Insect gut microbes have been shown to provide nutrients such as essential amino acids (EAAs) to their hosts. How this symbiotic nutrient provisioning tracks with the host’s demand is not well understood. In this study, we investigated microbial essential amino acid (EAA) provisioning in omnivorous American cockroaches (Periplaneta americana), fed low-quality (LQD) and comparatively higher-quality dog food (DF) diets using carbon stable isotope ratios of EAAs (δ¹³C_EAA). We assessed non-dietary EAA input, quantified as isotopic offsets (Δ¹³C) between cockroach (δ¹³C_{Cockroach EAA}) and dietary (δ¹³C_{Dietary EAA}) EAAs, and subsequently determined biosynthetic origins of non-dietary EAAs in cockroaches using¹³C-fingerprinting with dietary and representative bacterial and fungalδ¹³C_EAA. Investigation of biosynthetic origins ofde novonon-dietary EAAs indicated bacterial origins of EAA in cockroach appendage samples, and a mixture of fungal and bacterial EAA origins in gut filtrate samples for both LQD and DF-fed groups. We attribute the bacteria-derived EAAs in cockroach appendages to provisioning by the fat body residing obligate endosymbiont,Blattabacteriumand gut-residing bacteria. The mixed signatures of gut filtrate samples are attributed to the presence of unassimilated dietary, as well as gut microbial (bacterial and fungal) EAAs. This study highlights the potential impacts of dietary quality on symbiotic EAA provisioning and the need for further studies investigating the interplay between host EAA demands, host dietary quality and symbiotic EAA provisioning in response to dietary sufficiency or deficiency.