Researchers have learned that obese people have lower than average levels of a hunger-regulating hormone.

Hunger and appetite are both complex phenomena, regulated by various hormones. A team at Imperial College London reports that a gut hormone called PYY3-36 is deficient in obese people.

They have also found that by 'topping' up the hormone levels they could reduce both perceived appetite and calorie consumption of both the lean and overweight volunteers by around a third, for a period of 24 hours.

PYY3-36 is released from the gut as a response to eating and it sends a signal to the brain saying that a meal has been eaten. Maybe a lack of this hormone triggers people to overeat and put on weight? There is now the intriguing possibility that PYY3-36 could be used as a new therapy for obesity, regulating people's feelings of hunger so they feel full once they have eaten.

Source

New England Journal of Medicine 3rd September 2003