Food cameras would help to fight under-reporting in nutrition studies, researchers say.
The fatter we get as a nation, the more we seem to lie about what we eat, a study has shown.
Researchers have long known that people in nutrition surveys "under-report" what they ate they day before. Now a New Zealand study has shown how big the energy and credibility gap really is - and that it's expanding.
They suggest that food-cams or smartphone applications may be needed to counter the problem.
The 2008/2009 Adult Nutrition Survey found the average person's weight had risen by 4kg-5kg since the previous survey, in 1997. That would reflect an increase of around 400-500 kiloJoules of energy intake per day, Luke Gemming, of Auckland University, and his colleagues calculated.
However, the average daily intake reported by the participants decreased by 1300kJ for men and 400kJ for women, they said in the European Journal of Clinical Nutrition.
Around a fifth of men and a quarter of women under-reported what they ate and drank. That was a rise in the prevalence of under-reporting of 75 per cent among men and 19 per cent among women, compared with 1997.
Under-reporting rates in the new survey rose in line with body mass index: 16 per cent for those of normal BMI to 25 per cent for the overweight group and 30 per cent for the obese.
There was also an ethnic gradient, from 24 per cent for Europeans to 32 per cent for Maori and 34 per cent for Pacific people.
The researchers said forgetfulness was one reason.
A co-author of the paper, Professor Cliona Ni Mhurchu, said there was also a degree of "deliberate under-reporting" of high-energy foods.
"If they think it's not in line with what's considered healthy, people will not mention a Mars bar or the fact they had McDonald's on the way home from work."
The researchers outlined other likely factors, such as the increasing television and internet portrayal of "slim-body image or health-related content - for example reality television focused on weight loss, cosmetic surgery, makeovers and modelling".
These could lead to low self-esteem, depression and eating problems - and all were related to under-reporting.
Mr Gemming said the under-reporting of energy intake had serious implications in terms of gaining Government support for nutrition and health campaigns to fight obesity.
To improve the accuracy of food intake surveys, he is investigating the use of small cameras worn around the neck which take a picture every 20 seconds. In a small pilot study, participants' self-reported energy intake increased by 12.5 per cent after viewing the images.