“Bring back rationing.” This is not an exhortation by The Times, but a suggestion from Rachel Davy, of Staffordshire University, reported in our Public Agenda pages this week. It is instructive on two counts: it highlights the undeniable fact that, on average, we eat more than we used to (and probably more than is good for us). It also captures in a single phrase the equally undeniable fact that the terms of the national debate on obesity have drifted into the realm of the ridiculous.

Dr Davy is entitled to polemicise. The Government is not, but in framing proposals on diet and exercise it has nonetheless bracketed the word “obesity” with “crisis” and “epidemic” as if the public had no sense of proportion and an infinite capacity to be alarmed. A crisis is what is unfolding in Darfur. An epidemic is what is cutting an appalling swath through the people of Swaziland. The problem with the British waistline is its gradual, non-lethal, reversible growth.

Some important truths can be teased from the recent histrionics on the average Briton’s body mass index (BMI). It is, indeed, on the rise, and the rate of increase may be accelerating. The number of people with a BMI of more than 30 — the current official definition of obesity — has trebled or quintupled over the past three decades, depending on which survey is consulted. Obesity heightens the risk of diabetes, heart disease and other health complications, but its incidence should not be exaggerated. Fear is also not good for your blood pressure.

Efforts to improve school food should be unstinting, and the latest proposals to double schoolchildren’s “entitlement” to organised exercise from two to four hours per week are welcome in principle. But Tony Blair, who has given the four-hour plan his backing, should expect scepticism. His Government has already set a target of two hours’ exercise per child per week, and conspicuously failed to meet it. Furthermore, new research indicates that school PE makes little difference to children’s calorie counts. Most of those not forced to exercise at school do so at home or on the way to or from school. A simple way to increase the amount of energy, and calories, used each day would be to encourage more children to walk to school.

Mr Blair should also resist any temptation before the election to resort to obesity hyperbole. The context for this debate is not one of mass bingeing on junk food for want of any other sort of food. Nor is it a national failure to exercise for want of sports facilities. The context is one of plentiful choice in terms of food and exercise; plentiful healthcare (the NHS’s serious failings notwithstanding), and the sort of sustained prosperity of which chubbiness has always been a symptom.

On balance, Britons are healthier than they have ever been, and attempts to police their diet and behaviour other than in schools, for which the Government is directly responsible, are not only impractical and unwarranted; they are unethical. Getting young people to eat and exercise properly is a serious responsibility, but it is ultimately a parental responsibility.

It is, likewise, for adults to decide if they or children in their charge are going to fill up on the snacks and fast food which have become ideological morsels for those fond of central planning and non-stop nannying. A sensible diet and regular exercise are obviously to be recommended, but public paranoia is not a healthy option.