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A new, very detailed study showing that the most greedy consumers of organic foods have fewer cancers than those who never consume them illustrates the difficulty of establishing a cause-and-effect relationship to evaluate diet. food and health.
It is indeed impossible to prove beyond a doubt, in a laboratory, that a given food reduces the risk of developing a disease as complex as cancer.
"The diet is complex," AFP Nigel Brockton, director of research at the American Institute for Cancer Research (AICR), told AFP.
"We would never make a recommendation based on a study, even if it is statistically significant."
The researchers, along with the French team behind Monday's study, must then follow an important test group and wait for the onset of cancers in some subjects.
They then hope that, after the fact, they will be able to isolate a specific behavior among all those who are sick and who made the difference.
Thousands of studies on diet and disease have been conducted for decades.
Even the most important conclusions are sometimes disputed, like the one from 2013 that would have shown the considerable benefits of the so-called Mediterranean diet in the fight against heart disease.
This study was removed from the prestigious New England Journal of Medicine earlier this year after criticizing the methods used.
Only one major study on the link between organic foods and cancer had been performed before the latest effort published in the Journal of Internal Medicine of the American Medical Association (JAMA).
This 2014 research, known as the Million Women Study, used a test group of 600,000 Britons. There was no overall difference in cancer risk between those who ate organic foods and those who did not eat them.
He only found that lovers of organic foods were less likely to develop non-Hodgkin's lymphoma.
– Questionnaires and self-declarations –
So, how do we approach the new study of the French team?
It is certainly more detailed than the Million Women study, although it covered 69,000 women – about 10% of the sample size.
The hypothesis is that organic food lovers consume fewer pesticides in their fruits, vegetables and grains, which reduces their risk of cancer, as some pesticides are suspected to be carcinogens.
After being recruited for the NutriNet-Sante study, volunteers completed a questionnaire on various topics (income, physical activity, smoking habits, body mass index, etc.).
They also reported three times the amount of organic food that they had eaten during the previous 24-hour period.
The researchers separated the participants into four groups, based on their consumption of organic foods. They then counted the number of cancers in each group, over an average period of four and a half years.
In a quarter of people who reported consuming the most organic products, the risk of cancer was 25% lower than the quarter that had never eaten organic food.
In absolute terms, this translates into an increase in cancer incidence of 0.6 percentage points – an additional six out of every 1,000 patients.
– & # 39; One study at a time & # 39; –
The only statistically significant correlations were a reduction in the number of breast cancers in postmenopausal women and a sharp decline in the incidence of lymphoma.
The authors of the study took care to correct their results to reflect the fact that organic food consumers were on average richer, less obese and smoked less than those who did not consume them. .
But other invisible factors, whether environmental or lifestyle-related, could also have played a role – the usual problem of diet and exercise studies.
"People who deliberately consume organic food, to the point of reporting it, are probably different in many ways," noted Brockton.
The AICR suggests a range of behaviors designed to reduce the risk of cancer – maintaining a healthy weight, exercise, healthy diet, not too much red meat – but does not recommend a specific type of food .
Monday's study also raised issues: traces of pesticides in the subjects were not measured, which sparked criticism from Harvard University's experts in the same issue of JAMA, which called for caution.
Julia Baudry, co-author of the study, told AFP that such measures were only taken for a small group of subsamples.
John Ioannidis, Professor of Disease Prevention at Stanford University, acknowledged for stating that most published studies were untrue, said that self declaration could be a problem in this case.
"Most people, including myself (Professor of Disease Prevention), would not be able to accurately say if I eat organic foods and how much / how often," Ioannidis told AFP. AFP.
"The study has a three per cent chance of finding something important and 97 per cent spreading silly nonsense."
For Brockton, "research advances from one study at a time".
"When we observe very consistent observations or associations with elements such as alcohol, red meat and body weight, while many studies show these phenomena repeatedly, within different populations, we have a much better confidence, "he said.
In the meantime, the American Cancer Society is urging people to eat more fruits and vegetables, whether organic or not.
© 2018 AFP
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