Led by Newcastle University in the UK, the international team of experts reports its findings in the British Journal of Nutrition. In what is thought to be the largest study of its kind, the researchers describe how they pooled and analyzed data from 343 studies comparing the compositional differences of organic and conventionally grown fruit, vegetables and cereals.
Researchers say organic foods contain more antioxidants and less pesticide residue than conventionally grown crops.
Study leader Carlo Leifert, professor of ecological agriculture at Newcastle University, says the evidence is "overwhelming," and shows that:
"[...] choosing food produced according to organic standards can lead to increased intake of nutritionally desirable antioxidants and reduced exposure to toxic heavy metals. This constitutes an important addition to the information currently available to consumers which until now has been confusing and in many cases is conflicting."
Prof. Leifert and colleagues say their findings suggest that by switching to organically grown crop foods, and foods made from them, people would consume additional antioxidants equivalent to eating between one and two extra portions of fruit and vegetables per day.
They would also be consuming less cadmium, one of the three metal contaminants - the other two being lead and mercury - for which the European Commission has given maximum permitted levels in food.
In their analysis, the team found cadmium levels were nearly 50% lower in organically grown than conventionally grown crops.
For a crop to qualify as "organic," the grower is not allowed to protect it with synthetic chemicals or feed it with certain mineral fertilizers (for example, nitrogen compounds, potassium chloride and superphosphate). This is in order to reduce environmental impact from nitrates and phosphorous, and to avoid pesticide contamination in groundwater.
Instead, organic crop growers are expected to give regular feeds of organic fertilizers, such as manure and composts, to enrich the nitrogen in the soil by rotating legume crops, and use non-chemical crop protection methods, such as crop rotation, mechanical weeding and biological pest control.
Findings contradict two earlier important studies
The findings contradict those of two important studies - one published in 2009 and the other in 2012 - that have found no substantial differences or nutritional benefits in organic over conventionally produced foods.
The 2009 study - which was commissioned by the UK Food Standards Agency (FSA) - was the first systematic review of the literature on organic food versus non-organic food. Researchers from the London School of Hygiene and Tropical Medicine concluded there was currently no evidence to justify selecting organically over conventionally produced foods on the basis of nutritional superiority.
The 2012 study was also a literature review on organic versus conventionally produced food. There, researchers from Stanford University School of Medicine also concluded that - apart from "weak evidence" of higher phenol levels in organic produce - there was no significant evidence pointing to nutritional benefits linked to the consumption of organic foods.
Prof. Leifert says research on organic versus conventionally produced food has been slow to take off the ground and more studies have been published since these reviews:
"We benefited from a much larger and higher quality set of studies than our colleagues who carried out earlier reviews."
For example, more than half the studies in the Newcastle-led analysis were not available to the team that carried out the 2009 FSA-sponsored study. Prof. Leifert and colleagues also argue that the Stanford study incorporated less than half the number of comparisons for most health-promoting nutrients.
Plus, because of the much larger quantity of data available, they were able to "use more appropriate statistical methods to draw more definitive conclusions regarding the differences between organic and conventional crops," says meta-analysis expert Dr. Gavin Stewart, a lecturer in Evidence Synthesis at Newcastle.
Nutritional benefit of organic food is yet to be studied
The study did not investigate the nutritional benefits of organic food, it compared the composition of organic food against conventionally grown food and found:
- Concentrations of antioxidants such as polyphenolics were between 18-69% higher in organically grown crops.
- Organic crops contained, on average, 48% lower levels of the toxic heavy metal cadmium.
- Concentrations of total nitrogen were 10% lower, nitrate 30% lower and nitrite 87% lower in organic compared with conventional crops.
- Pesticide residues were four times more likely to be found in conventional crops than organic ones.
Prof. Leifert says while the findings show without doubt there are compositional differences between organic and non-organic crops, there is now an urgent need for well-controlled human studies that pinpoint and quantify the health impact of switching to organic food.
Critical reaction says 'the evidence is unconvincing'
Meanwhile, there has been a critical reaction from some experts about the new findings. One of these is Dr. Alan Dangour, reader in Food and Nutrition for Global Health at the London School of Hygiene & Tropical Medicine, and first author of the 2009 FSA paper.
Dr. Dangour says the new review mixes good quality data with bad quality data in a way that is "highly problematic" and in his view significantly weakens the researchers' conclusions.In an "expert reaction" published by Science Media Centre, he also says the new study has overstated the significance of its findings, because there is no good evidence to support the idea that consuming more antioxidants and polyphenols has important benefits for public health, and there is no robust evidence that having more of them in the human diet reduces the risk of cardiovascular disease, cancer and other chronic conditions.
"All natural products vary in their composition for a wide variety of reasons," says Dr. Dangour, adding that the paper "provides no convincing evidence to refute our earlier finding," which he says were fully supported by the 2012 Stanford study, that there are "no important differences in nutrient content between organically and conventionally produced foods."
The study was funded jointly by the European Framework 6 programme and the Sheepdrove Trust.