DIET is a very powerful word: you only have to ask a publisher. Walk into any bookshop and the choice of diet books is bewildering - The Wine Diet, anyone? How the Rich Get Thin? The Duvet Diet? - it raises the heart rate just trying to choose. In January, that market is at its peak as thousands of women appraise their bodies critically in the dim light of a new year. It starts with a cold tape measure round the waist, head cocked to one side. A frown, a sigh and a resolution later, a new customer is born.

The slimming industry - encompassing books, foods, supported weight-loss programmes, supplements and surgery - is growing in inverse proportion to its clients' waistbands. And going by new research released today, it is a relatively stable market. The survey of 2500 British women found that one in 10 is currently trying to lose weight and, of them, more than a fifth consider themselves to be permanent dieters. For them, it's a case of once a dieter, always a dieter. So much so that the report says a woman can spend up to 31 years of her adult life, or an average of six months a year, on a diet.

Mori, which conducted the survey, polled 1446 people and carried out interviews with a further 1036. They found that three-quarters of those who began diets in the New Year will give up their resolutions. The average diet apparently lasts 5.48 weeks at any time of the year but fares less well after Christmas, lasting just three weeks. Half give up because of lack of will power, while a quarter claim the regime is making them moody. Typically, though, dieters carry on looking for solutions to their perceived weight problem - the average dieter, for instance, spends £150 on special diet foods.

Figures show that most products and services associated with weight loss and slimming, from books to cosmetic surgery, are on the increase. But what is fuelling this rise - a desire to embody a certain image, a bid to live a more healthy life or a combination of the two? And can money really buy a svelte waist and trim thighs?

Marion Hetherington, professor of biopsychology at Glasgow Caledonian University, believes the rise in obesity rates over recent years could be fuelling the growth, rather than failed dieting fuelling obesity. "The heavier people are, the more likely they are to buy dieting products - I think it's that way round," she says.

Among women who are of a healthy weight, media images of thin women and girls have some effect in making them want to slim down, but Hetherington does not see this across the board - it depends on the individual and their stage of life. Women in their 60s are unlikely to be too concerned about how much a leading actress weighs "but the average teenage girl is very aware of who Keira Knightley is and how much she weighs". Knightley, of course, is currently suing a national newspaper, saying it "dishonestly sought to mislead the public" about whether she has an eating disorder - something she has publicly denied.

A similar division is apparent when it comes to peer pressure, with younger women in their late teens and 20s more preoccupied than older women with what other women and their partner may think.

On the face of it, the fact that women are picking up and dropping diets in an ongoing cycle - sometimes, if the Mori survey is correct, over many years - would suggest that these diets aren't working.

Yet Hetherington counsels caution in drawing such conclusions. The very fact that people keep taking up new diets after falling off the wagon with a previous diet, she points out, suggests that the regimes are working, at least for as long as people stick to them. That encourages them to try again next time.

She highlights an alternative scenario: what if no-one dieted? Would that be healthier? Probably not. Dieting on and off is "not necessarily a bad thing", she says - it depends on the individual. "If people get into the habit of dieting in the summer and in January, say, for some that is normal behaviour."

However, she sees pitfalls. "It's hard to sustain a diet that's based on depriving yourself. Pleasure is important to human behaviour generally. If a person don't sustain the changed diet, because perhaps it's unrealistic, they'll likely put the weight back on again."

For Dr Kristen McNicol, a clinical psychologist at Glasgow Weight Management Service, which gives support to people who are overweight and trying to slim, yo-yo dieting is not a desirable long-term weight-maintenance strategy. Many of her clients have been through years of it. "The problem with traditional diets is they are unrealistic; they are too restrictive and inflexible. People can keep them up for days or weeks, but they are hungry and deprived on them. They crave foods that they are not allowed to have. It's like a pressure cooker building and can lead to bingeing and over-eating, which confirms their belief that they have no control over their own weight."

Much of Dr McNicol's work focuses on trying to change the way people think about losing weight. "One of the things that can drive extreme dieting is unrealistic weight-loss expectations," she says. Trying to achieve an "ideal weight" can be counterproductive. "We try to get people to change the really strict rules they live by. People tend to be very all-or-nothing about weight loss."

All this would appear to undermine the value of novelty diets, such as those that focus on cutting out food groups, dropping three dress sizes by next Tuesday or consuming only water in the evening. And there are clues to suggest that consumers are starting to accept healthy eating as opposed to quick-fix dieting messages. Demand for diet foods such as meal-replacement shakes and cereal bars is falling: a recent study by Mintel found that sales of such slimming foods had fallen by 27% over the past five years. They are worth £81m now, compared to £110m in 2001.

This trend, the move away from processed towards more "wholesome" foods, also chimed well with the launch of the low glycaemic index (GI) diet, one of the most popular dieting phenomena of recent years. The Low GI diet advocates eating a broad range of foods, including lots of fruit and vegetables, but choosing foods that release their energy slowly.

This year's dieting literature reflects that changing market, suggesting that publishers are responding to the needs of a savvier readership. The Wine Diet, for instance, by heart specialist Prof Roger Corder, might sound like a fad diet, but in fact simply advocates moderate red wine consumption and a balanced healthy-eating regime rich in fruit, vegetables, nuts and berries, that will result in slow but lasting weight loss. Another of this year's offerings, the Japan Diet, is once more about health as much as weight loss, advocating the consumption of lots of vegetables and fish with no portion control.

Cynthia McVey, who is a senior lecturer in psychology at Glasgow Caledonian University, is a prime example of a clued-up consumer who adapts the science to suit herself. She lost weight a few years ago by trying an adapted version of the Atkins Diet, a high- protein, low-carbohydrate regime. She had never dieted before - at school, she was so slim she was nicknamed Skinthia - but had been aware that her waistbands were getting tighter as a result of snacking between meals. She and two colleagues tried it together. "We were very careful - we had more fruit and vegetables than you would usually have," she says. "We saw the results quickly. I felt very well on it."

That was three years ago and she has never regained the weight, nor has she hopped from diet to diet. She now eats carbohydrates, but avoids refined ones, sticking to multigrain bread as opposed to white.

So "fad" eating regimes can last in the long term, if they are dovetailed into long-term healthy eating.

Hetherington believes there will always be a market for weight-loss products: "People like the notion of a rapid return for their money," she says. "But the problem with body-weight regulation is that it is not about the short term, and expenditure is just as important as intake."

Eat healthily and do more exercise - in the end, the mighty diet industry comes down to just those six words.