Ready for your lifestyle health check-up? According to a leaked white paper on health this week, every patient in Britain is to be offered a personal health consultation - a body MOT - by their GP. Patients, apparently, will be followed up by email and a new service, Health Direct, will offer health promotion advice.

You might think it sounds like a fabulous new advance, but I, for one, feel deeply cynical.

Superficially, offering people a check-up sounds good. Preventative medicine is, after all, the ideal. But in fact, we general practitioners have been sneakily doing this kind of lifestyle advice thing for ages - offering help to stop smoking, weight loss advice to those overweight, and sexual health advice - for example, where relevant, to use a condom along with the pill. It's been the kind of thing doctors have been opportunistically squeezing into consultations, and we know that some of this - for example, advice to stop smoking - does work.

But offering people a separate appointment to discuss their lifestyle is different. My cynicism rests on my concern that the very people who are existing on junk food, smoking heavily, overweight or drinking dangerously are the least likely to come along for an appointment to talk about what they are doing and how best to improve on it. The truth is that most smokers do realise it isn't a very healthy addiction - after all, in the UK, tobacco kills 300 people a day - and most people know if they need to eat more fruit and vegetables.

The real reason why some people have poor diets and smoke heavily is not one that can - or should - be easily medicalised. The problems run broader and deeper. Those most likely to come along for an MOT-style appointment are those who are highly motivated about their health - and probably don't need much help to stop smoking or change their diet anyway - if they need to at all.

However, the other question is whether using doctors' and nurses' time purely to offer lifestyle advice is a good use of resources. For many years, the NHS has been grappling with the idea of "evidence-based medicine" - which in effect, means that we are meant to be using things that work and not using things that don't. While this isn't always possible - sometimes there simply is not the science to back up one treatment over another - it means that there is a bigger emphasis on using the limited resources of the NHS to best effect. This can only be a good thing.

But I can't find any science behind this MOT idea - which is not part of the new GP contract. From what I can gather, general practices would have to be reimbursed separately for these health checks as an extra service. But since staff and time are all limited, the question for many may be - if we offer these appointments, then what will we not do instead?

Of course, in an ideal world, we would know that this shift of resources would be worthwhile, because we would end up with a very healthy populace who would have less heart disease and cancer, thus balancing the books. Of course this is what I - and, I reckon, every other doctor in the country - would want: but the problem is that we seem about to pour in resources without knowing if giving out this kind of lifestyle advice in this way actually works.

Take, for example, the Healthy Eating Line. This was a Scottish telephone advice service set up as part of a major advertising campaign - costing £1.7m - to try to improve on the traditionally unhealthy Scottish diet. However, calls were so infrequent, that at one point, it cost £120 to fund just one call to the service.

However, if this lifestyle advice was part of an investigation to find out how best to improve the long-term lifestyles of those people most likely to benefit, then I could see the point of it. This isn't likely to happen without direct targeting of patients. Offering a lifestyle check-up to everyone who wants one may not be the way to do it.

The idea of targeting people most likely to benefit is behind the National Institute for Clinical Excellence's (Nice) new guidance on dental check-ups, due to be released tomorrow. From having the recommended six-monthly check ups, it seems that it will be now recommended that those with healthy teeth are to have fewer appointments, and those who have a history of gum disease or unhealthy teeth, more frequent appointments. This seems to make sense - the people who need most dental care get it, based on research.

In 2002, the Audit Commission made what seems like a fairly rational judgment regarding how best to fund NHS dentistry. For example, they reported that 11% of the NHS family dental budget was spent on scaling and polishing. The commission noted: "For most people this procedure is mainly of temporary cosmetic benefit, and rarely prevents or cures gum disease."

Time and money have to be wisely spent. Crucially, however, Nice is recommending more research - fully supported by the British Dental Association - into how frequent check-ups should be. I don't see why an evidence-based approach should only apply to dentistry. The public shouldn't be expecting anything else.