Glucose regulation in birds

Birds maintain higher plasma glucose concentrations (PGlu) than other vertebrates of similar body mass and, in most cases, appear to store comparatively very little glucose intracellularly as glycogen. In general, birds are insensitive to the regulation of PGlu by insulin. However, there appears to be no phylogenetic or dietary pattern in the avian response to exogenous insulin. Moreover, the high levels of PGlu do not appear to lead to significant oxidative stress as birds are longer-lived compared to mammals. Glucose is absorbed by the avian gastrointestinal tract by sodium–glucose co-transporters (SGLTs; apical side of cells) and glucose transport proteins (GLUTs; basolateral side of cells). In the kidney, both types of glucose transporters appear to be upregulated as no glucose appears in the urine. Data also indicate that the avian nervous system utilizes glucose as a metabolic substrate. In this review, we have attempted to bring together information from a variety of sources to portray how glucose serves as a metabolic substrate for birds by considering each organ system involved in glucose homeostasis.