Across the world, scientists are re-designing foods to make them better for us.

They are using science to create products that are part food, part medicine: TV dinners that could prevent cancer.

Their aim is to increase levels of chemicals that are good for us - and remove those that are bad.

Just desserts?

One of the world's largest food companies, Nestle, has tested and developed desserts that boost the body's immune system.

They contain bacteria that liven up the body's natural defences, and many are already on sale in the UK.

Nestle is also investigating ways of making its heat-up dinners better for us by adding natural chemicals that might combat some cancers.

"People don't have time to grow their own food. They expect to buy a range of tasty dishes which incorporate all the different fruits and vegetables, but also provide nutritional benefits," says Dr David Richardson, chief scientist at Nestle UK.

Doctors and TV programmes have been urging us to improve our health through better diet for decades.

But despite some improvements, too many people - especially those on lower incomes - still eat badly.

If each of us ate two pounds of fruit or vegetables every day nutritionists estimate that some cancer rates could be cut by a third.

But sadly many of us succumb to less nutritious fare. The big idea is to compensate for poor diet by adding chemicals to food.

Most of these chemicals occur naturally in food we already eat. But, according to some scientists, not in high enough quantities to be effective.

People now eat fewer greens

Researchers at the Institute of Food Research in Norwich have shown that some of the chemicals in broccoli prevent the development of cancer cells in test tubes.

With the help of colleagues they have improved on nature, growing a super broccoli with higher than normal levels of this chemical.

Dr Ian Johnson said: "If we can identify and understand the protective mechanisms in such foods, we could increase people's intake of protective factors without them having to eat large quantities of vegetables."

The next step is to see if the broccoli slows the development of cancer in humans by first feeding them broccoli and then testing their blood for early traces of disease.

Engineering 'nutraceuticals'

The Norwich researchers are producing these vegetables using traditional breeding techniques, but others believe they can do better by genetically engineering 'super veg'.

Giant biotechnology firm Monsanto's latest idea is to engineer vegetables that have medicinal properties; so-called 'nutraceuticals'.

They believe that by selling some of the potential health benefits of genetic engineering - they can make the technology more palatable to the public.

"In the next 40 or 50 years, there are going to be twice as many people on this planet. We're not going to be able to afford to give everyone the treatment they receive today," said Monsanto's Dan Verakis.

"We believe that doctors will be able to prescribe certain foods to you that have increased or decreased nutritional properties which will prevent you from getting a particular disease."

New foods have huge commercial potential

In many ways, food companies are doing now what the health food industry did in the early 1990s. They are using a new understanding of nutrition to package up vitamins and minerals and sell them as products that are supposed to improve our health.

But critics say nutraceuticals are a marketing gimmick designed to increase profits rather than improve health.

"We're confusing the health of the consumer with the motives and morals of the producer. It's the companies that want to get these things down our throats, it's not that we necessarily need them," said Professor Tim Lang of the Centre for Food Policy.

The potential is huge. In the US, the market for foods that contain nutraceuticals is worth $4bn, with growth in sales expected to rise by between 5% and 8% over the next five years. And in the UK, sales have already reached £200m.

Unilever is researching 'mood food'

While some might be prepared to accept foods that might improve their health, there is more controversy over plans to develop foods that affect mood.

Unilever are to begin a research programme - part-funded by the government - to develop a slimming food containing boosted levels of a chemical once used to treat depression.

The chemical, tryptophan, occurs naturally in meat, dairy products and some fruit, and most of us consume on average 800 grams of it in our normal diet.

Dr Phillip Cowan, working with Unilever, believes that slimmers would feel less hungry and miserable if they ate a food that contained boosted levels of tryptophan.

If all goes well, later this year he plans to see if the food helps people who are depressed.

"It could have a useful role in a number of common food disorders and could help people cope with stress," he said.

Unilever has no plans to sell an anti-depressant food, but the mere development of the product - even for research purposes - worries some.

"The Orwellian nightmare is that the state or large companies could control the mass mood," said Professor Lang.

"I don't think it's being fanciful to say we're on what could turn into a slippery slope where products are sold to try and compensate for problems in society."

At the moment all these foods are lightly regulated, with laws merely requiring firms not to make false claims.

The food industry wants to keep things as they are, but calls for tighter control will grow as the distinction between food and drugs becomes increasingly blurred.