processes.

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A diet may appear high in nutrients on paper while still showing poor nutrient utilization if digestion, absorption, or metabolic accessibility is impaired.

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Studies evaluating canine digestibility commonly measure:

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• apparent total tract digestibility (ATTD)

• ileal digestibility

• amino acid digestibility

• metabolizable energy

• stool quality

• fecal metabolites

• microbiome responses

 

Research demonstrates that nutrient composition alone does not fully predict nutrient utilization (Hendriks et al., 2015; Templeman & Shoveller, 2022). Critically, apparent total tract digestibility (ATTD) metrics can mask poor-quality protein sources, as hindgut bacterial fermentation converts undigested amino acids into ammonia and other metabolites, artificially lowering fecal nitrogen measurements. 

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Protein Digestibility and Amino Acid Availability

 

Protein digestibility is one of the most extensively studied aspects of canine nutrition because amino acid availability directly influences muscle maintenance, immune function, gastrointestinal health, and metabolic regulation.

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Research comparing protein sources demonstrates substantial variability in digestibility depending on:

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• ingredient source

• processing method

• anti-nutritional compounds

• amino acid composition

• heat exposure

• fiber interactions

 

Differences in protein digestibility among meat and fish substrates have been demonstrated using canine digestibility assays (Faber et al., 2010).

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Amino acid bioavailability may differ substantially even among diets with similar crude protein levels (Hendriks et al., 2015).

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Digestible indispensable amino acid scoring research suggests that ingredient quality and amino acid accessibility are more clinically relevant than crude protein percentage alone (Templeman & Shoveller, 2022).

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Hydrolyzed proteins may improve amino acid accessibility while reducing antigenic stimulation in dogs requiring elimination diets or gastrointestinal support (Hsu et al., 2023).

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Animal vs Plant Protein Digestibility

 

Modern canine diets increasingly incorporate plant proteins, legumes, pulses, yeast proteins, soybean products, barley protein, and mixed protein systems.

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Studies evaluating plant- and animal-based diets demonstrate that digestibility can remain high in carefully formulated diets, though amino acid profiles and protein quality may differ.

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Both animal- and vegetable-protein diets can achieve substantial digestibility, though true nutrient availability varies across formulations (Cargo-Froom et al., 2019).

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Both vegan and animal-based extruded diets have demonstrated acceptable macronutrient digestibility in healthy adult dogs under controlled conditions (Liversidge et al., 2023).

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Legumes and yeast proteins can function as alternative protein sources in canine nutrition when formulations maintain amino acid adequacy. (Reilly et al., 2021)

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However, plant proteins may have lower digestible indispensable amino acid scores for certain essential amino acids than high-quality animal proteins (Templeman & Shoveller, 2022). This is driven by both inherent amino acid profiles and the presence of antinutritional factors such as phytates, which chelate essential divalent minerals, and protease inhibitors, which actively inhibit enzymatic protein breakdown unless deactivated by precise thermal processing. 

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This distinction is clinically relevant for:

• growth

• muscle preservation

• senior dogs

• therapeutic diets

• chronic disease management

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Processing Methods and Digestibility

 

Processing methods substantially influence nutrient availability, microbiome responses, stool quality, and metabolizable energy.

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Comparative studies evaluating raw, fresh, mildly cooked, retorted, freeze-dried, and extruded diets demonstrate measurable differences in digestibility and fermentation profiles.

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Minimally processed and fresh diets often demonstrate high apparent digestibility and altered fecal metabolite profiles compared with extruded diets (Algya et al., 2018; Tanprasertsuk et al., 2021; Geary et al., 2024).

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Differences in amino acid digestibility and metabolizable energy have also been reported among frozen raw, freeze-dried raw, mildly cooked, and retorted diets (Geary et al., 2025).

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This divergence in energy utilization highlights a common gap between label predictions and biological reality. Standard pet food labels typically estimate Metabolizable Energy using predictive mathematical formulas like the Modified Atwater equations, which assume fixed baseline digestibility coefficients for protein, fat, and carbohydrates. However, because feeding trials show that fresh or minimally processed diets often achieve significantly higher actual digestibility coefficients than standard formulas assume, standard labels often underestimate the true caloric values that dogs absorb from highly digestible formulations. 

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However, higher digestibility does not automatically indicate superior clinical outcomes or nutritional completeness. Diet safety, formulation balance, microbial risk, and nutrient adequacy remain essential considerations.

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Extrusion itself may improve starch gelatinization and nutrient accessibility in some formulations while simultaneously reducing the availability of heat-sensitive compounds depending on processing intensity.

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Carbohydrate Digestibility and Glycemic Responses

 

Carbohydrate digestibility influences metabolizable energy, glycemic response, stool quality, fermentation patterns, and satiety.

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Common carbohydrate sources used in pet foods vary considerably in their nutritional value and digestibility (Cho et al., 2024), and shifts in the ratios of dietary macronutrients can significantly alter overall organic matter digestibility coefficients (Akhil et al., 2023). 

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Pulse-based diets may alter glycemic responses and taurine metabolism while still maintaining acceptable digestibility in healthy dogs (Quilliam et al., 2021).

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Fiber inclusion can either improve or reduce digestibility depending on:

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• fiber type

• fermentability

• particle size

• inclusion rate

• nutrient density

 

Cassava fiber, microalgae supplementation, enzyme additives, and xylanase supplementation have all demonstrated measurable effects on digestibility and fecal metabolites in controlled canine feeding trials (Souza et al., 2023; Cabrita et al., 2023; Jadhav et al., 2023; Carneiro et al., 2025)

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Mineral Bioavailability and Micronutrient Accessibility

 

Mineral digestibility and bioavailability are increasingly recognized as important components of nutritional adequacy.

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Studies evaluating selenium accessibility, organic mineral supplementation, and mineral digestibility suggest that ingredient interactions and processing methods significantly affect mineral utilization.

Selenium accessibility varies with diet composition and ingredient interactions (Van Zelst et al., 2015).

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Differences in mineral digestibility and physiological responses have also been reported in dogs fed organic mineral-based diets. (Chang et al., 2025)

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These findings are clinically important because laboratory nutrient inclusion levels may not reflect true biological availability.

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Digestibility and Gastrointestinal Disease

 

Digestibility becomes especially important in dogs with:

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• chronic enteropathy

• malabsorption

• inflammatory bowel disease

• pancreatitis

• postoperative recovery

• gastrointestinal sensitivity

 

Highly digestible diets reduce intestinal workload and may improve nutrient absorption efficiency in compromised gastrointestinal systems (Lenox, 2021).

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Semi-elemental, hydrolyzed protein diets may help achieve clinical remission in dogs with chronic enteropathy (Manchester et al., 2023).

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Therapeutic gastrointestinal diets can also influence microbiota composition, intestinal permeability, fecal metabolites, and immune regulation (Santos et al., 2025).

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Digestibility therefore functions not only as a nutritional concept but also as a therapeutic strategy in veterinary medicine.

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Age, Breed Size, and Individual Variation

 

Digestibility is not identical across all dogs.

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Research demonstrates that nutrient utilization may vary according to:

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• age

• breed size

• gastrointestinal physiology

• activity level

• disease state

• microbiome composition

 

Measurable differences in digestibility associated with aging and life stage have been reported across multiple studies (Weber et al., 2003; Kim et al., 2021; Schauf et al., 2021; Lee et al., 2025).

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Gastrointestinal physiology and microbiome composition also differ substantially across dog sizes, from Chihuahuas to Saint Bernards (Deschamps et al., 2022).

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This variability helps explain why dogs may respond differently to the same diet despite similar nutrient labels.

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Stool Quality, Microbiome Effects, and Fermentation

 

Digestibility studies frequently assess stool quality because fecal characteristics provide indirect indicators of nutrient utilization and gastrointestinal tolerance.

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Multiple studies report associations between digestibility and:

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• fecal consistency

• fermentation products

• microbiome composition

• short-chain fatty acid production

• intestinal metabolites

 

Specific bacterial families associated with protein and energy digestion in dogs have been identified (Bermingham et al., 2017).

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Fresh, raw, and mildly processed diets may produce distinct microbiome and metabolomic profiles compared with extruded diets. (Do et al., 2021; Geary et al., 2024)

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However, alterations in microbiota composition do not automatically indicate clinical benefit or harm without corresponding clinical outcomes.

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Digestibility Testing Methods and Limitations

 

Digestibility research uses several methodologies, each with important limitations.

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Common methods include:

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• apparent total tract digestibility

• ileal digestibility

• rooster assays

• in vitro simulations

• in-home feeding trials

 

Methodological complexities involved in canine digestibility testing have been highlighted in both laboratory and in-home feeding studies (Hagen-Plantinga et al., 2014; Bos et al., 2022).

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In vitro digestibility models may not fully replicate canine gastrointestinal physiology in vivo, while in-home feeding trials introduce variability in compliance and feeding consistency.

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Apparent digestibility also differs from true digestibility because fecal nutrient measurements include endogenous losses.

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These methodological differences complicate direct comparison between studies.