ghrelin

noun

ghrel·​in ˈgrel-ən 

: a 28-amino-acid peptide hormone that is secreted primarily by stomach cells with lesser amounts secreted by other cells (as of the pancreas) and acts to stimulate appetite and the secretion of growth hormone

Weight reduction brought about by caloric restriction causes the ghrelin level to increase, suggesting that ghrelin might contribute to the drive to eat that makes long-term success with dieting so rare.

—Jeffrey S. Flier and Eleftheria Maratos-Flier, The New England Journal of Medicine