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A consumer survey in France found that those who bring organic products to their table suffer less cancer. The debate is reborn, because for other scientists, it is "impossible to prove categorically" this relationship.
A large study published this week showed that the largest consumers of organic foods in France developed fewer cancers than those who had never used them, which illustrates the difficulty of establishing a causal link between diet and health.
It is impossible to categorically prove in the laboratory that this or that food reduces the risk of contracting a disease as complex as cancer.
Researchers need to monitor a large number of people for a period of time and determine which of them are developing cancer, in the hope of documenting the specific behavior of those who become ill a posteriori.
Thousands of studies on diet and various diseases have been conducted for decades. Even the biggest ones are sometimes questioned, like the famous work that showed in 2013 the beneficial effects of the Mediterranean diet against heart disease and the fact that this year has been removed from a prestigious medical journal due to problems methodological.
In the case of organic foods, only one large study had previously evaluated its effect on cancer, the Million Women Study, with 600,000 Britons in 2014.
He found no difference between consumers and non-consumers of biologics in terms of overall cancer risk, but found a reduction in the risk of cancer, non-Hodgkin's lymphoma.
The new French study, involving among others the University of Sorbonne, the National Institute of Agricultural Research and the National Institute of Health and Medical Research, is more detailed, although it has fewer participants, about 69,000, mainly women
Their results were published Monday in the Jama American magazine.
The hypothesis is that consumers of organic products ingest less synthetic pesticides in fruits, vegetables and grains, thus reducing their risks, while some pesticides are suspected to be carcinogens.
After their inclusion, the NutriNet-Health study volunteers completed a questionnaire (income, physical activity, smoking or non-smoking, body mass index, etc.) and declared that a biological food had been eaten in the last 24 hours.
The study divided participants into four groups based on the type of ingested organic food. Then, the number of cancer cases in each group was counted in four and a half years on average.
In a quarter of those who reported consuming a majority of organic foods, the risk of cancer was 25% lower than in the fourth part, which never consumed them. In absolute terms, the increase is only 0.6 percentage points, or six additional patients per 1,000 people.
– "complex" –
The study found a statistically significant correlation only for breast cancer in postmenopausal women and for lymphoma, including non-Hodgkin's lymphoma.
The authors were keen to correct their results, taking into account that consumers of organic products were on average richer, less obese and less smokers.
But other invisible, environmental or lifestyle factors can also have an impact. This is the typical problem of these studies.
"People who deliberately consume organic products, to the point of declaring them, are probably different from others," said Nigel Brockton, director of research at the American Institute for Research Against Cancer (AICR).
This expert recommends, instead of a particular type of food, a set of practices to reduce the risk of cancer: normal weight, physical activity, healthy diet, not too much red meat …
"The diet is a complex thing," he says. "We would never make a recommendation based on a single study, even if it is statistically significant," he adds.
Other aspects of the study were also blamed: pesticide traces were not measured in participants, which sparked Harvard experts' criticism in the same number of Jama. Co-author Julia Baudry told AFP that this was only done in a small subsample.
The declarative aspect of the study is also problematic for John Ioannidis, professor emeritus of medicine at Stanford, who is known to have claimed that most published studies were false.
"Most people, including myself, can not say exactly how much organic food they eat," he told AFP. "The study has a 3% chance of finding something important and 97% spreading absurd and ridiculous results."
As in the case of the consumption of red meat or cigarettes, many other studies in the same direction will be needed to conclude on the effect of organic nutrition.
Meanwhile, the American Cancer Society continues to advocate the consumption of fruits and vegetables, organic or not.
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