Not long ago, I popped into a big Sainsbury's near the office for lunch. There, almost the first thing I encountered were three shelves of prepacked fruit salad. You can have Fresh Classic Fruit (melon, apple, pineapple, kiwi and blueberries); Fresh Pineapple Chunks; or Fresh Mango Chunks. The use-by date on the Fresh Melon Selection, "ideal as a refreshing snack", proclaimed that it had another two days of viability, but already the cubes of watermelon were blurring. Apples, as Eve might have said, have to be the original convenience food - pick, wash (or rub briskly along the leg of your jeans), eat. No need to peel, no mess. So, strangest of all were the little pillows of vacuum-packed apple slices, thin wedges of Pink Lady at 59p for 80g, or £7.38 per kg - nearly three-and-a-half times more than it would cost to pick up an intact Pink Lady, a few aisles down.

And yet, increasingly, we don't seem to mind. The idea of fruit being a shortcut to health has taken such a hold on our imaginations that the European market for what the industry calls "fresh, valued-added produce products", almost negligible a decade ago, is now worth €3bn (£2.2bn). Much of that is accounted for by packaged salads, but packaged fruit is quickly catching up. According to the market research company TNS, sales of chilled, prepared fruit in the UK have increased by 80.6% in the past two years (this doesn't include fruit packs not taken home, eg those eaten at work).

"Along with organics and fair-trade, fresh-cut is probably one of the most exciting parts of the fresh produce trade at the moment," says Mike Knowles, editor of Eurofruit magazine, adding, somewhat superfluously, "it's enabling retailers to add value and increase their sales."

But is the fruit boom really so good for our health? There can be few of us who haven't clocked, at some level, that the government wants us to eat five portions of fruit and vegetables a day, and "a lot of the fresh produce suppliers to retailers in the UK have become aware of the growing demand and are really starting to invest in the machinery to cut fruit and prepare it," says Knowles. "Fresh produce tends to be seen as quite a mundane product category, not an exciting or sophisticated market, but in the past few years the kind of value that suppliers can add has made it more exciting for the trade and, I would also say, for the consumer."

Surprisingly, quite a few nutritionists seem to agree, with caveats. Obviously, the ideal is that we buy whole fruit, but, as Richard Faulks, of the Institute of Food Research, argues, packaging fruit salads might entice people actually to eat the fruit they buy. "Certainly in the past, fruit was seen as a necessary purchase, but not as a necessary food item," he says. "It goes in the fruit bowl and [often] sits there until it gets thrown away. Given the option of a plate of meat, potato and two veg, the instruction has always been to eat up your meat, because that's the expensive item. You'd never get, 'Eat up your vegetables, because those will do you good and won't make you fat.' It wasn't seen as providing energy in value-for-money terms by a lot of people - particularly people at lower socioeconomic levels, who can't afford a lot. You go for energy because that's what makes you survive. It's seen as OK to buy a cabbage and then throw it away, or a lettuce and throw it away, but you'd never do that with a joint of beef."

Making fruit more expensive, more of an event, the theory goes, makes it more likely to be eaten. "You'd throw away a mouldy apple, but not a mixed fruit salad. You've paid a bit more for it." Prepackaged, sliced fruit is attractive to children, and to single people, too, who might struggle to get through a whole pineapple or melon or watermelon - and especially all of the above, if they wanted some variety - before it went off.

However, our increasing concerns about food miles, about preservation, about exploitative labour practices, still apply - in fact, apply with a vengeance. Packaged fruit is often not only grown but also cut up abroad (I had mangoes for lunch the other day that were prepared in Ghana). Therefore, the tricky process of slowing down spoilage becomes even more difficult.

A couple of years ago the consumer magazine Which? discovered that Sainsbury's sliced apple bites - sold as 100% apple, no preservative - were being dipped in citrus solution, or AS1 (ascorbic acid), to stop them from going brown. Many manufacturers use lemon juice; others use a treatment called NatureSeal, which contains vitamin C and calcium. Food USA magazine recently reported on the use of bacteria on the cut surfaces of fruit to fight the pathogens than can form there.

Guardian journalist Felicity Lawrence, in her book Not on the Label: What Really Goes into the Food on Your Plate, points out that "between 1992 and 2000, the period in which sales of bagged salads took off, nearly 6% of food-poisoning outbreaks were associated with ready-to-eat salads and prepared fruit and vegetables." The Health Protection Agency says it does not track which of these outbreaks applied specifically to fruit, but it is as well to remember that it's less likely to happen to plums, say, than rocket. "The inside of a fruit is sterile," says Faulks. "Once you take the skin off and [as long as] you cut it with a clean knife, you shouldn't have any contamination.

I mention that I have heard that peeling fruit removes much of the goodness available, because most nutrients are to be found near the surface. "No. There tends to be a slightly higher concentration of some things under the skin, but if you eat an apple most of the nutrients come from the flesh, not from the skin," says Faulks. "The skin is a tiny fraction of the whole thing. The concentration [of nutrients] per unit mass might be higher, but the amount of mass overall, by comparison to the whole apple, is very small. It's a misinterpretation."

It is true that there is some diminution for other reasons, however, as Lawrence argues, the concentration of nutrients in fruit and vegetables has dropped with the introduction of artificial fertiliser (because soils fed only with artificial fertilisers containing nitrogen, phosphate and potash gradually lose their vital trace elements). Between the beginning of the second world war and the early 1990s, the iron content of vegetables dropped by 27%, the magnesium content by 24%. And a Which? magazine investigation published in June 2004 discovered that pre-cut fruit and vegetables often suffered marked decreases in vitamin C content. Much was made of Asda sliced runner beans, which contained only 11% of the vitamin C they would have had when fresh, while mango from Marks & Spencer and melon slices from Safeway contained less than half the vitamin C than might have been expected. (The exceptions were apple slices, which, being treated, had eight times the expected amount, and pineapple slices, which appeared to have four times the expected amount.)

The Which? study into this issue seems to be the only known one in the UK, and experts are not entirely convinced of its importance. "Whenever you cut up fruit or put it into cold storage, or whatever you do when you've got a prepared product, you are going to lose nutrients," says Faulks. "There's no question about that. But the level of loss is relatively small." So, in the Which? survey, a Somerfield citrus fruit medley retained 75% of the expected vitamin C, while the kiwis in an Asda tropical fruit salad retained 62%.

"The fact that people are eating more fruit because it's convenient and readily available is the more important thing," says Anna Denny, a nutrition scientist at the British Nutrition Foundation. "The main thing does seem to be that people struggle to eat enough fruit and veg in this country, so any form of pre-prepared fruit and veg is a great way of helping them to reach five a day. "A lot of innovations are seen as totally negative," says Faulks. "This one, I think, in terms of nutrition, is positive."

The one thing that is in nobody's interest to say is this: fruit just doesn't provide that much nutrition in the first place. If you believe the nutrition industry, every week produces some new superfood, often a fruit: blueberries, pomegranates, acai berries. The fact is that fruit consists of water, sugars (normally about 10%), some vitamin C, and some potassium (thought to be good for controlling blood pressure). And that's kind of it. Pineapple, for example, has only got about 10mg of vitamin C per 100g (which means a 80g standard portion would only have about 12% of RDA) and is mainly water and sugar. In a typical supermarket fruit medley of 150-200g, at least 15g will be sugar, and the other major constituent water. If it's a citrus medley, there will be about 40mg per 100g of vitamin C, if not, there will be about 10-20mg.

"It's a myth that fruit is packed full of vitamins and minerals," says Tom Sanders, who is director of the Nutritional Sciences Division at King's College London. "The foods packed full of micronutrients are grains, seeds and nuts, the peas and things." Bagged salad? "It's mainly water. Dark green vegetables are a good source of some vitamins, such as vitamin A and folate, but lettuce hasn't got much going for it at all. The really sad thing is that we don't eat enough vegetables, such as cabbage, spinach and broccoli."

In May, the Observer reported that dietitians have become so worried about claims being made for so-called superfoods that they convened a debate on the subject at the Science Museum. It may be claimed that particular exotic berries boost IQ, energy and immunity, but the only science even vaguely backing this up is that they contain folic acid, which does boost brainpower, but is present in many foods. The antioxidants in pomegranate juice, which supposedly fight diseases as different as cancer and arthritis, actually only last in the body for an hour. Wheatgrass, that standby of the trendy juicebar, is said to be rich in detoxifying chlorophyll, but every green vegetable and leaf in the world contains cholorophyll - which is not, in fact, absorbable by our bodies.

"The term 'superfoods' is at best meaningless and at worst harmful," Catherine Collins, chief dietician at St George's Hospital in London, told the paper. "There are so many wrong ideas about superfoods that I don't know where best to begin to dismantle the whole concept."

Nor do dietitians have much time for the rise of the smoothie, sales of which have increased by 523% in the past five years. They are expensive, says Sanders, "and bloody holier than thou". With whole fruit, the cell structure is still intact, and you swallow pieces. They take longer to digest and the sugar in them is released slowly, rather than the rapid spike in blood glucose produced by drinking juice, or a smoothie. "If you liquidise it into goo it's just like drinking ordinary Coke. Or worse, actually," he says. "It's still a sugary drink. A lot of people on diets don't realise that if they're drinking loads of apple juice or orange juice, it's got a lot of calories in. If you drink a litre of apple juice a day, it'll be 400 calories." Saunders particularly objects to labelling that implies that drinking these concentrates substitutes for three or four portions a day: "They don't. They only count for one."

Meanwhile, the British Dental Health Foundation warns that drinking or eating fruit should be kept to mealtimes, rather than indulged in throughout the day. This is because the sugar (combined with citric acid in oranges, grapefruits or pineapples) attacks tooth enamel, which then needs some time to recover; ie, if you eat many boxfuls of clementines, all the time, you may begin to do actual harm to your teeth.

This is not an incitement to throw the baby out with the proverbial bathwater. The reason apples are good for you is mainly the cellulose and vitamin C; chewing gives a feeling of satiety and promotes saliva secretion, which is good for your teeth; and because, in the real world, they tend to come as part of a deliberate lifestyle. "People who eat apples probably ride a bike and don't smoke," says Sanders. Except for the truly fanatic, they are also more likely to eat them in moderation.

The added irony, in the topsy-turvy world of supermarkets, is that rich desserts often cost very little, while fruit, especially organic, fairtrade, and prepared fruit, is marked up. At the Sainsbury's where I got my lunch, I could have had four 100g creme brulees for 44p, two tiramisu for 98p, and six chocolate mousses for 69p - or a grand total of 11.5p each, making those Pink Lady slices, gram for gram, four times more expensive. We are, more or less willingly, paying through the nose for a particularly 21st-century version of virtue. "You're made to feel worthy, and therefore you're made to pay a premium for it," says Sanders. "Supermarkets have a lot to answer for in the obesity debate."

"The way you've got to look at fruit is that it's better to eat fruit than biscuits, cakes and puddings, because there's very little energy value in it and it's not fattening," he says. "A bit of sugar gives you a lift and takes the pangs of hunger away. But it's not full of all sorts of other nutrients as well. That's a myth"