GCG and obesity due to melanocortin 4 receptor deficiency: In a study on female individuals with obesity, the circulating levels of eight specific amino acids (isoleucine, leucine, lysine, methionine, phenylalanine, proline, tyrosine, and valine) were positively correlated with the plasma GLP-1 levels [89], and in experimental models, a range of specific free amino acids has been demonstrated to induce GLP-1 secretion with different potencies and through different mechanisms [90,91,92,93,94,95].