The mystery of why some people stay slim while others get fat may be partly explained by differences between the way individuals' brains measure the pleasure of eating.
Overweight people may actually find fatty or sugary food less satisfying than thin people, which leads them to overeating as a way of compensating for the relative lack of enjoyment.
A study of the brains of young women and adolescent girls found significant differences in the way thin and overweight individuals responded to drinking a chocolate milkshake, compared with a tasteless drink.
Using a scanner that measured the amount of activity in the "pleasure centre" of the brain, and a genetic test to determine an individual's genetic makeup, scientists could even predict which of the women would put on weight during the following year.
The findings support previous work suggesting that a key factor determining whether someone is likely to stay fat or thin is dopamine, a neurotransmitter released in the brain's pleasure centre when someone eats tasty food.
"Although recent findings suggested that obese individuals may experience less pleasure when eating, and therefore eat more to compensate, this is the first prospective evidence for this relationship," said Eric Stice of the University of Oregon in Eugene.
The genetic test used by the scientists "counted" the dopamine receptors in the brain's pleasure centre.
Those women carrying the genes for fewer dopamine receptors seemed to have to eat more to trigger the same sort of pleasurable response as those women born with more dopamine receptors.
When the scientists followed up the women for a year, they found those with fewer receptors were more likely to put on weight, Dr Stice said.
"Obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit. People with fewer receptors need more of a rewarding substance such as food or drugs to experience others' level of pleasure."
Scientists monitored brain activity in two groups of females - 43 students aged between 18 and 22, and 33 adolescent girls aged between 14 and 18.
Each individual in the study, published in the journal Science, also took a test for a gene variant known as Taq1A1, which is linked with a lower number of dopamine receptors in the brain.
Dr Stice said that the findings point to the importance of psychological factors under the control of brain chemistry in determining whether someone is likely to be predisposed to putting on weight in an environment where high-calorie food is freely available.
"Understanding the abnormalities of activation of reward circuitry in response to eating is critical to helping people regulate their weight because dopamine is the primary neurotransmitter in the reward pathways of the brain," Dr Stice said.
"Although people with decreased sensitivity of reward circuitry are at increased risk for unhealthy weight gain, identifying changes in behaviour or pharmacological options could correct this reward deficit to prevent and treat obesity."
Cara Bohan of the University of Oregon said the study is the first on obesity to use brain scanners, gene tests and to follow people up over the course of a year, and added: "The findings suggest certain biological factors may impact one's risk for weight gain, which is important to better understand how we can intervene and prevent obesity."