More than five million people joined fitness centres last year. It was a 12% increase on the figures for 2000 and numbers are predicted to grow again this year, as the gym holds out its eternal promise of bodily salvation. Gym membership has increased every year since the initial boom of the early 80s when Jane Fonda urged us all to pull on the leggings and go for the burn.

What has increased just as inexorably is our collective body mass. In 1980, 14% of Britons were seriously overweight. Now, more than 40% are. While the ever expanding fitness industry succeeds in convincing us that it has the solution, the sums do not add up.

So why is the government drawing up plans to get more people, including children, into fitness centres? As part of its ambitious Game Plan initiative, it is proposing that those on low incomes receive vouchers to attend and others get tax-free membership. GPs, meanwhile, will be able to prescribe fitness-centre sessions. As a way of defusing Britain's obesity time bomb, it appears to have a logic, and the idea has been supported by employers and business analysts. But obesity experts are more sceptical.

"Obesity is a global problem and not one that can be solved through institutional exercise programmes. Half the British population is overweight - are you going to get them all into the gym every week?" asks Neville Rigby, director of policy of the International Association for the Study of Obesity.

While there is still a consensus that inactivity rather than overeating is responsible for the current obesity epidemic, the terms of the debate are shifting. Exercise as most people understand it is no longer the main focus. It is activity that matters, says Rigby. While the difference may seem a matter of mere semantics, in practice it is the crucial distinction between going somewhere special to get and keep fit until you leave and living a life that does it for you until you die.

"We have lost so much physical activity from our everyday lives that an hour or two in the gym a week can't possibly compensate. I don't know of a single society in the world where the gym or equivalent has made any difference to a national obesity problem."

According to the new thinking, institutionalised exercise is rather like crash dieting: it is a means to an end that can rarely be sustained. Gym attendance relies on discipline and a concomitant belief that somehow the more you spend financially, the more you will expend calorifically. Lose the discipline or re-evaluate the cost/ benefit, and your membership lapses.

While exercise is clearly vital, Rigby and others agree, there is an absurdity about the way we try to do it: driving to the gym to get on a treadmill or exercise bike; standing on the escalator on the way to a step aerobics class. And then going home to what they euphemistically call "compensatory behaviour" - having a session in front of the telly, or eating a sugary snack.

Clearly, a core of devotees do demonstrate that the gym can be a very effective way to build muscle, lose fat and increase stamina. But they are in the minority. A number of consumer surveys have put the gym drop-out rate at more than 80% in the first eight weeks of signing up. Dr Melvin Hillsden, lecturer in public health at University College London, says his figures are more optimistic, showing that 60% of gym members are still paying their subscriptions after a year. But while they may be paying up, are they still turning up?

Payment of the membership fee clearly does not always equate with gym attendance. A recent survey by Cahoot online bank identified gym subscriptions as typical of the direct debit payments that ex-users forget to cancel. Then there are those who keep paying in absentia because, well, tomorrow is another day.

Membership does not determine regularity of attendance, either. A Mintel survey has found that one in five gym members goes just once a month or less. This equates to 100 calories a week. You could burn up more energy than that doing some gardening every weekend or taking a brisk 20-minute walk every week for a month. These are just the kind of activities that the proponents of the "active living" approach endorse.

Jaap Seidell, chair of the public health and prevention task force of the European Association of the Study of Obesity, says: "Most people find high-intensity activity very difficult to sustain and the benefits of lower-intensity exercise have been greatly underestimated."

He points to a study by compatriot Klaas Westertup of the human biology department at Maastricht University, who showed in clinical trials that short periods of intense physical activity had less effect on overall energy expenditure in a day than routine and regular low to moderate activity, such as gardening or playing with your kids in the park.

It sounds unlikely, but Seidell says this has been proven in rigorous laboratory tests. International comparisons prove the case, too. In the Netherlands, 50% of people regularly walk and cycle, compared with under 10% in the UK. Only 12% of Dutch people are obese. Gym membership, as you might guess, is no higher in the Netherlands than it is here.

The familiar recommendation to do three 20-minute bouts of high-impact aerobic exercise a week, cited as gospel for many years, was actually designed for performance athletes. It was never meant for the average man or woman in the street, which is why most find it so difficult to sustain.

So how much moderate-grade walking, cycling, digging or carrying do you have to do to be sufficiently active? And what if you don't have a bike or a garden or a local shop?

The new consensus is half an hour five times a week. But at the core of this approach is the principle of integrating activities: they accomplish tasks you would be doing anyway, and can be chopped into smaller chunks and distributed throughout the day.

"It's about making active choices," says Charlie Foster of the Health Promotion Research Unit at Oxford University. You can order your shopping online and use 20 calories, push a trolley around the supermarket for an hour and use 10 times that or make several trips to your local grocers, carrying the shopping back, and expend 300. These numbers are nothing next to the 1,200 calories it cost our hunter-gatherer ancestors to hunt, kill, skin and butcher an animal for a few meals. "But you have to take into account the cumulative effect: over a week, walks to the shop and similar activities all add up."

The biggest calorific impact you can make, experts agree, is by changing the way you get about: don't drive if you can cycle; don't cycle if you can walk; don't walk if you can run; don't use the lift if there's a staircase. Making roads safer would have a far bigger impact on our collective health than building more gyms. Game Plan does note the need to get more people walking and cycling - in a single sentence in a 240-page document.

The World Health Organisation, in its recent report A Physically Active Life Through Everyday Transport, is more dogmatic: increasing the opportunities for walking and cycling, it insists, would have a bigger impact on the inactivity epidemic than any other single measure. Pilot projects are now running across Europe to test the theory.

Foster, who used to work in primary care, says the most common reason patients gave for not doing organised exercise was lack of time. Game Plan notes the same problem. But adopting the "everyday activity" approach has the potential to eliminate that ubiquitous excuse.

The fitness industry is worth £682m and is predicted to grow to £1.6bn in five years' time. A year's gym membership will rarely cost you less than £200 a year. You may lose some weight; but pound for pound, does it add up?

Case study 1 Kevin Kemplen, 40



"The first time I joined a gym, I hardly went. But the second time I was determined to make it work. I'd just come back from a holiday in the US and was 15 stone. I attended fanatically for a year, lost over a stone and got very fit. Then I suppose I thought, 'I'm OK now' and got a bit complacent. I haven't been for ages. I'm still paying every month for it. I suppose I feel I don't want to lapse and pay a rejoining fee. "Meanwhile, I'm keeping fairly fit by doing things that are more sociable and less tedious - playing tennis two or three times a week and jogging. Although I still find the idea of sitting on a stationary bike and watching MTV mind-numbingly boring, there are some exercises you can do at the gym that are difficult to reproduce outside. And if I cancel I know I won't go again. So, although it's a bit mad to write the gym owner a free cheque every month, I'm keeping my options open."

Case study 2 Nicoela Casalotti, 34



"I joined a gym after I had my baby. I'd put on four stone in my pregnancy and wanted to get rid of it. But of course I found that, with a baby, I had very little time. I was able to take him in the pool at the centre, but other users were hostile to that. Also, I found the whole gym experience sterile and uninspiring. Then my husband, who runs a tricycle courier company, bought me a trike. I go everywhere on it. It makes doing everyday things with Miles much easier, it elevates my mood, removes stress and seemingly without trying I was back in size 10 clothes within nine months - and without dieting. If anything, I probably eat more now because I use up so much energy on the bike. But it doesn't feel like work and it doesn't take any extra time. I doubt I'll join a gym again."