The study, conducted by researchers at University of Cambridge headed by Dr Sadaf Farooqi and Dr Paul Fletcher, found that leptin, one of the key hormones responsible for reducing hunger and increasing the feeling of fullness, also controls the fondness for food.
As a part of their study, the researchers studied patients with a rare genetic disorder resulting in a complete lack of leptin. These patients tend to eat excessively, like all types of food (including really bland foods) and develop severe obesity. After treatment with leptin, their hunger is reduced, they become more choosey about food and they lose weight.
The boffins recorded the brain activity of the participants using Functional Magnetic Resonance Imaging (fMRI) on showing them pictures of food to find out how the brain reacted.
They noted that the parts of the brain - known collectively as striatal regions - are activated or "light up" in response to different pictures.
The pattern of brain activation in response to pictures of food was compared to that seen with pictures of non-food items such as trees, cars, and boats. Some of the foods were really appetising (chocolate cake, strawberries, pizza) while others were rather bland (cauliflower, broccoli).
Striatal regions have previously been linked to pleasant and rewarding emotions and desires. When the patients were treated with leptin, responses to food pictures in these areas were reduced.
One of the striatal regions - the nucleus accumbens - was especially responsive to pictures of foods that people find more appetising. For example, its activity was higher in response to a picture of chocolate cake than to a picture of broccoli. In healthy volunteers, activation of the nucleus accumbens by appetising foods was only found when the person was hungry (following an overnight fast).
In the leptin deficient patients, the nucleus accumbens showed this distinctive response (greater for well-liked foods) when patients were hungry (following an overnight fast) but also after they'd just eaten. After treatment with leptin, the response in these patients normalised.
Taken together, these findings have important implications for the understanding of how two key systems - the pathways that control hunger and fullness and the brain processes involved in liking and wanting foods - may interact.
Dr Farooqi, University Department of Clinical Biochemistry, said: "While body weight remains stable for many people over a long period of time, other people gain weight very easily. More studies are needed to find out how these brain responses vary in people with weight problems in general. Research is needed to find out how leptin triggers other chemicals in the brain and how alteration of these pathways contributes to overeating and obesity.
"Understanding how brain systems interact with hormones that signal hunger and energy stores will provide us with a more complete picture of factors controlling eating behaviour and will hopefully take us beyond some of the prevailing and simplistic assumptions about why some people have difficulties in controlling how much they eat.
"Such understanding will be a key step in the prevention and treatment of obesity. Importantly, the finding that the liking of food is biologically driven should encourage a more sympathetic attitude to people with weight problems," he added.
The study was funded by the MRC and the Wellcome Trust.