You report an apparent impossibility – people are eating less, but getting fatter (Recession has hit spending on food, says survey, 4 November; Editorial, 4 November). You might have smelled a rat. The research by the Institute of Fiscal Studies is based on government surveys of expenditure. But food purchasing surveys, like food consumption surveys, depend on people telling researchers honestly what they buy and eat ("self-report data"). Such methods are vulnerable to what is known in the trade as "under-reporting". In plain English, people lie. They claim to buy and eat a healthier diet than they actually do. In this age of obesity, they lie about calories.

Happily, with diet surveys we have an independent biochemical measure of calorie intakes ("double-labelled water"). It is expensive and cannot be used on everyone. But it shows that adults under-report calories by 25%, adolescents by 34%. In one 1980s survey, 40% of women claimed they were eating less than is necessary to stay alive. We are not actually eating less. We are just telling researchers that we do. In contrast, as the saying goes, the tape measure does not lie.

The IFS team did not even attempt to correct its findings for under-reporting. Like many before, they performed sophisticated statistical manipulations on poor primary data. And hinted at dubious conclusions as a result.

The sad fact is that nutrition science has not solved a fundamental problem – measuring accurately how much people actually eat. Until we have cracked that issue, it is premature to conclude that our weight problem is down to sloth, gluttony, genes or anything else.
Jack Winkler
Former professor of nutrition policy, London Metropolitan University