People who are overweight or obese often under-report what they eat when they're asked to reveal their diet to health professionals.
This was the conclusion of a review of existing evidence by a team of researchers, which included University of Newcastle Associate Professor of Nutrition and Dietetics Tracy Burrows.
Dr Burrows said this under-reporting of the amount of food and drink consumed "often makes it hard to assess what changes can be made" to a person's diet.
The review was related to studies that measure and interpret the diet of people who are overweight or obese.
"It's important to assess diet as accurately as possible," Dr Burrows said.
She said most study methods rely on self reporting, which might include a survey, food record or reporting total food and drink intake over 24 hours.
But the review shows that many people don't report everything they eat and drink.
Dr Burrows said it was not clear if the misreporting was related to denial or a lack of awareness.
"We know dietary assessment is so important and there is a need to do it well to help make practical recommendations," she said.
"The reasons for misreporting are likely to vary. A lack of awareness and denial are factors for some individuals," she said.
She added that not remembering and wanting to be seen to meet social standards were also factors.
"I don't think many individuals openly report their intakes of specific foods like junk foods and alcohol."
She added that people also report that they eat more healthy food than they actually do.
When discussing diet, overweight people have a tendency to make comments such as "I eat healthy", "I eat well" or "I don't eat much junk food".
Asked about this, Dr Burrows said this mindset could affect all people.
"That is why diet assessment is important as it starts the process of awareness."
People who undergo such assessment are often surprised to learn what food is considered healthy and unhealthy.
She said misreported diets made it harder to help people improve their health and wellbeing.
"If clinicians such as dietitians don't have the right information, it's difficult to help people make the best choices," she said.