Glutamate can be produced in the brain via astrocytes and is also a component of human diets.69–71 The gut and associated microbes have a high capacity for dietary glutamate; it is converted into several important molecules in the gut including proline, arginine, glutamine, and GABA.71,72 The enzyme, glutamate decarboxylase (GAD), catalyzes the transformation of glutamate into GABA.30,73 The GAD gene is found in the genomes of several key AD-associated bacteria including Bacteroides, Bifidobacterium, Alistipes, Blautia, and Lactobacillus. This evidence concerns the gene GAD1 and Alzheimer disease.