The number of obese adults is increasing
The brain's response to food is linked to future weight gain in women, US researchers report in Science.
Brain imaging showed those who had the weakest response to drinking a chocolate milkshake were most likely to have put on weight a year later.
Poor responses to food were also associated with a gene controlling the brain's response to dopamine - a chemical controlling pleasure.
It backs previous work showing obese people may get less pleasure from food.
Two separate studies, one in 43 female students aged 18 to 22 and another in 33 teenage girls aged 14 to 18, measured activation in a certain part of the brain (the dorsal striatum) when drinking chocolate milkshake or a tasteless drink.
The researchers also tested for a particular genetic variant - TaqA1 - which is linked to fewer dopamine receptors in the brain.
A year later, those with the "blunt" responses to the milkshake and the genetic variant were most likely to gain weight.
Dr Eric Stice, from the Oregon Research Institute, said although recent studies had suggested that obese people may experience less pleasure when eating and eat more to compensate, this is the first study to link that with future weight gain.
"The evidence that this relation is even stronger for individuals at genetic risk for compromised signalling in these brain regions points to an important biological factor that appears to increase risk for obesity onset."
He added: "It is possible that behavioural or pharmacological interventions that correct this reward deficit may help prevent and treat obesity."
Dr David Haslam, a GP and clinical director of the National Obesity Forum, said there were interesting parallels with the findings and addiction.
"There is a debate about whether you can have a genuine addiction to food.
"But someone say on low dose heroin becomes resistant to the dopamine response and needs more and more.
"This is very interesting but it doesn't really help us now with tackling obesity," he said.