Kathryn Bradbury and colleagues in Oxford's Cancer Epidemiology Unit found no evidence that regularly eating a diet that was grown free from pesticides reduced a woman's overall risk of cancer.
The researchers asked around 600,000 women aged 50 or over whether they ate organic foods as part of the Million Women Study. They looked at how many of the women developed 16 of the most common types of cancer in a nine year period following the survey. Around 50,000 women developed cancer in this period.
The scientists found no difference in overall cancer risk when comparing 180,000 women who reported never eating organic food with around 45,000 women who reported usually or always eating organically grown food.
When looking at the results for 16 individual types of cancer they found a small increase in risk for breast cancer but a reduction in the risk for non-Hodgkin lymphoma in women who mostly ate organic food, although these results could be partly due to chance and other factors. The results are published in the British Journal of Cancer.
Study author Professor Tim Key of the University of Oxford said: 'In this large study of middle-aged women in the UK we found no evidence that a woman's overall cancer risk was decreased if she generally ate organic food.'
Pesticides are widely used in agriculture and there are concerns that they could increase the risk of cancer but so far the evidence has not been strong enough to give us any clear answers. Conventionally grown fruit and vegetables sometimes contain very small amounts of pesticides but there is no evidence that these small amounts increase the risk of cancer in people who eat them.
Cancer Research UK's health information manager Dr Claire Knight said: 'This study adds to the evidence that eating organically grown food doesn't lower your overall cancer risk. But if you're anxious about pesticide residues on fruit and vegetables, it's a good idea to wash them before eating.
'Scientists have estimated that over 9% of cancer cases in the UK may be linked to dietary factors, of which almost 5% are linked to not eating enough fruit and vegetables. So eating a well-balanced diet which is high in fruit and vegetables – whether conventionally grown or not – can help reduce your cancer risk.'
The Million Women Study is funded by Cancer Research UK and the Medical Research Council.