So it is possible to be both fat and fit? We are so used to heaping those who are fat with other value-laden judgements – that they are lazy, indulgent and weak-willed – that we neglect the possibility they may be carrying a few extra pounds and still be perfectly healthy.

This is the important message of Katherine Flegal's research. Waistlines have expanded so far and so fast in the West over the past 20 years that it has created something close to panic. It has been cast as the biggest public health problem that we face – but it may not be.

If you are overweight, smoke, eat junk food, and take no exercise then you are heading for an early grave. That much is clear. All these risk factors combined increase your chance of heart disease and cancer. If, however, you are little chubby but have a fundamentally healthy lifestyle, the outlook is a lot rosier.

The findings will not come as a complete surprise to experts because earlier studies have shown that being moderately overweight is not necessarily a bad thing. Indeed, the ideal weight rises with age as people need a little extra padding in their later years to see them through periods of sickness, when they lose their appetite and shed pounds. Statistically, what counts as overweight when you are 40 or 50 may be an ideal weight when you are 60 or 70.

But in its apparent reversal of the orthodox message on the risks of weight gain, the study also demonstrates the dangers of over-simplifying complex public health messages. We have seen it before, with alcohol – it was only in the 1980s that doctors conceded a little of it did you good (by reducing the risk of heart disease) – and more recently with sunlight and skin cancer after doctors conceded that some people were at risk of vitamin D deficiency if they covered up too much. We look to scientists for simple rules by which to live – but the truth is nearly always more complicated.

Still, we should not get overexcited by the latest findings. The researchers looked at deaths linked with being overweight but not at disease or disability, which generally increase with extra pounds. There is no question that the speed with which average weights are rising in the UK and US is of major concern.

To take one condition, diabetes, we know that the risks of developing it increase proportionately with increasing weight. There is an explosion in diabetes in Britain and around the world. Before diabetes kills, it destroys the eyes and the circulation, leaving its victims blind and limbless (through amputation).

It may be, as Dr Flegal has observed, that the risks to health of being overweight are declining thanks to advances in drug treatments to control cholesterol levels and blood pressure, and better care for a range of other obesity-related conditions. If that is so, then it would clearly be better to prevent the problems in the first place than to rely on medicine to cure them.

Nevertheless, it is reassuring to learn that obesity is not the first but the seventh biggest killer in the US – behind smoking, drinking, infections, toxins and pollutants, cars and guns.

The findings provide an opportunity to get the risks in perspective. We are right to be worried about the rising levels of diabetes, but probably wrong to worry about the legions of men and women with body mass indexes hovering around the 27 or 28 mark, which labels them overweight, but who are taking regular exercise. If they are frightened into thinking they are putting their health at risk, they may be condemned to yo-yo dieting in a struggle to meet the ideal weight with a BMI of between 20 and 24, which is worse for their health.

This study does not give carte blanche to eat more cream cakes. But it does encourage a slightly saner, and more humane, view of those of a larger size.