GCG and Alzheimer disease: The objective of this narrative review is to synthesize the current evidence on the humoral, endocrine, immune, and neural communication mechanisms linking the gut and brain in AD and highlight newly discovered GBA messengers such as microRNAs, extracellular vesicles, T-cells, and the intestinal hormones, including emerging neuroprotective role for glucagon-like peptide-1 (GLP-1).