Joan Smith: The fat deserve the warnings that my father was never given

It has been condemned as NHS rationing, but the decision that obese people in East Anglia are to be denied replacement joints is the right policy for the wrong reasons. It smacks of punishing overweight people and raises questions about treatment for other largely self-inflicted conditions, such as alcoholism or sports injuries. Why should the NHS treat a patient who breaks a leg playing football, then deny a replacement hip to someone just because he or she happens to be obese?

There are clinical reasons for such a decision, in that pre-existing conditions affect outcomes; obese people are at greater risk from a general anaesthetic and the joint may collapse again because of the patient's excess weight. This distinction has been obscured by an admission from the East Suffolk Primary Care Trust that the motive behind rationing some procedures at Ipswich hospital is "unashamedly financial" and prompted by a £47.9m deficit, but it isn't negated by it. On the contrary, it means that a debate which the NHS, government ministers and public should have engaged in years ago is finally emerging.

There is already rationing in public health care, in the form of waiting lists and restrictions on prescribing expensive drugs. But there is a larger problem, which goes to the heart of what the NHS does: with certain not-entirely logical exceptions, such as screening for cervical and breast cancer, it tends to rely on patients reporting their own illnesses. In that sense, the NHS has from its inception been a national illness service, not sufficiently engaged with maintaining health and disease prevention, perpetuating a situation in which unhealthy habits aren't addressed early enough. The NHS treated my father's lung cancer, but I don't recall doctors challenging his smoking habit until it was too late.

Since his death, things have improved where smoking is concerned, although the illogical and in some cases bad-tempered resistance to a complete ban in public places demonstrates how quickly people start hurling insults about the "nanny" state. It's a lazy accusation, based on the bizarre proposition that governments have no interest in the health of their citizens - if that were the case, why the NHS? - and ignores the effects on non-smokers of secondary smoke in the workplace. Nor does it take into account the way in which cigarette and drinks manufacturers, supermarkets (castigated last week for special offers which promote unhealthy foods) and the advertising industry bombard people with messages encouraging behaviour that damages their health.

The US has an even more unhealthy population than the UK but we are catching up fast, to the point where I'm not sure we can afford this laissez-faire attitude for much longer. The ethical problem with restricting NHS treatment for certain groups of people is that it is an intervention too late; primary care trusts that take this route are warning patients that they have a life-shortening condition only when it is having a major impact on their lives and reducing the efficacy of surgery. For most obese people, it is by this stage extremely difficult to lose weight: patients with weakened joints cannot undertake strenuous exercise.

Private hospital providers were quick last week to remind us that medical insurance covers a range of procedures, regardless of the patient's weight, but that merely demonstrates that they are making the same mistake as the NHS. Restricting access to treatment for patients with avoidable conditions represents a failure of the public health system, but it is also a long-overdue challenge to the flawed assumptions on which that system was built.