Metabolic sensing neurons integrate a variety of signals and respond to glucose levels, fatty acids, insulin, leptin and other hormones and their metabolites. The brain uses this data to control appetite and food intake.
Some obese people experience a genetically reduced sensitivity of metabolic sensing neurons. This may be the target of future anti-obesity drugs. Exercise tends to improve other signalling pathways (insulin, leptin, growth hormone) and most likely improves overall metabolic sensing as well.