Dietary inclusion of spent coffee grounds (SCG) shifts black soldier fly larvae towards catabolism: insights from metabolomics and nutrient mass balance.

Abstract

Black soldier fly (Hermetia illucens, BSF) larvae efficiently convert organic residues into nutrient-rich biomass for use as animal feed, but the metabolic responses to nutritionally challenging substrates remain poorly understood. In this study, the effects of incorporating spent coffee grounds (SCG) into the BSF larval diet were evaluated by combining growth performance metrics, fatty acid and amino acid mass balance analysis and untargeted nuclear magnetic resonance (NMR) metabolomics. SCG inclusion reduced larval performance and overall nutrient retention. Metabolomic profiling confirmed diet-driven multivariate shifts in central metabolism after partial least squares discriminant analysis (PLS-DA, p < 0.001), with SCG-fed larvae exhibiting markers of intensified lipid and protein catabolism (acetylcarnitine and β-alanine, respectively), which may represent changes arising from altered glycolysis. Together, these results indicate that although SCG provides usable nutrients, its incorporation imposes metabolic constraints that ultimately limit larval growth. Our findings inform biowaste valorisation in BSF bioconversion and reveal metabolic plasticity of insect larvae to nutritional stress.

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