KEY POINTS:
They are expensive, sometimes pocket sized and often advertised as "immune boosters" or "gulps of goodness".
So-called "functional foods", containing added ingredients claimed to be good for health, are increasing in popularity.
Consumers have been persuaded that additives can be good for them as well as bad.
Margarines
that reduce cholesterol, yoghurts to improve digestion and sports drinks that boost energy are among the top sellers.
But researchers warn that the medicinal effect of the foods, also known as nutraceuticals, could rebound to produce unexpected side-effects. They are banned in some parts of the world because of the risks.
Walk down any supermarket aisle and you will see omega-3 fortified eggs, pro-biotic yoghurt drinks, sleep-inducing milk, and selenium-enriched bread.
New functional foods due for launch include the first cosmetic yoghurt, made by a French manufacturer, which it is claimed will "nourish the skin from the inside." Soft drinks are planned with added vitamins and minerals. Mineral waters with added vitamins already sell well in the US. A one-shot drink claimed to aid slimming which contains an extract of pine nuts containing hormones that increase feelings of fullness is already on sale in the UK.
A pasta enriched with grapeseed extract said to protect the retina in diabetics, anti-ageing olive extracts and cranberry chewing gum to prevent gum disease are among other products on the way.
Millions of people are consuming the new foods and swallowing the added medicinal ingredients that they contain but they are not being monitored in the same way as for the launch of a new drug. Even data on their sales is limited. Researchers from the National Institute for Public Health and the Environment in Bilthoven, the Netherlands, say in one three-month period 200 new products were launched in the UK.
According to market analysts Datamonitor the number of people buying functional foods could exceed five million this year.
Writing in the British Medical Journal, Nynke de Jong, project director at the Duth Institute and colleagues, focused on the potential risks of cholesterol lowering margarines and yoghurts.
These products, he wrote, could trigger reactions in people taking statins - drugs that do the same job but act more powerfully - which might actually increase their risk of heart disease, the Dutch experts say.
The margarines contain plant sterols which lower cholesterol but when eaten by people taking statins, the level of plant sterols in their blood is raised.
There are concerns that this could increase the thickening of the arteries - and the risk of a heart attack - and Canada has banned the sale of these products.
The authors say there is no evidence so far that functional foods cause harm, but the data is limited to five or six years of use.
They write: "There is little understanding of the circumstances under which the foods are eaten [and] whether target groups are reached.
"Very little is known about exposure, long term or otherwise, and safety under free conditions of use, and whether and how functional foods interfere with drugs designed for the same target."
The researchers call for a systematic monitoring programme to provide consumers with "practical and unbiased information" about when, how and if to eat functional foods.
In Holland, monitoring of the effect of cholesterol-lowering margarines over five years has shown they can stabilise cholesterol levels in individuals which would otherwise increase naturally with age, an effect they describe as "modest" but potentially beneficial.
"We need to find out more about what functional foods can contribute in relation to the promises made by the manufacturers," they say.
- INDEPENDENT
