FGF21 graphic (IMAGE) Pennington Biomedical Research Center Caption When the liver detects low levels of protein, it activates cells that produce FGF21 and release the hormone into the bloodstream. FGF21 is then carried to target cells (neurons) in the brain. Those cells hit the alarm button when they see FGF21, signaling that protein intake is too low.the brain reacts by sending out a complex set of directions that, among other things, stop the mouse from growing, alter food intake, and increase the number of calories burned. Credit Pennington Biomedical Research Center Usage Restrictions none License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.