What is sad is that at a time when this country is richer than ever and ought to have better schools than ever, we have far more fat people than ever — a dangerous explosion of flab. Last week the Department of Health issued a report grimly called Forecasting Obesity to 2010 and its findings were grotesque. Within four years, it predicts, a third of all adults — 13m people — will be obese. So will 1m children.

Obese means not just podgy, but dangerously, disablingly, distastefully fat, as in American fat.

This is not just shocking; it has also happened shockingly fast. As the report says, a third of all men will be obese by 2010; in 1993 the figure was only — if one can say only of such a large figure — 13%, rising to 24% in 2004.

The same is true of women, although the rate is rising more slowly; 16% were obese in 1993, 24% in 2004, and the trend is expected to rise until 2010. The proportion of boys who were obese stood at 17% in 2003 and is predicted to rise to 19% by 2010, while among girls it is expected to increase more swiftly from 16% to 22%.

This presents an awkward challenge to libertarians. The libertarian assumption is that we should all be free to do what we want, as far as possible, and if some people’s lifestyle choices involve snacking on deep-fried Mars bars and triple-processed cheeseburgers, other people have no business interfering, still less the government.

Besides, there is the embarrassing fact that those who eat and drink junk do so for cheap comfort and because they are either too poor or too ignorant (or both) to prepare healthy food. It doesn’t come well from the consumers of steamed organic asparagus and free-range ducks’ breasts to criticise those who can manage only frozen reconstituted chicken nuggets and sugary baked beans.

However, obesity does not concern only the obese. It concerns all of us. Obese parents produce obese children, and obesity places a crippling burden on the National Health Service, quite apart from the many personal miseries involved. Currently 10% of NHS resources are spent on diabetes (two-thirds of which is the avoidable type 2 associated with obesity) and this could easily double within the next four years to 20%.

This is quite apart from the increased risk among the obese of heart disease and other serious illness. More young people are being diagnosed with type 2 diabetes, something previously seen only in people over 40. In these circumstances even the most swivel-eyed libertarian would probably agree, for once, that something must be done and even perhaps by the government.

Curiously enough, however, in one of the few areas where our ever-intrusive government might for once justifiably intrude, new Labour does almost nothing. Possibly as a result of the ferocious lobbying of the food industry, ministers restrict themselves to making repetitive noises about healthy living and “small changes” that won’t cost anybody anything.

Tony Blair said last month that if the food industry did not agree to limit junk food advertisements by 2007 he would bring in mandatory rules, but he has said that before and more than once. Besides, why not bring them in straight away? His government has persistently ignored the demands of the Commons health select committee for a traffic light system of food labelling, enabling shoppers to make informed choices.

England’s chief medical officer warned in this year’s annual report that public health budgets were being raided to deal with deficits. That is the reality behind government talk of raising public awareness.

I have never been convinced that government health education has any effect. Despite the “five-a-day” campaign, only a quarter of people in England eat vegetables every day. About half of overweight men are in denial; they don’t see themselves as overweight, according to the report.

There is nothing complicated about being thin. Being fat is usually the result of eating too much junk food and taking too little exercise. Being thin means eating much less food, avoiding junk food altogether and taking exercise every day. It may be that nothing can be done about the plague of obesity; there is a growing epidemic in Europe and worldwide. Perhaps affluence is a disease to which only the fortunate few are immune. But if anything could be done about it, it would have to be radical.

Nobody who craves cheap comfort food will willingly give it up. But if over-processed, over-refined food and junk food were to become expensive while healthy fresh food became cheap — the opposite of the case today — people would be forced to eat well. This could be done through taxes or subsidies. Alternatively, you could ration unhealthy food.

There could be a public campaign against fattening food, just as there was against smoking, aimed at making everyone ashamed of consuming anything naughty but nice. I am just as greedy as anyone else but I have come to think of cakes, biscuits, crisps, sweets, white bread and puddings as more or less toxic. Foods like this should have health warnings — “cake can kill”. They are not just unnecessary, empty calories; they interfere with your blood sugar levels, affect your appetite and your mood; they may even induce food addiction. The same applies to alcohol: more than a modest amount makes you fat, interferes with your mood and is often addictive.

Just as there would need to be financial incentives to eat well, there should also be inducements to take exercise. The cost should be subsidised or declarable against tax. Employers should be required to give workers time off to go to the gym or jog. We could imitate the Japanese and have mass group exercises at work every day.

And that is the problem. Obesity, one of the trials of affluence, can be solved only, if at all, by the kind of interventionism that has been discredited by the failure of socialism. Liberty is indivisible; it belongs to the ignorant and the low paid just as much as to anyone else. Perhaps obesity is one of the many prices of liberty. Fat is a freedom issue.

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