Very extensive research, over many decades, has identified a number of biochemical routes that can influence appetite in humans.
For an overview, see: Mechanisms of Appetite Regulation
In particular, the so called 'hunger hormones' leptin and ghrelin, are known to play a crucial role in the biochemical feedback mechanisms involving the brain - especially the hypothalamus. Pleasure-seeking and hedonic responses to food intake are also influenced, for example, by endorphins, dopamine, and endocannabinoids. The feedback system is so complex, involving not only bio-chemical, but psychological and cultural factors, that an overall inclusive picture is not yet agreed.
“Appetite regulation by the brain undoubtedly depends on signals received from the periphery. The stimulus to eat, at first claimed to originate in the stomach, is now proposed to be a consequence of departures from homeostatic levels of glucose, lipids, or amino acids. Debate continues about whether changes in the availability of each of these metabolic fuels give rise to separate feedback signals or whether this information is integrated to produce a single, common, stimulus to eat.”