Sex differences in dietary leucine utilization revealed by ¹³C tracing in the cockroach Blaptica dubia

Sex differences in amino acid acquisition and allocation are fundamental determinants of insect life-history strategies, yet the fate of assimilated amino acids after ingestion remains largely unknown. Here, we used ¹³C stable isotope tracing to investigate sex-specific patterns of dietary leucine utilization in the cockroach Blaptica dubia. Adult males and females were fed artificial diets containing either ¹³C-labeled or unlabeled leucine for 14 days, after which food consumption; body, gut, and fecal mass; gut length; fecal isotope enrichment; and tissue-specific ¹³C incorporation were measured. ¹³C enrichment of CO₂ was also measured. Females consumed less food than males but had substantially greater fresh and dry body mass, longer alimentary canals, and larger gut mass. Despite consuming more food and having significantly lower body mass (almost half that of females), males did not show greater ¹³C enrichment in somatic tissues; no significant sex differences were detected in the brain, fat body, muscle, or respired CO₂. In contrast, male reproductive tissues showed significantly higher isotope enrichment than female oothecae, suggesting differences in the timing and dynamics of nutrient allocation to reproduction. Male feces showed significantly greater ¹³C enrichment than female feces. In males, fecal ¹³C enrichment did not differ between days 7 and 14, whereas females showed greater enrichment on day 14, consistent with slower food transit. Together, the larger, longer alimentary canal and lower fecal ¹³C enrichment in females, combined with their lower food intake, are consistent with greater leucine retention and utilization and may indicate more efficient nutrient assimilation in females.