Professor Richard Mithen, from the Institute of Food Research Norwich, has surveyed the research results and concluded that "one mustn't underestimate how complex it is to do the studies to support health claims". He added that studies across the world have shown that this area is far more complex than hoped.
"Straight vitamin and nutritional supplements cannot reproduce the health benefits of a diet rich in fresh produce. It's a real challenge to understand the biological activity of these compounds," he said.
Phytochemicals, or secondary metabolites, are compounds synthesised by plants but not involved in the plant cell's primary metabolism. They, together with plant-derived vitamins and minerals, were put under scrutiny at the recent Camden & Chorleywood Food Research Association's Nutritional Quality of Fresh Produce Conference.
The types of phytochemicals associated with promoting and maintaining health are: polyphenolic compounds, found, for example, in soya beans, red wine grapes and onions; carotenoids such as carotene and lycopene in carrots and tomatoes; fatty acids (eg GLA in evening primrose); and the sulphur-containing compounds in brassicas, particularly the glucosinolates.
Mithen demonstrated that the phytochemical content was determined by a complex interaction between genetic and environmental factors - from variety differences through to storage and even domestic processing, in addition to the human gut reaction as food is broken down.
In summing up the complexities involved in determining the workings of plant-derived health-enhancing compounds in the human body, conference chairman Professor Geoff Dixon said: "This is all very good news for the fresh produce industry - we will never find synthetic substitutes to end up with a magic pill."