Nile tilapia digest dietary non-starch polysaccharides (NSP) to a measurable degree — a meta-analysis of 95 estimates found a mean apparent digestibility coefficient (ADC) of 24.3%, with 88% of cases positive — yet the enzymatic origin of this digestion is unknown: fish appear to lack the cellulases, xylanases, beta-glucanases and beta-galactases needed to cleave the beta-glycosidic bonds of NSP, and no study has determined whether hydrolysis is due to endogenous fish enzymes, autochthonous gut microbiota, or transient microbes ingested with the feed. The mapping from NSP chemistry to digestibility remains qualitative (cellulose is nearly inert while soluble NSP are partly digested), and no validated model predicts NSP digestibility from NSP type, diet composition, fish size, feeding level, ingredient processing, or water temperature. Follow-up work from 2020 to 2026 has profiled volatile fatty acids and lactate along the tilapia gut in vivo and quantified the low (about 5%) energy utilisation efficiency of digestible NSP, but as of late 2025 the digestion mechanism is still described only as 'possibly' microbial fermentation. The problem is to identify the hydrolytic and fermentative agents of NSP digestion in Nile tilapia and to establish a validated quantitative prediction of NSP apparent digestibility for use in feed evaluation.

Background

Non-starch polysaccharides (NSP) are the carbohydrates of plant cell walls other than starch and simple sugars: cellulose, hemicelluloses such as arabinoxylans, pectins, and various oligosaccharides. In monogastric animals (animals with a single-chambered stomach, including fish), NSP have traditionally been treated as indigestible 'fibre' of no nutritional value. As fishmeal is replaced by plant-derived ingredients in aquaculture feeds, dietary NSP levels rise, so the question of whether fish can extract energy from NSP has become practically important. Nile tilapia (Oreochromis niloticus) is a warm-water omnivorous cichlid and one of the world's most farmed fish; it tolerates high-carbohydrate diets and has a relatively long gut. Digestibility in this field is quantified by the apparent digestibility coefficient (ADC): the fraction of an ingested nutrient that does not appear in the faeces, measured in vivo using inert dietary markers (indigestible reference substances added to the feed so that nutrient disappearance in the faeces can be located). A meta-analysis compiling all available tilapia digestibility studies (Maas, Verdegem, Wiegertjes and Schrama 2020) found that NSP is not inert in tilapia: mean NSP ADC was 24.3% across 95 cases, 88% of cases showed positive NSP digestibility, and NSP digestibility contributed measurably to energy digestibility — although higher dietary NSP also worsened the feed conversion ratio (feed intake per unit of growth, FCR) and reduced the digestibility of crude protein, fat and energy. This creates an enzymatic puzzle. Fish digestive enzymes (alpha-amylase, disaccharidases, alpha-glucosidases) hydrolyse alpha-glycosidic bonds, as in starch; the beta-glycosidic bonds linking the long NSP chains require cellulase, beta-xylanase, beta-glucanase and beta-galactase, which are scarce or absent in the fish gastrointestinal tract (as compiled in Maas et al. 2020). Cellulase activity has been reported in some carp species, and bacterial strains isolated from tilapia intestines show cellulase activity in vitro, but it is unknown whether the responsible microbes are permanent (autochthonous) gut residents or are ingested transiently with detritus and feed, and whether any hydrolysis at all is endogenous. The digestion of NSP is assumed to proceed by anaerobic microbial fermentation producing volatile fatty acids (VFA — mainly acetate, propionate and butyrate) and lactic acid, which are absorbed by the gut; in vivo VFA profiles along tilapia gut segments exist in only a handful of studies, and lactic acid was measured in vivo for the first time only in 2021. Related warm-water omnivores also digest NSP — African catfish (Clarias gariepinus) reach NSP ADCs of up to 56% (Leenhouwers, ter Veld, Verreth and Schrama 2007) — and recent work shows that digestible NSP supports growth with an energy utilisation efficiency of only about 5%, versus about 73% for digestible starch (Dey et al. 2025), so mispredicting NSP digestibility materially biases diet energy evaluation. What is missing is, first, identification of the agents that actually hydrolyse NSP in the tilapia gut, and second, a validated quantitative mapping from NSP chemistry and diet and fish variables to NSP digestibility.

Problem Statement

The problem, as posed in the 2020 meta-analysis of Maas, Verdegem, Wiegertjes and Schrama, is to give a mechanistic and quantitative account of non-starch polysaccharide (NSP) digestion in Nile tilapia (Oreochromis niloticus), sufficient for its use in feed evaluation. Concretely: (1) identify the agent(s) responsible for hydrolysing the beta-glycosidic bonds of dietary NSP in the tilapia gastrointestinal tract — endogenous fish enzymes, autochthonous gut microbiota (permanent gut residents), or transient microbes ingested with the diet — and characterise the associated fermentation, that is, the in vivo profiles of volatile fatty acids and lactic acid along gut segments and their absorption and metabolic fate; (2) establish how NSP apparent digestibility depends on NSP chemistry — cellulose versus hemicelluloses/arabinoxylans versus pectins versus oligosaccharides, including structure, molecular complexity, and soluble versus insoluble behaviour; and (3) deliver and validate a quantitative predictive rule for NSP apparent digestibility coefficient (and its digestible-energy contribution) from diet composition, NSP type, fish body weight, feeding level, ingredient processing, and water temperature, where 'validated' means that the rule is fitted and its predictive accuracy reported against the published NSP apparent-digestibility measurements for tilapia — including the 95 meta-analytic estimates compiled by Maas et al. 2020 and subsequently published values — under a stated error metric and out-of-sample or cross-validation protocol. These three strands are jointly mandatory, and together they replace the current qualitative treatment of NSP as a single inert fraction with a mechanism-based, quantitative component of tilapia feed formulation. (Maas et al. 2020 also identified two adjacent questions — the interspecific generality of NSP digestibility among warm-water omnivorous fish, and the molecular mechanisms of beta-glucan (yeast- and cereal-derived glucose polysaccharides) immune modulation — which are separable research objectives outside this problem's scope.)

References

  1. Maas RM, Verdegem MCJ, Wiegertjes GF, Schrama JW (2020) Carbohydrate utilisation by tilapia: a meta-analytical approach. Reviews in Aquaculture 12(3):1851-1866. DOI: 10.1111/raq.12413. https://doi.org/10.1111/raq.12413
  2. Maas RM, Deng Y, Dersjant-Li Y, Petit J, Verdegem MCJ, Schrama JW, Kokou F (2021) Exogenous enzymes and probiotics alter digestion kinetics, volatile fatty acid content and microbial interactions in the gut of Nile tilapia. Scientific Reports 11:8221. DOI: 10.1038/s41598-021-87408-3. https://doi.org/10.1038/s41598-021-87408-3
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