Organic kiwi fruit is much healthier than its conventionally counterpart, a US study has concluded.

Researchers from University of California, Davis found that organic kiwi fruit had much higher levels of polyphenols and antioxidant activity.

Polyphenols help to reduce cholesterol and improve blood circulation, while antioxidants help to neutralize free radicals that can damage cells.

The study, which is published in the journal Chemistry and Industry, also revealed that organic kiwi fruit had higher levels of vitamin C.

Lead researcher Maria Amodio said: "It is possible that conventional growing practices utilize levels of pesticides that can result in a disruption of phenolic metabolites in the plant that have a protective role in plant defence mechanisms.

"All the main mineral constituents were more concentrated in the organic kiwifruits, which also had higher ascorbic acid and total phenolic contents, resulting in a higher antioxidant activity.”

Last month, a Manchester Business School study concluded that the environmental benefits of organic food production are not "clear-cut".

"There is no clear evidence in environmental terms to support locally-sourced rather than globally-sourced shopping," the study said.

"For some foods, global sourcing might be a better option for the environment."

However, the study findings were attacked by the Social Association, the group which campaigns for organic farming.

The Soil Association says there is clear evidence that there are more nutrients in organic produce.

Figures from the Social Association show that organic food sales in the UK jumped 30 per cent to £1.6bn in 2006.

Research group Mintel predicts that the UK organic market will reach a value of £2bn by 2010.