Not a day goes by without another warning of the dangers of too much food, too much fat, too much alcohol. Not a day goes by without another warning of the dangers of anti-fatness drugs, anti-fat operations and crash diets. Gluttony is surpassing smoking as public health enemy number one. But tackling Britain's fatness-creating culture will be much more tricky, its consequences more far-reaching, than the 50-year progress towards a smoking ban. Our entire economic and social system will have to shift if obesity is ever to be checked.

For two months, I have been at the front line, trying to lose weight in what chief government scientist Sir David King called our 'obesogenic environment'. (King conducted the Foresight inquiry into obesity, published in October.) For years, I resisted the idea that I was fat. I worked out. I play tennis and golf (execrably). Friends reassured me that while I was big, I wasn't really fat. But this year's summer holiday photographs were even more troubling than usual. Then came the trigger moment: a friend in his fifties died unexpectedly of a heart attack in the arms of his wife. It could have been me. I finally owned up to the idea that I was too big.

So, a couple of Wednesdays later, I found myself the only man at the meeting of the local WeightWatchers group. The Little Britain sketch is all too real. A cross-section of friendly women, united in being overweight, assembled to be weighed and then face half an hour of exhortation, homily and advice. This alternated between farce and fascinating tips.

But WeightWatchers is right. The ritual of the weekly weigh-in, the plotting of your weight reduction on a chart, the half-hour lecture and the sharing of the last week's trials and tribulations form cornerstones in the programme. For modern life is stacked against the weight watcher. Try to buy a remotely healthy or low-calorie meal in a station, football stadium or most pubs and restaurants and you might as well cry for the moon. It is no surprise that obesity has doubled in the past 25 years and by 2050 we will be mainly an obese society.

The WeightWatchers subculture is a tiny and very fragile protection against vast forces. It is telling that the clients are overwhelmingly women, for you cannot begin to make any inroads into your weight unless you own up to both the truth that you are fat and the necessity of doing something about it. But if women are more likely to understand this truth than men, the statistics show how hard they struggle; as a group, they are just as obese as men.

The Foresight report highlights how overwhelming the environment is. I tried sporadically to slim before WeightWatchers, but it did not work. There is cheap alcohol. There is the coffee, croissant and muffin culture. Almond croissants are as addictive as a packet of cigarettes. There is the pressure on time that tempts us into short car journeys rather than walking. Food is eaten on the run or consumed socially with vast amounts of alcohol. As the Foresight document says, obesity is a consequence of 'abundance, convenience and underlying biology'. Human beings, like pub dogs, are still programmed with the instinct to keep eating, even when we don't need to any more. Our civilisation has yet to help us curb our primitive appetites and that is the scale of the challenge.

In some respects, it is worse still. The Foresight document is exhaustive in listing the system-wide changes needed to tackle obesity, both psychological and physiological. They are about our eating patterns in childhood (the poor are more likely to suffer than the rich) and the opportunities for exercise and sport. They are about the production, manufacture and distribution of food. They are about how we organise our cities.

While much of this is historical, what has changed is the commercial dynamics of the food and drink industry. It has become a standard bearer of what leading American economist Robert Reich calls 'supercapitalism'. The quest for growth in volume and margins is relentless, driving higher portion sizes, more fat and sugar and carelessness about the addictiveness of ingredients. It means minimal attention to labelling and a war against the 'traffic light' scheme proposed by the Food Standards Agency meant to signal the fat and sugar content.

It is the weight watchers' enemy, the animator and creator of British obesity. The food and drink industry, including some supermarkets, wants to keep labelling non-standard and firm-specific and it is winning this argument with the European Commission.

We also need a comprehensive assault on how towns are planned and transport organised. Initiatives have to work locally, regionally, nationally and at European level. It is as big a challenge as climate change. As for me, I have lost 13 pounds and feel better than I have for 20 years, but it is a fragile achievement. In today's environment, most weight loss is quickly reversed. We need change and we need it urgently.