Can isoproteic diets with different amino acid profiles modulate the amino acid composition of the black soldier fly?

The nutritional composition of black soldier fly (Hermetia illucens; BSF) larvae is highly dependent on feed substrates, yet the independent effects of substrate protein levels and amino acid composition remain poorly understood due to the use of complex mixed feeds. This study employed a simplified isoproteic model using three purified peptones (meat, soybean, and casein) at two protein levels (124 g/kg and 46 g/kg DM) to investigate larval metabolic responses. The results reveal that BSF larvae employ a dual regulatory mechanism in response to varying nutrient inputs. The total amino acid (TAA) profile exhibited compositional homeostasis, where all essential amino acids were affected solely by dietary protein levels rather than amino acid composition, ensuring stable nutritional value across substrates. In contrast, the free amino acid (FAA) pool displayed functional plasticity; while total FAA content remained relatively constant (20.0-27.2 g/kg DM), its composition reflected both passive accumulation of dietary markers and selective patterns that diverged from substrate availability. Elemental composition showed a similar differentiated regulation: Zn, B, and Mo exhibited substrate-dependent bioaccumulation, whereas Fe was maintained under strict homeostatic control. These findings indicate that BSF larvae maintain core nutritional quality through TAA rigidity while utilising FAA and elemental plasticity to adapt to substrate variations. This dual strategy provides compositional insights into insect nutritional homeostasis and demonstrates that BSF biomass can be reliably tailored through substrate formulation for specific feed applications.