Obese people may overeat because they experience less satisfaction from the taste of food, scientists believe.
New research has found that the taste of food produced less of a pleasureable sensation in the brains of those who are obese and that they may have to eat more to compensate.
In particular the eating of tasty food produced less dopamine in the brain, a hormone which promotes positive feelings of happiness and lowers anxiety.
Scientists feel that it might be that overweight people have to eat more or "comfort eat" in order to produce the same feelings of contentment.
"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, lead author and a psychology researcher at The University of Texas.
His team, which included scientists from Yale and Oregon, scanned the brains of 76 volunteer women in two studies as they drank chocolate milkshakes.
They found that the women who were obese or prone to being obese had deficiencies in the "reward circuitry" or striatum of their brains.
When checking a year later it was discovered that those with poor "reward circuitry" were more likely to have gained weight even if they shown no signs of obsesity at the beginning of the study.
Dr Stice found obese people showed less activation in the striatum relative to lean people. He also found individuals with a blunted response were more likely to show unhealthy weight gain.
The researchers believe that the findings could help predict who is prone to weight gain in the future and provide treatment and advice to help them avoid it.
"The study is novel because it is the first to use brain response to food to try to predict future weight gain," said Dana Small, associate professor at Yale.