Just when you thought you knew what was good for you, here comes 7Up with a makeover. It's no longer yellow, but pink, no longer lemon and lime, but mixed berry - and it's no longer a health and fitness disaster zone, Cadbury-Schweppes will argue, because it's full of added calcium and vitamin C. The US launch of 7Up Plus last summer was seen by food analysts as a slightly desperate attempt to pep up a flagging brand. But it's well within the trend. In a climate of growing concern over fat, salt and sugar in food, health is the big new sales pitch. "Wellness will be to the food business what convenience was over the last 15 years," said Brock Leach, chief innovations officer for PepsiCo, in an interview with the UK based trade journal New Nutrition Business.

It's not exactly new. For years, we've sat at the breakfast table, gluey-eyed, reading and rereading the fortification claims on the sides of our cereal packets. Most of us probably know that our cornflakes contain a healthy dose of riboflavin, niacin and thiamine, even if we don't have a cluewhy we need them. And we probably feel slightly reassured that we're getting at least a quarter of the recommended daily allowance of the vitamins we need from a bowlful.

Kellogg's has been adding vitamins to its cereals since the 1930s. The rest of the food industry is finally cottoning on. "Nutraceuticals", it seems, is the growth area of today. Food is no longer for taste, hunger and convenience - it is for health. There's food - and there's functional food, sold implicitly as better than food, which may make you feel healthier and stave off diseases, ranging from allergies to Alzheimer's, or so the manufacturers would have us believe. As is always the case with food, however, the full facts in a digestible form are not always to be found written on the packet. From probiotics to plant sterols, there is evidence of some benefit under certain circumstances, but not necessarily what we all need or what all of us think we're getting.

Fortification of food in the UK began in the most respectable way possible, in wartime. It was an answer to the anxieties of government public health officials over the restricted diet of a population tightening its belt and digging for victory. Margarine replaced butter and was required, as it is to this day, to contain vitamins A and D. White flour still by law has to have added thiamine, nicotinic acid, calcium and iron to replace the nutrients lost in the processing. Neither margarine nor white flour has to list the added vitamins and minerals, although every other fortified food must do so.

More recently, the government consulted on whether to add folic acid to our flour. All pregnant women are advised to take folic acid tablets because it can prevent neural tube defects such as spina bifida in their babies. But dosing the entire population was this time thought to be a step too far. The decision has been put off for the time being, because of concerns that some women, for instance avid spinach eaters, may end up with too much folic acid, while population groups who do not eat bread and other flour-based products would get none of it. But if vitamins and minerals are good things, as we all believe, then can you actually have too much of them? It's a fraught question.

Denmark says you can. Public health officials have just banned Kellogg's from adding iron, calcium, vitamin B6 and folic acid to 18 breakfast cereals and cereal bars. Those are some of the same vitamins and minerals that we have consumed for decades. But the Danes say Kellogg's is proposing "toxic" doses which, if eaten on a regular basis, could possibly harm children's livers and kidneys and damage the foetus in pregnant women. Kellogg's deny there was any problem and point out this is the way they fortify cereals around the world.

The Danish authorities have based their decision on the upper safety limit for vitamins and minerals set down by the European Commission's scientific committee on food and the European Food Safety Authority. A high proportion of Danes - around half the adult population and 70% of children aged four to 10 - take multivitamin tablets every day. Breakfasting every morning on more vitamins and minerals would be overload, they say.

An EU directive on upper safety limits will come into force in the UK on August 1 this year. It will effectively ban certain ingredients - including minerals such as boron (which is claimed to alleviate some arthritis symptoms) and silicon (which allegedly helps prevent atherosclerosis, or furring up of the arteries) - from supplements because they are not on a list of tested and approved substances. And it is hugely controversial. Celebrities such as Paul McCartney, Elton John, Cliff Richard, Carole Caplin and Zoë Ball have lined up to demand that the bureaucrats here and in Brussels leave vitamin and mineral supplements alone. Actress Jenny Seagrove, among others, went to the High Court in January to support the health food manufacturers and retailers in their successful bid to challenge the directive. They won permission to take their challenge to the European court of justice, which will hear their case this month.

At issue, say campaigners, is the availability of hundreds of supplements containing natural ingredients that people have taken to keep themselves healthy or fight illness for hundreds of years. This threat, widely publicised under the slogan SOS (save our supplements) in health food stores around the country, generated a petition of over a million signatures.

"Our argument is that the directive is disproportionate," says David Adams of the Health Food Manufacturers Association. "It basically is initiating a positive list of ingredients that could be used in food supplements. That omits some 100 ingredients that have been used in the UK for hundreds of years."

Some of these will be particular forms of the ingredient. There may be several types of selenium listed, but not organic selenium, for example. Manufacturers can submit a dossier of evidence for the safety and efficacy of their product, but the Food Standards Authority says it could cost up to £250,000 per product. "It has been estimated that up to 5,000 products might have to come off the shelves," Mr Adams says.

And then there are those maximum limits. The UK government's ad hoc expert committee on vitaminsand minerals set the trend in 2003 with a report which specified upper safety limits for all the common vitamins and minerals. It upset a lot of people who believe they are protecting themselves from cancer with high doses of vitamin C, or those taking high levels of vitamin B6 for premenstrual syndrome. The daily limit put on vitamin C, which in extreme doses can cause diarrhoea and possibly kidney stones, was 1,000mg and on B6 - which has been linked to sensory neuropathy (nervous system dysfunction) - was 10mg.

Unfortunately, the large-scale studies that could prove the benefits or dangers of food supplements have not been done - their supporters say that's because there's no money to be made. The health food and supplement industry may be growing, with £335 million spent every year in the UK on vitamins and minerals, but it is still small fry when compared with the wealthy drug companies.

There is no doubt extra vitamins help those who don't get enough. Young prisoners are set to be given vitamin pills in an attempt to improve their behaviour - not because the pills would act in isolation, but because many prisoners habitually choose junk food over any healthier options and may well be deprived of certain essential nutrients.

But it would be wrong to think that food supplements can be a substitute for good food. Many people take vitamin C because it contains antioxidants, which combat the free radicals that damage cells in the body, contributing to ageing and cancer. Yet the vitamin C in a piece of fruit is responsible for only a minority of the antioxidants it contains - the rest comes from plant material. That's why the recommendation for a healthy diet is to eat five portions of fruit and vegetables a day - not to take multivitamin tablets.

Last October, the Lancet published a review of the evidence from the (few and often not brilliant) studies that have been done looking at the effects of antioxidant supplements - including beta-carotene, vitamins A, C and E and selenium - on oesophageal, gastric, colorectal, pancreatic and liver cancers. The low-quality trials suggested the supplements had no effect, but seven high-quality trials indicated that people on supplements actually had higher mortality rates. Only selenium appeared to be helpful - a mineral that is ingested less in the UK since we began to get our flour from Europe, which has selenium-depleted soil, instead of Canada.

The review was accompanied by a cautious Lancet editorial and attracted strong criticism from food supplement campaigners, but the message would appear to be that more work is needed to establish what is safe. The assumption that vitamins and minerals equal health could lead a lot of people down another dangerous path, according to the Consumers' Association, which is concerned about companies that add vitamins to high fat, sugar and salty foods. Good does not cancel out bad on this occasion.

They back the European Commission, which will regulate health claims. "We're very supportive," says their spokesperson Michelle Smyth. "The underlying principle that we want to see is that foods high in fat, sugar and salt should not be fortified. The majority of products are breakfast cereals, but tinned pasta is there too and some biscuits. It sends confusing messages." Some products actually suggest if you eat them, you will be healthier, she says. "There shouldn't be a health incentive to eat that product."

On their hit list are products such as Kellogg's Frosties, with 38g of sugar and 0.6g of sodium per 100g and HP Bob the Builder pasta shapes in tomato sauce, containing 0.5g of sodium per 100g.

The rush to add vitamins and minerals to processed food is one aspect of the boom in nutraceuticals. In parallel to this, but more interesting, is the development of foods that exist to do you good - the functional foods.

The Japanese arguably started it. Yakult, a recent phenomenon here, owes its existence to a medical researcher called Minoru Shirota who became interested in intestinal bacteria and its potential for delivering a healthy life. In 1930, he isolated a lactic acid bacterium robust enough to survive the digestive process, which later became known as Lactobacillus casei Shirota. He put it in a fermented milk drink and launched Yakult on Japan. It reached Europe in 1994.

Probiotics have been a huge success. The theory behind them - that we all have "good" bacteria in the gut which help fight off the "bad" but which get depleted by antibiotics and our sterile, super-hygienic environment - is appealing.

But do they work? Yes, says Jeremy Hamilton-Miller, professor at the Royal Free in London and an acknowledged expert - but not all of them and not always as claimed.

"There are genuine health advantages of taking the appropriate one," he says. "But there are an awful lot on the market and many of them have not displayed any proven health benefits." Certain specific organisms have been shown in studies to be effective in reducing the numbers of colds children catch, for example. There was also some very interesting work, published in the Lancet, that showed that babies given probiotics were less likely to become allergic as they got older. But there are thousands of different organisms. Just because there are bacteria in your yoghurt does not mean you are automatically getting a health boost, says Hamilton-Miller.

Some of the products out there are useless, he says. His advice is to buy only those that advertise on television and radio, where claims have to be substantiated before they are broadcast. That includes some of the biggest brands, such as Yakult and Danone.

The particular organism which has been most studied - and the one used in the allergy research - is not in any of the products available in the UK. Lactobacillus GG is massively used in Finland, but both the yoghurt and the horsesized pills containing it that were launched in the UK bombed.

There seems to be no stopping probiotics, but cholesterol-fighting margarines such as Benecol have had a harder time of it. They contain plant sterols, which are substances that occur naturally in some foods, such as many fruits, vegetables, nuts, seeds, cereals, beans and other edible plants containing oils.

These plant sterols are almost identical to cholesterol in their chemical make-up. They work by blocking the absorption of cholesterol from our food during the digestive process. They also impede the absorption of cholesterol manufactured in the liver.

According to the Food Commission, there aresome doubts about the effectiveness of these margarines. Certainly, they reduce cholesterol by 10-15% in trials where people are eating them three times a day as part of a controlled diet. But among those who buy them from supermarkets and eat them in a more haphazard way, the cholesterol reduction is less than half that. The British Heart Association warns people with potential heart problems not to buy their Benecol or Flora pro-activ and get complacent, saying: "It is vital that people also eat plenty of fruit, vegetables and fish, stop smoking and increase their activity to reduce their overall risk of coronary heart disease."

Omega-3-fortified eggs have appeared on our supermarket shelves in cartons stating that omega-3 is proven to improve the health of the brain, eyes and the heart. The British Nutrition Foundation is very supportive. "Many people do not eat fish and fish stocks are unsustainable in the long term," said Hannah Theobald, nutrition scientist. "Diets rich in omega-3 fatty acids are associated with a reduced risk of heart disease."

But the eggs, launched in 2003, have not exactly taken off. Julian Mellentin, director of the Centre for Food and Health Studies, which analyses the global nutrition business, says that some specific health claims put people off, because they perceive them to be aimed at sick or unhealthy people.

"What's overlooked is that some of the world's most successful functional brands carry no health claims at all," he says in an article for Nutraceuticals World. "Yakult, for example, a $2.3 billion brand, talks only about 'wellness from the inside' and 'a healthy start to every day' as does Danone's Actimel."

Even more ambitious functional foods are on the cards. Belgium and Ireland have seen the launch of a fermented dairy drink called Zen, containing 90mg of magnesium - 30% of the recommended daily intake - which is said to help relaxation. Phytopharm, a company that develops drugs from plants, is working on a plant extract to suppress the appetite, which could be added to milkshakes. European Union-funded research is looking at the potential for preventing Alzheimer's by adding specific lipids to elderly people's diets.

The possibilities appear to be endless. The good news is that the food industry has taken on board today's health issues, which is having an impact on their unhealthier products too - both salt and fat levels are dropping, although sugar has a way to go. Meanwhile, there is an increasing amount of fortified and functional food to try for those who feel they need more than their ordinary diet can provide.

It is important, however, that we know what we're being sold in the name of health. Omega-3 may be a very good thing, but omega-3 from oily fish is far superior to that from plants, and costs a lot more too. And guess which type is contained in the spreadable margarines that make big claims to improve the health of your heart? Not the most effective, expensive kind.

Traffic-light colour coding for fat, sugar and salt may be round the corner, which can only be a good thing for those of us who buy ready meals and other processed foods to fit in with our frenetically paced lives. But when we grab what looks like a healthier product, we need to be sure that we're not being sold unhealthy food by stealth, sexed up with a health claim or two - extra vitamins do not cancel out the damage done by sugar. And we also want to know, when we choose our low-fat spread that it really will protect our heart or lower our cholesterol. Maybe it's time to think about some health warnings on our health foods too.

What works? Vitamin C may be worth taking in a pill if you are coming down with a cold. Studies have not shown that it can prevent colds, in spite of the theories of Nobel Laureate Linus Pauling which have passed into folklore, but it does seem to lessen their impact.

Vitamins B6 and B12, together with folic acid, are important in the processing of homocystiene, a substance resulting from the breakdown of protein. Some studies suggest high homocystiene levels are linked to a higher risk of heart disease and stroke, but the jury is still out. Most people can get enough of these vitamins from their diet, as long as they eat meat and fish. Vegans and vegetarians may benefit from supplements.

Vitamin D helps regulate calcium and phosphate in the body which are important for healthy bones. Pregnant women are advised to take 10 micrograms (0.01mg) per day. Anyone who doesn't get around 15 minutes of sunlight a day or eat oily fish such as salmon and tuna may benefit from extra vitamin D in tablet form.

Vitamin E is most plentiful in plant oils. It's been suggested it could have a protective effect in prostate cancer and help prevent atherosclerosis. If you don't eat olive oil you may want to take vitamin E.

Folic acid prevents neural tube defects such as spina bifida and should be taken if planning a baby and during pregnancy.

Selenium has been found to protect against gastrointestinal cancers. It is found in brazil nuts, fish, meat and eggs but not so much in bread since Canadian flour, rich in selenium, was replaced by European flour, which is not.

Cod liver oil, that ancient family remedy, contains important omega 3 essential fatty acids. Official advice is to eat two portions of oily fish, such as sardines, herring and salmon, a week, but supplements may be worthwhile for those who can't face it. Omega 3 fatty acids help prevent heart disease and are good for pregnant and breastfeeding women because they help the baby's nervous system to develop.