Obese people may find it more difficult to refrain from sweets than those who are thin, because of a dysfunction in their brains, a new study suggests. As young people reach adulthood, their preferences for sweet foods typically decline, researchers said. But for people with obesity, new research suggests that the drop-off may not be as steep and that the brain's reward system operates differently in obese people than in thinner people, which may play a role in this phenomenon, they said. "We believe we may have identified a new abnormality in the relationship between reward response to food and dopamine in the brains of individuals with obesity," said Yanina Pepino from Washington University in the US. "In general, people grow less fond of sweet things as they move from adolescence into adulthood. Also, as we age, we have fewer dopamine receptors in a brain structure, called the striatum, that is critical to the reward system," said Pepino. Researchers found that both younger age and fewer dopamine receptors are associated with a higher preference for sweets in those of normal weight. However, in people with obesity, that was not the case in the study. Researchers studied 20 subjects with healthy weights and compared them with 24 people considered obese, each of whom had a body mass index of 30 or higher. The study volunteers were 20 to 40 years old. Participants received drinks containing varying levels of sugar to determine the degrees of sweetness each individual preferred. Researchers then conducted positron emission tomography (PET) scans to identify dopamine receptors linked to rewards in each person's brain. Dopamine is the main chemical in the brain that makes us feel good. The PET scans showed that although there was a relationship between the dopamine receptors, preference for sweet things and age in lean people, that pattern did not hold true in the brains of obese people, researchers said. "We found disparities in preference for sweets between individuals, and we also found individual variations in dopamine receptors - some people have high levels and some low - but when we looked at how those things go together, the general trend in people of normal weight was that having fewer dopamine receptors was associated with a higher preference for sweets," said Tamara Hershey from Washington University. This was not true in the obese subjects. The relationship between their ages, sweetness preferences and dopamine receptors did not follow the pattern seen in people who weighed less, researchers said. "What is clear is that extra body fat can exert effects not only in how we metabolise food but how our brains perceive rewards when we eat that food, particularly when it is something sweet," said Hershey. The findings were published in the journal Diabetes.