Products grown without using toxic chemicals, which can cost twice as much as a result, contain higher levels of flavonols. These vegetables have 20 per cent more of the antioxidants, which prevent damage to the body, an Irish study suggests.
It comes five years after a major review of more than 200 studies concluded that organic food offered no nutritional benefit.
However, the new Teagasc Food Research Centre, Ashtown, investigation is the longest-running study to address the issue.
The findings will be welcomed by organic farmers, whose claims their food is more nutritious have previously been rubbished by the government watchdog.
How was the study carried out?
Researchers assessed the levels of flavonoids and antioxidants in 'Red Baron' and 'Hyskin' onions between 2009 and 2014.
These forms of the vegetable are known to be high in quercetin - hailed by scientists for its cancer-fighting properties.
A range of crops, grown either organically or using pesticides, were measured for their levels of the compounds, according to the report published in the ACS' Journal of Agriculture and Food Chemistry.
What did they find?
As well as being higher in flavonoids, the researchers also found that antioxidant activity was higher in both varieties of organic onions.
To be certified as organic, farmers are unable to use the strong chemical weedkillers and pesticides on their vegetables.
Instead, they use natural forms of growing crops in time for their harvest, such as biological pest control and rotation.
However, due to this method of farming being less efficient, it costs more to produce - explaining the higher price of such vegetables.