Obesity in the UK has almost trebled in the last 20 years, and the World Health Organisation has described the condition as a 'worldwide epidemic'. So, why has it suddenly ballooned in the UK?

Britain has become a land of fad diets and health clubs but we seem to just keep piling on the weight. Health Secretary, John Reid has called for a national debate involving the public, NHS and food industry on how to tackle 'worrying trends' in obesity - and the findings are to form part of the proposals to be put forward in a Government White Paper this summer dealing with issues such as advertising bans and tighter food labelling.

Mr Reid stressed that the Government cannot force people to be healthy with fears growing that strict regulation will lead to accusations of them interfering in the public's daily life and ability to make choices.

But it's obvious that something needs to be done. An epidemic by definition alerts us to the fact that obesity is sudden and widespread and, most importantly, undesirable. Is this seemingly infectious phenomenon inevitable, or where should we point the finger?

Changing lifestyles
As a nation, our lifestyles are becoming increasingly sedentary. In a recent research study into the lifestyles of 15 EU nations, the UK was only the sixth most active, with 60 per cent of the population deemed to have a sedentary lifestyle. The term 'sedentary' was determined by how much (or little) of their leisure time people spent on physical activity.

According to the World Health Organisation (WHO), sedentary lifestyles not only increase obesity and diabetes, but also double the risk of cardiovascular diseases and exacerbate all causes of mortality.

The WHO cites overcrowding in cities, poverty, crime rates, pollution, a lack of green spaces and safe areas to exercise as core reasons for low levels of activity, and is calling on governments to promote physical activity and prioritise the development of safe public recreation facilities.

Another factor in our lack of physical activity is the use of technology in place of manual work. Machines, computers and automated gadgets are part of the workforce.

So, if we're not exercising, what are we doing? According to a recent survey by the Broadcasting Standards Commission and the ITC, more than 80 per cent of British adults watch TV at least daily, with the majority (61 per cent) watching between two and five hours a day.

Poverty and obesity
Low- and middle-income families are the worst affected by obesity and its inherent diseases. In less affluent areas of the country there is less access to good exercise facilities and information about the positive benefits of physical activity.

The lack of exercise in these less affluent neighbourhoods is compounded by the fact that investors are unwilling to provide recreational facilities because of the threat of vandalism, the lack of expendable income amongst local families and the low level of interest in exercise in general.

The cost of eating well is also an issue with lower-income families. Fast-food chains are constantly competing to slash the price of their fat-loaded foods - with some outlets offering an adult burger-and-fries meal for less than a couple of quid.

The hidden danger is that, while a burger and fries might seem a reasonably modest meal, research conducted by the Medical Research Council has found that most fast food is very 'calorie-dense', meaning that people unwittingly eat far more calories than they would estimate are contained in the portion size.

So while fast food is cheap, fresh foods can be costly and organic ranges are pricier still. Another consideration is the decline of local mini stores and independent greengrocers and butchers. With many small businesses disappearing and being replaced by out-of-town supermarkets, it's harder to get hold of healthier foods.

Ironically, although the best bargains and multi-buy deals are to be found in supermarkets, some householders - particularly those without cars - simply can't afford to travel there and back. It's no wonder, then, that some families will rely on a high-fat, high-carbohydrate chip butty to fill them up.

Supply and demand
The question of why there's so much convenience food on offer in our supermarkets is up for debate. Advertisers say that they're responding to public demand; the public says it's responding to advertising; supermarkets say they're responding to consumer feedback. There's no doubt that the supermarket giants, for the most part, hold the key to what we eat. They know what we want: food that's tasty, visually appealing, cheap and quick to prepare: they can provide it, but at the cost of much of it being loaded with fat to bring the price down.

New-style loyalty cards, offered by supermarkets such as Somerfield, collect data on which products are most frequently bought by cardholders. As a result, these are the products most likely to be re-stocked and discounted for higher sales. This suggests that if we bought enough healthy items, they would eventually become the supermarkets' mainstays and more likely to be subject to special offers.

Cooking - a lost art?
With supermarkets offering around a 1,000 new convenience ranges each year, even those who can afford to buy and cook from fresh tend to rely on ready-meals to some extent, and 'real' cooking is becoming a lost art, despite endless cookery programmes on TV.

Women, traditionally the main providers of main meals, now make up a greater percentage of the workforce than ever before. According to figures released by the Office of National Statistics, 55 per cent of women with children under the age of five now have full- or part-time jobs. Add in the fact that we work some of the longest hours in Europe, and some convenience foods are, frankly, a more attractive option than spending what little leisure time remains in cooking from scratch.

In addition, for those in higher income brackets, the social pressure to eat out often comes into play, especially amongst adults without children who have the expendable time and income to spend.

Tax breaks for the fit
In a bid to tackle the growing problem of obesity, the government is looking at ways to provide tax relief for the fit. Businesses could be given tax allowances for providing gym equipment or subsidised gym membership for employees, and for replacing snacks and drink machines with health-food canteens and water coolers.

Members of the public could benefit from the incentive, too, by agreeing with their GPs to undertake a certain amount of exercise. The proposals suggest that if overweight patients can provide proof of increasing their exercise, they may become liable for tax breaks.