What did you have for breakfast? I said, what did you have for breakfast? Toast? How many pieces? And what did you have on them? I've told you this before, chocolate spread isn't good for you. Didn't you read the label?

When did the contents of your breakfast become my business? I'll tell you exactly when, my friend. And where. It was on the July 5, 1948, in Park Hospital in Trafford. Now, put down that coffee. I am getting worried about your caffeine intake.

The past couple of weeks in the obesity debate have been surreal. The Health Secretary said, yes he really did, that the increasing tendency of people to be overweight was “a potential crisis on the scale of climate change”. This was good news for the staff of Weight Watchers. Hey, one of them might win the Nobel Peace prize! But to me Alan Johnson's remark, and the respectful response it received, suggested we've finally lost the plot altogether.

There is a fundamental difference between obesity and climate change. It's so clear that I almost hesitate to point it out. If you drive a sports utility vehicle you harm my climate as well as yours. If I eat another muffin it only has an impact on me. And despite the use of the term epidemic to describe the increase in obesity, muffin consumption is not catching.

Yet this simple point seems to have been lost in a welter of articles about why so many of us are obese and how we can stop this trend. The Right argues that staying trim is a personal responsibility, the Left that Big Food is poisoning us all for profit. Is there room in this debate to point out that the freedom to be fat is a fundamental liberty that needs to be protected?

Now, very few people want to be overweight. Being overweight is a cost. It is uncomfortable, makes activity more difficult and takes years off your life. But where there is a cost, there is usually someone willing to pay it. Fat people make a trade-off between the damage of being overweight and the pleasure of the activities that make them pile on the pounds. And I believe that this trade-off is made more or less consciously. I find ridiculous the idea that, in a tragic misunderstanding caused by complicated labelling, millions are mistakenly eating doughnuts without realising that they are fattening. We realise. Of course we realise.

When asked to choose between a longer life and a fatter one, it is assumed that only a fool would choose to live it short but live it large. Why?

We have moved smoothly from the debate on smoking to the debate on obesity without pausing to reflect that at least some of the rise in obesity may be among those responding to campaigns by replacing unhealthy cigarettes with unhealthy eating, making the same trade-off but with a different vice.

There are two responses to this argument. The first concerns children. Children are the “passive smoking” of the obesity debate. And I quite agree that parents have a responsibility to teach their children how to eat healthily and exercise properly, just as they should make sure their children wash, and cross the road safely. It is perfectly reasonable for the State and wider society to help parents with this responsibility and express concern if they fail particularly badly.

But even here a degree of humility is in order. The State is busy giving lectures to parents. We are so sure that society can sort things, with TV advert bans and so forth, while lazy, deadbeats are rearing their children to the sound of a pinging microwave. Meanwhile, as Jamie Oliver's television programmes on school dinners showed so well, among the worst, most unhealthy, meals served to children are those being cooked by employees of the Government.

Then there is the second objection to my right to be fat. And this is the one that goes back to Park Hospital in Trafford on July 5, 1948. On this day, in this place, Aneurin Bevan formally declared the National Health Service open for business.

It never takes long in an argument over obesity, as indeed was the case over cigarettes and alcohol, for the cost to the NHS to be brought up. Think that the trade-off you wish to make between that extra portion of dinner and an extra ten minutes of life is your concern alone? Think again. If you make yourself ill I have to pay, so I have a big stake in your big steak. (Unless, of course, you have private health insurance. Following the logic of the argument, if you are a BUPA client, please go ahead and eat up. You can get as fat as you like.)

This argument is routinely deployed. But when you think about it, isn't it remarkable? Here I am, middle-aged, middle-class, professional and definitely among those who pay more for the NHS than the NHS spends on me. I am financing the healthcare of many others and am happy to do so. But excuse me if I don't find it a little bit irritating to be informed that because I have agreed to this social compact it gives everyone else the right to tell me what to eat.

Where does this end? If it is reasonable to argue that people shouldn't be fat because it harms the finances of the NHS, then it would be reasonable to argue that these people shouldn't choose low-earning occupations either. We need their tax money.

Would I be entitled to insist that charity workers quit their jobs and work in the City, so that they can help to pay for my healthcare with their increased earnings? Can I start lecturing people who play dangerous sports that they should stay at home in case they inflate my bill?

Just about any form of activity has an impact on health. So there is nothing that is immune to the NHS cost argument. Everything that you do, everywhere you visit, every bite you eat, every drop you drink, the job you choose, every road you cross is everyone else's business. This is the road to serfdom.

daniel.finkelstein@thetimes.co.uk

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