Obese people underestimate the amount of sugar they eat, making studies into the condition based on self-reporting very unreliable, UK researchers say.
Not everyone reports accurately how much sugar they eat
But a new urine test has been developed which can for the first time work out how much sugar people have consumed.
In a study of hundreds of volunteers, researchers compared what people said they ate with data from urine tests.
The findings appear in the journal Cancer Epidemiology, Biomarkers and Prevention.
"These results show what many have suspected for some time: obese people are not able to tell us what they actually eat," said Professor Sheila Bingham who led the team from the Medical Research Council and University of Cambridge.
"If we are to tackle the scourge of obesity, both exercise and diet need to be taken into account."
Linked after all?
Studies have suggested that eating more sugar is not linked to obesity.
But the report says these conclusion were based on studies which relied on self-reporting and that their findings show these to be inaccurate.
"The spot urine and blood tests established that obese people consume more sugar and less Vitamin C than their thinner counterparts, but this did not show up when asked," said Professor Bingham.
"Although obese people may have a less active lifestyle than people of normal weight, reports about what they ate were less accurate than those from their normal weight counterparts."
Dr Colin Waine of the National Obesity Forum said the tendency to under-report food intake among the overweight was a major problem for medical practitioners.
"Used sensitively, this test could be a great tool in helping patient and practitioner work out what's going wrong and talk through what could be done about it."