Last week, the UK health secretary declared that we are in a grip of an obesity epidemic, but does the evidence stack up? Researchers debate the issue in the British Medical Journal.

Claims about an obesity epidemic often exceed the scientific evidence and mistakenly suggest an unjustified degree of certainty, argue Patrick Basham and John Luik.

For example, the average population weight gain in the United States in the past 42 years is 10.9kg or 0.26kg a year. Yet, between 1999-2000 and 2001-2002, there were no significant changes in the prevalence of overweight or obesity among US adults or in the prevalence of overweight among children.

Furthermore, they say, the categories of normal, overweight, and obese is entirely arbitrary and at odds with the underlying evidence about the association between body mass index and mortality.

For example, the study on which the bands for overweight and obesity in the US are based found that the death risks for men with a body mass index of 19-21 were the same as those for men who were overweight and obese (29-31). Other studies have shown negligible differences between body mass index and death rates.

The association of overweight and obesity with higher risks of disease is equally unclear, they write. And, despite supposedly abnormal levels of overweight and obesity, life expectancy continues to increase.

They suggest that some public health professionals may have deliberately exaggerated the risks of overweight and obesity, and our capacity to prevent or treat them on a population wide basis, in the interests of health. They warn that this has unwelcome implications for science policy and for evidence based medicine.

But Robert Jeffery and Nancy Sherwood argue that a large body of scientific evidence shows that obesity is a major global health problem.

In the US, the prevalence of obesity in 1976-80 was 6.5% among 6-11 year olds and 5% among 12-17 year olds. In 2003-4 it was 19% and 17% respectively. Europe can also expect to see the numbers of overweight and obese children rising by around 1.3 million a year by 2010.

The risks of obesity on many serious health conditions including high blood pressure, diabetes, heart disease and some forms of cancer, are also serious and well established, they write.

Most health economists and epidemiologists agree that the contribution of obesity to current healthcare costs is high and that it is likely to get much higher. Some have argued that we may even see real falls in life expectancy within a few decades, they add.

In summary, a large body of evidence documents that over-nutrition and obesity are a major global health problem, say the authors. With the continuing rise in obesity and limited treatment efficacy, options for averting a poor public health outcome seem to rest either on the hope that scientists are wrong in their projections or speedy investment in the development of more effective public health measures to deal with it. They think the second option a more prudent scientific and policy choice.