Fuel monitoring in the liver or hepatic portal area was historically implicated in the control of eating. According to this view, a common denominator of nutrient metabolism such as the intracellular ATP/ADP ratio was supposed to modulate eating through changes in hepatic vagal afferent signaling. More recently, this hypothesis has been questioned because hepatic parenchymal vagal afferent innervation is scarce and because experimentally induced changes in hepatic fatty acid oxidation often failed to produce changes in eating. Accumulating evidence suggests that small intestinal enterocytes rather than hepatocytes may serve as energy flow sensors in the control of eating. These recent developments are discussed here and an outline is given of the challenges of this promising new concept.