Fish Digestive Capacity: Definition and Methods

The nutritional value of a diet and its bioavailability in fish depend on three primary capacities: (a) ingestion, (b) digestion, and (c) absorption. Among these, digestive capacity, defined here as the total enzyme activity available to hydrolyze the bonds of dietary macromolecules to obtain hydrolysis products that are ultimately converted into absorbable micromolecular units, establishes the upper limit for the bioaccessibility of nutrients. To clarify usage and measurement, we conducted a systematic SCOPUS survey (January 2020–June 2024; 62 relevant articles). Most studies either omit a clear definition of digestive capacity or conflate it with digestive organ morphology or isolated enzyme activities. We compared indicators and assay conditions (substrate type, pH, temperature, and expression of units), revealing significant inter-study variability. Based on this synthesis, we propose four operational definitions: (a) Extract Theoretical Volume (ETV)—calculated volume of extract, considering both the solvent volume (SV) used for tissue homogenization and the tissue’s water content; (b) digestive capacity (U)—the total catalytic activity present in the digestive tract at the moment of sampling, where 1 U is the amount of enzyme catalyzing the formation of 1 µmol of product per minute under species-specific physiological pH, ionic strength, and temperature, with the total activity expressed as U fish−1, U organ−1, or U g−1 fish or U g−1 organ, enabling direct comparisons across studies; (c) Digestive Processing (DP)—the total number of bonds hydrolyzed during a given digestion time, whether instantaneous or over a defined period; and (d) Digestive Processing Index (DPI, U-min or U-h), which integrates digestive capacity over time. This framework provides a harmonized checklist for assay standardization and advances comparative studies in fish digestive physiology.