GCG and obesity due to melanocortin 4 receptor deficiency: However, fructose cannot directly promote insulin secretion because of the absence of GLUT5 in beta cells, and fructose intake stimulates glucagon-like peptide-1 (GLP-1) secretion via GLUT5-containing L-cells in the gut, thereby increasing insulin secretion, an effect more pronounced in individuals with obesity [80, 128, 129].