THANK you for dining with us this evening.

Since madame et monsieur are clearly of the big-boned persuasion, allow me to tell you about our special menu. For starters we have spicy contempt soup followed by cod in a white wine and scorn sauce, and for dessert a cheeky little trifle sprinkled with mockery and loathing. After dining, do visit our Tub of Lard Lounge where health experts are waiting to dispense advice on exercise. Enjoy.

Matters have not yet come to such a pass that obese people are turned away from restaurants for fear of upsetting the rest of the clientele, but the way society is going it might only be a matter of time. A glance at some newspapers this week could have left one with the impression that the greatest threat to the solvency and general well-being of the nation comes not from the fall in the oil price, the rise of fundamentalist extremism, or the weapons of mass destruction up the road, but a mother and daughter from Kirkcaldy.

Janice Manzur weighs 26 stones. Her daughter, Amber, is 17 stones. Since neither is able to work due to obesity, they receive benefits totalling £33,500 a year. Their story came to light in the same week as a new report was published which states that it is not just stuffing our faces which is killing us, but sitting on the sofa while doing so. The Royal College of Physicians and Surgeons of Glasgow (RCPSG) reckons that 2,500 deaths in Scotland can be linked to people not exercising enough. By "enough" they mean just 30 minutes a day. Besides the personal tragedy of families losing loved ones before their time, the couch-potato culture is costing the economy £660 million. Also this week, researchers at the University of Cambridge concluded that being inactive was more likely to kill folk than being obese.

Any way this cookie crumbles, those who are obese, inactive, or both, are entering the cross hairs of officialdom and society as never before. Being fat and unfit is the new smoking and boozing. One need not be a member of a royal college to know that the number of overweight and unhealthy people is increasing. A walk down any high street will show that. Those who believe, however, that the answer lies simply in naming and shaming, or dispensing lectures from on high, have probably been eating too many of those delicious brownies with the peculiarly moreish kick.

Obesity is on the rise for many reasons. In the main, the food industry's massive and increasing reliance on sugar, the inability to access cheap, good food, and cook it, and sedentary lifestyles have combined to make the average bod bigger than it was a generation ago. Those spare, healthier citizens of the post war era have been replaced by ranks of jelly bellies who watch television programmes about cooking while inhaling microwaved mini pizzas.

That much is clear. What, though, are the reasons behind the fall in physical activity? The shift from physical labour to office work means most of us spend our days going from office chair to sofa via a car seat. Adding to this are other factors. In Scotland particularly it takes a brave heart to go out jogging of a January evening. Private gyms are expensive, council run ones are over-subscribed (usually with people who could afford to go private), and it can cost a lot to kit oneself out with the kit required for the gym. Allied to this, gyms tend to be full of people who have already lost weight and are fit. They do not seem like welcoming spaces to those whose bodies are works in progress. But what about walking? That is free. True, but a good waterproof coat, or the heating required to dry clothes later, is not.

The costs involved in eating well and staying healthy have helped to make size a class issue. A person is more likely to become fat and unfit if they are on a low income, are poorly educated, and enjoy few prospects in life. Life is rubbish, they eat rubbish, and the situation becomes worse as their health, physical and mental, declines. Those who have a healthy income, in contrast, have the means to buy good, fresh food. Feeling healthier and fuelled by higher self-esteem, they invest further time and money in taking exercise. These life lessons are passed on to their children, and so it goes on, down the generations.

Those who are truly privileged, of course, have the easiest time of all. In the same week as the Manzurs are pilloried, we are invited to smile indulgently at the confession by David Cameron, the Prime Minister, that he put on a few pounds over Christmas. To shed the excess weight he has ditched bread from his diet. While chewing over that, one might like to ponder how his figure also benefits from those beautifully cut suits he wears. If Dave's regime did not convince you that fitness and weight are class issues, there was also the grisly sight of one of the world's most famous yo-yo dieters, Sarah, Duchess of York, flogging a blender, and some fresh dietary advice, on American television.

Never let it be said, however, that politicians are not addressing these matters. The Tories and their Coalition partners have certainly done their bit for trimmer figures and exercise by cutting the benefits of the poor and forcing them to walk to food banks. How is that for a Bogof health offer? As for the Scottish Government, it has joined Westminster in bringing in a policy of free school meals for all in primaries one to three. What a warm, fuzzy feeling this surely prompted among those keen to spout about ending stigma and ensuring chances for all. A truly radical plan, however, would have used the cash to target more help towards families who are genuinely in need, by adding, say, dinner clubs to breakfast clubs. Instead, those who can afford to send their children to school with a packed lunch, or lunch money, will benefit by £330 a year.

Government has a crucial part to play in improving health and wellbeing, but the hard yards must ultimately be done by individuals. For all the billions of words written on diets and exercise, the boring, essential truth comes down to one sentence: Eat less, eat well, move more. To that end, GPs should be encouraged, as the RCPSG suggests, to ask patients how much exercise they take. Scots have already shown a willingness to change their ways on smoking and drinking, and activity levels are increasing, but the improvements are not taking place universally. It will not be easy to change this. The Royal College is on to something with exercise, though. If ways and means can be found to make exercise more attractive or achievable, then the obesity problem becomes easier to crack. A person who exercises is a happier person, and a happier person eats less. Consider how much would be saved - not least in the bill for anti-depressants - if exercise rather than inactivity became the drug of choice for all Scots.