CHICAGO - The appetite-suppressing hormone leptin may do more than just reduce one's hunger: it might also regulate one's fondness for food and thus figure high in overeating and obesity, a study released Thursday said.

After experimenting on two people with a rare congenital leptin deficiency, British researchers found that providing them with additional levels of the hormone appeared to diminish the amount of pleasure they associated with food, as well cutting their sense of hunger and increasing feelings of fullness.

The finding suggests that our enjoyment of food is partly biologically-driven and may ultimately explain why some people have to work harder to control their weight than others, the authors of the paper said.

"While body weight remains stable for many people over a long period of time, other people gain weight very easily," said Sadaf Farooqi, a researcher in the department of clinical biochemistry at the University of Cambridge.

"More studies are needed to find out how these brain responses vary in people with weight problems in general."

"The appetising properties of food have been shown to have a strong effect on the centres of the brain linked to rewarding emotions and desires. These areas of the brain light up when individuals lacking leptin are shown images of food.

To measure the pair's psychological response to food, the researchers used brain scans to track the activity in these pleasure centres as they were shown pictures of food.

They took scans before and after a meal and during the two phases of the study -- in their normal state, and after supplementary leptin therapy.

In their normal leptin-deficient state, the reward centres of the volunteers' brain lit up in response to images of chocolate cake, pizza, strawberries, cauliflower and broccoli, regardless of whether they were hungry or not.

After treatment with leptin, their reaction was more discriminating and closer to that seen in healthy volunteers, who tend to show that kind of pattern of brain activity only when they are hungry and presented with images of appetising foods.

Similarly, post-leptin therapy, the nucleus accumbens, the part of the brain that seems to be particularly attuned to the prospect of appetising food, lit up predominantly when they had nothing to eat overnight and were hungry, and in response to foods they like.

The results indicated that leptin acts on the neural circuitry in a brain region called the ventral striatum, diminishing the perception of how rewarding food is while enhancing the ability to feel full after eating.

The study appears in the journal Science. - AFP