Organic fruit and vegetables may be better for the heart and general health than eating conventionally grown crops, new research has found.

A ten-year study comparing organic tomatoes with standard produce found that they had almost double the quantity of antioxidants called flavonoids which help to prevent high blood pressure and thus reduce the likelihood of heart disease and strokes.

Alyson Mitchell, a food chemist, who led the research at the University of California, believes that flavonoids can also help to stave off some forms of cancer and dementia.

She found that levels of quercetin and kaempferol, both flavonoids, were on average 79 and 97 per cent higher, respectively, in organic tomatoes. Her findings are due to be published in full in the Journal of Agricultural and Food Chemistry.

Dr Mitchell said that previously it had been hard to make comparisons between organic and conventionally grown produce because of difficulties in comparing soil quality, irrigation practices and the handling of harvested produce. But for this study researchers used data from a long-term project in which standardized farming techniques were used to reveal trends in crop productivity.

The team believes that the different levels of flavonoids in tomatoes are due to the absence of fertilizers in organic farming.

Plants produce flavonoids as a defense mechanism; they are triggered by nutrient deficiency. Feeding a plant with too many nutrients, such as inorganic nitrogen commonly found in conventional fertilizer, curbs the development of flavonoids. The lower levels of flavonoids in conventional tomatoes were caused by “over-fertilization”, the research team concluded.

The Soil Association is now pressing the Food Standards Agency to review its guidance on the merits of organic as opposed to conventional fruit and vegetables. Peter Melchett, its policy director, said that there was now a rapidly growing body of evidence which showed significant differences between the nutritional composition of organic and non-organic food.

Recent research in Europe found that organic tomatoes contained more vitamin C, B-carotene and flavonoids than conventionally grown tomatoes. Organic peaches and organic apples were also found to have more antioxidants. Lord Krebs, the former chairman of the Food Standards Agency and now Master of Jesus College, Oxford, said that even if such benefits existed, higher flavonoid levels did not make organic food healthier. “This depends on the relevance of the differences to the human body,” he said. “Tomato ketchup has higher levels of lycopene [a strong antioxidant] than either organic or conventional tomatoes. So if you wanted lots of lycopene you should eat tomato ketchup.”

The Food Standards Agency, however, has commissioned a three-year study into the benefits of flavonoids. It said: “There is accumulating evidence that dietary flavonoids. . . may in large part explain the cardiovascular disease benefits of increased fruit and vegetable intake.”