The study shows obese and thin people all fib about food to the same amount regardless of the number on the bathroom scale and this could be undermining national health advice.
Researchers innovatively took into account the amount of energy a person burns in a day with everyone misreporting how many calories they consume by an average of 900 calories.
It found that as obese people burn more energy doing day-to-day tasks, they do not lie about food more than slimmer people.
Although the gap in reported meals and actual intake was bigger in obese people, they actually burn more calories than non-obese people.
This casts doubt on the official guidelines that claim Britain’s bulging waistlines are due to obese people not telling the truth about their diet.
And comes amid controversy sparked by the Government backtracking on a proposed ban on 'buy one get one free' junk food deals and a 9pm watershed for sugary snacks.
The researchers hope the findings - published in the American Journal of Human Biology- will shake up guidelines on energy intake.
It was led by Professor Gavin Sandercock from the School of Sport, Rehabilitation, and Exercise Sciences.
The research team said: “The gap between reported intake and actual expenditure was bigger in obese adults than normal-weight adults but not because they lied about how much they had eaten instead it was because they expended much more energy each day than their thinner peers.
“Bigger bodies need more energy every hour of the day and particularly during physical activity because moving your weight is hard work.
“We used an innovative mathematical model to correct for the difference in body size between obese and non-obese adults.
“When we took into account the different body size and the different energy needs they have there was no difference in how much they underreported their food intake
“The idea that obese people lie about their food intake is wrong - it's simply that as energy requirements increase with a larger body size there is more error between what people report and what they actually eat.”
The study looked at 221 adults with an average age of 54 and a range of body shapes.
Researchers asked them to keep a food diary and then the researchers checked how much energy they consumed by using radioactive water and testing the urine of participants.
Although obese people misreported how much they ate by an average of 1200 calories and slimmer participants by 800 calories they actually burnt 13 per cent or 400 calories more energy.
Everyone lied, whether they were obese or non-obese, about how much they consumed by the same amount - claiming they consumed 1,800 calories on average.
As a result of the study Professor Sandercock is calling on the government to overhaul its advice.
He said: “Public health recommendations have historically relied heavily on self-reported energy intake values
“Recognising that the measures of energy intake are incorrect might result in the setting of more realistic targets.
“Additionally, changing the narrative around obese people fibbing about their energy intake might change the focus to investigating dietary risk factors for obesity, such as foods with high-energy density, processed foods, high-fat low-fibre foods and sugary beverages, all of which drive a high energy intake.”
900 calories in numbers
3 – McDonald's Cheeseburgers
5 – Pints of lager
7 – Packets of ready salted crisps
18 – Apples
300 – Cherry tomatoes