Dietary Nitrogen-Free Extract as a Confounding Nutritional Factor in Growth and Digestibility Studies of Arapaima gigas: A Meta-Analysis

Abstract

Arapaima gigas is a carnivorous freshwater species with high dietary protein requirements and increasing relevance for aquaculture. However, nutritional studies with this species frequently use experimental diets with varying carbohydrate fractions that are not explicitly controlled. Nitrogen-free extract (NFE), which represents soluble dietary carbohydrates, may therefore influence growth and digestibility responses and act as a confounding nutritional factor. The objective of this study was to evaluate the effects of dietary NFE on growth performance, feed efficiency, and nutrient utilization in A. gigas through meta-analysis. A systematic literature search was conducted using peer-reviewed and grey-literature sources. Studies were included when they provided quantitative information on dietary composition together with growth-performance indicators or digestibility coefficients. Twenty studies met the eligibility criteria and were included in the quantitative synthesis. Specific growth rate (SGR), feed conversion ratio (FCR), and apparent digestibility coefficients (ADC) of dry matter (DM), crude protein (CP), and gross energy (GE) were analysed using standardized mean differences (Hedges’ g) under random-effects models. Digestibility analyses were restricted to plant-based energetic and protein ingredients because animal-derived raw materials contribute minimal NFE. Higher dietary NFE levels were associated with reduced SGR, while the effect on FCR was more sensitive to study structure and within-study dependence. Higher NFE levels were also associated with reduced DM and GE digestibility, whereas the effect on CP digestibility was not statistically significant and showed substantial heterogeneity. Sensitivity analyses supported the robustness of the SGR, ADC DM, and ADC GE responses. The available evidence suggests that dietary NFE represents an important source of variation in nutritional studies involving A. gigas. Although heterogeneity among studies and variation in ingredient composition limit causal inference, improved control and reporting of dietary NFE may reduce confounding effects and improve interpretation of growth, feed efficiency, and digestibility responses in future feeding trials.

The authors' abstract, as published at the source. Fishes, 2026 · DOI ↗