Tomatoes contain compounds which are good for the heart
Organic fruit and vegetables may be better for you than conventionally grown crops, US research suggests.
A ten-year study comparing organic tomatoes with standard produce found almost double the level of flavonoids - a type of antioxidant.
Flavonoids have been shown to reduce high blood pressure, lowering the risk of heart disease and stroke.
Writing in the Journal of Agricultural and Food Chemistry, the team said nitrogen in the soil may be the key.
Dr Alyson Mitchell, a food chemist at the University of California, and colleagues measured the amount of two flavonoids - quercetin and kaempferol - in dried tomato samples that had been collected as part of a long-term study on agricultural methods.
These findings also confirm recent European research, which showed that organic tomatoes, peaches and processed apples all have higher nutritional quality than non-organic
Peter Melchett, Soil Association
They found that on average they were 79% and 97% higher respectively in the organic tomatoes than in the conventionally grown fruit.
New Scientist magazine reported that the different levels of flavonoids in tomatoes are probably due to the absence of fertilisers in organic farming.
Flavonoids are produced as a defence mechanism that can be triggered by nutrient deficiency, such as a lack of nitrogen in the soil.
The inorganic nitrogen in conventional fertiliser is easily available to plants and so, the researchers suggests, the lower levels of flavonoids are probably caused by over-fertilisation.
Flavonoids have also been linked with reduced rates of some types of cancer and dementia.
The Food Standards Agency says there is some evidence that flavonoids can help to reduce the risk of cardiovascular disease and they are currently carrying out a study to look at the health benefits in more detail.
However, a spokesperson said there was no evidence that organic food was healthier.
"Our long-standing advice on organic food is there can be some nutrient differences but it doesn't mean it's necessarily better for you."
For example, a recent study found that organic milk had higher levels of omega-3 fatty acids, but the FSA points out that these short-chain fatty acids do not seem to have the health promoting benefits offered by long-chain omega-3 oils found in oily fish.
Peter Melchett, Soil Association policy director said: "We welcome the now rapidly growing body of evidence which shows significant differences between the nutritional composition of organic and non-organic food.
"This is the second recent American study to find significant differences between organic and non-organic fruit.
"These findings also confirm recent European research, which showed that organic tomatoes, peaches and processed apples all have higher nutritional quality than non-organic."
"As further scientific evidence emerges from new research looking at differences between organic and non-organic food, the Soil Association will be asking the FSA to keep their nutritional advice to consumers under review."