Doctors are guilty of wrongly believing that obese people are simply lazy, research suggests.
Obesity has many causes
Researchers at Yale University said the findings highlight the difficulty in tackling the stigma around obesity.
Many obese people complain that others believe they are overweight simply because they eat too much or fail to exercise.
This is despite the fact that obesity can be caused by a variety of other factors, such as genes and environment.
Dr Marlene Schwartz and colleagues carried out psychological tests on 389 professionals who treat and study obese people.
They found that younger professionals, in particular, were most likely to have unfavourable stereotypes of obese people.
Workers who did not deal directly with obese patients were also inclined to see them in an unfavourable light.
"On both implicit and explicit measures, health professionals associated the stereotypes lazy, stupid and worthless with obese people," said Dr Schwartz.
"The stigma of obesity is so strong that even those most knowledgeable about the condition infer that obese people have blameworthy behavioural characteristics that contribute to their problem, i.e. being lazy," she said.
"Furthermore, these biases extend to core characteristics of intelligence and personal worth."
Dr Ian Campbell of the UK's National Obesity Forum said he was not surprised by the findings and said they would probably be replicated if the test was carried out on British doctors.
"It is disappointing but it is not surprising to see that health professionals have the same ingrained prejudice against obese people as the general public," he told BBC News Online.
"It is becoming increasingly clear that as much as 80% of people who are obese are predisposed genetically.
"Although it is very rare to find a case where obesity is purely genetic, there are many cases where it is not in the patient's control."
He said it was unhelpful for doctors to be biased against obese patients.
"If a doctor already has an attitude towards a patient, then they are unlikely to understand the difficulties they have in achieving even minor differences or becoming more active.
"It is important for doctors and other health professionals to show understanding and enthusiasm for change.
"If a doctor is biased against the patient's efforts, then it is unlikely to have a good outcome."