Organic food: does eating it cut cancer risk?

May 28, 2014 at 1:16 pm | Posted in Feature Articles | 1 Comment
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Organic food: does eating
it cut cancer risk?

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A recent study published in the British Journal of Cancer (BJC) found that, in a group of over 600,000 British middle-aged women followed for just over 9 years, no decrease in cancer risk could be observed between women who described themselves as “always” or “usually” eating organic food, and women who described themselves as “never” eating organic food (Bradbury et al. 2014).

The finding was picked up by almost all major media outlets in the UK, usually under headlines along the lines that eating organic does nothing to reduce cancer risk among women (the Daily Mail) or that women who eat organic food are no less likely to develop cancer than women who do not (the Guardian and the Telegraph).

Three people were quoted in the media coverage: Professor Tim Key of Oxford University, one of the study authors; Dr Claire Knight, Cancer Research UK’s health information manager (CRUK funded the research and owns the British Journal of Cancer); and Peter Melchett, Policy Director of the Soil Association, the UK’s largest body promoting organic agriculture. Key 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.” Knight said: “This study adds to the evidence that eating organically grown food doesn’t lower your overall cancer risk.”

Melchett was critical of the study, suggesting that the researchers were too quick to dismiss an observed 21% decrease in non-Hodgkin’s lymphoma among women who ate organic food, pointed out that reasons for eating organic extend beyond cancer risk to bee protection and prohibition of genetically modified organisms, and also that CRUK’s advice that people wash vegetables if they were concerned about pesticide exposure would not work for systemic pesticides or pesticides in processed foods such as bread.

It would be reasonable to assume that Melchett’s response comes from concern that people reading about the study might be discouraged from eating organic food. The Cancer Prevention and Education Society (publisher of Health & Environment) also recommends that people eat an organic diet, or at least limit as far as possible their exposure to pesticides. We are therefore going to look at exactly what can be concluded from the study and consider whether or not this supports any sort of a case against the purchase and consumption of organic food.

What the study did

Historically, researchers investigating the prospective health benefits of an organic diet have mainly examined either the consequences of an organic diet on levels of pesticides in people’s bodies or differences in nutritional value between conventional and organic produce.

A systematic review of this data conducted in 2012 concluded that while there was good evidence that an organic diet reduces pesticide exposure, there was a lack of compelling evidence of increased nutritional value in organic food (Smith-Spangler et al. 2012): only three studies had examined clinical outcomes in relation to organic diet, finding no significant differences between incidence of allergic reactions or food poisoning in relation to conventional or organic diets.

This leaves a substantial research gap in terms of whether or not the reduced exposure to pesticides resulting from an organic diet translates into a measurable health benefit. It is to this which the BJC study was designed to contribute.

The BJC study is an analysis of data generated by the Million Women Study, a national investigation of the health of UK women over the age of 50. The Million Women Study periodically collects data on the health and lifestyles of over a million women volunteers in order to uncover connections between reproductive and lifestyle factors and women’s health (Million Women Study, 2013). The variety of questions being asked (see for example the third questionnaire, sent to participants 8 years after recruitment) means the study is large set of health and lifestyle data for analysis in prospective epidemiological research.

One of the questions in the surveys concerns the frequency with which the respondent eats organic food (possible responses being “never”, “sometimes”, “usually” or “always”). This allows a rough distinction to be made between people who eat more organic food and those who never eat it. When this is set against the health records of the respondents, in theory it enables researchers to observe differences in health outcomes between the group which eats only a conventional diet (which we shall call the “never-eats”) against the group which eats a larger proportion of organic food (the “usually/always-eats”).

Question 50 of Survey #3 in the Million Women Study.

Question 50 of Survey #3 in the Million Women Study.

What the study found

The study found that 30% of the women surveyed say they “never” eat organic, 63% “sometimes” do, and 7% either “usually” or “always” eat organic (the “usually” and “always” groups were combined throughout the analysis, so we do not know from the study what percentage stated they “always” eat organic).

The study found that the usually/always-eats were almost exactly as likely to get cancer as the never-eats: the relative risk (RR) of cancer for the usually/always-eats was 1.03 with a 95% confidence interval (CI) spanning 0.99 to 1.07. Additional analyses showed that usually/always-eats were a bit more likely to get breast cancer (RR of 10.9, CI: 1.02-1.15) than the never-eats and somewhat less likely to get non-Hodgkin’s lymphoma, although by exactly how much it was difficult to say (RR of 0.79, CI: 0.65-0.96).

The authors state that the results for non-Hodgkin’s lymphoma and breast cancer both need to be interpreted cautiously. Since the study looked at a large number of cancers, it is much more likely that any individually significant results are the result of chance rather than reflective of a real difference in cancer risk between the two groups. (This is because the researchers were looking for relationships which had a better than 95% chance of being true; because they did this may times over, they greatly increased the probability that any of the individual relationships they observed would be spurious. Rather like a 20-sided dice, if you rolled it once and got a 20 you might think something significant had happened, but if you rolled it 20 times and got a 20, you should be less likely to think something significant had happened – though you could not rule out that it had.)

Discussion points

The two issues worth discussing in detail are whether or not the difference in organic food consumption between the never-eats and the usually/always-eats is great enough to allow the study to detect a subsequent difference in cancer risk between the two groups, and the extent to which a study of middle-aged women allows inferences to be drawn about the putative health benefits of eating organic.

How often is “usually”? It is not clear from the study what it is the respondents mean when they say they “usually” eat organic. Does that mean 20% of their diet is organic? That they eat exclusively organic vegetables but not organic meat or dairy products? That 80% of their diet is organic? That they do not know exactly how often they eat organic but they usually try to do so? And so forth.

It is also not clear how long before the start-point of the study that participants had been eating organic diets. Given that cancers resulting from chronic exposure to low levels of environmental pollutants can take a long time to emerge, it could be the case that 10 years of eating organic since the age of 45 (which would mean participants starting a usually-organic diet in around 1993, when this was much more difficult than 10 years later) may be of small significance compared to eating only conventional food up to the age of 45.

The problem is, the less often the usually/always group actually eats organic, the more similar they are in pesticide exposure to the never group. And the closer they get, the more difficult it becomes for the study to observe subsequent differences in health outcomes – particularly if the effect being studied is small, as the contribution of organic diet to reduced cancer risk may be.

The data would be easier to interpret if an always-eats group was compared to the never-eats group. The likelihood is that the always group is too small to be usable (this would explain why the usually and always groups were merged). Hard data from blood samples taken from the respondents would allow the assumed difference between the never-eats and the usually/always-eats to be validated but this was not available to the researchers.

Without being able to get a fix on the real difference in diet between the two groups which are being compared, it is very difficult to get a sense of the likelihood that the study design was either unable to detect a real effect or correctly observed that there is no real difference in cancer incidence between the two groups. Since the difference in lifetime diet between the two groups could in fact be negligible, the possibility that this study in fact yields a false negative (that it finds nothing when in reality there is an effect) needs to be considered.

Relevance of the study group. The study only looked at middle-aged women. This group may not be particularly susceptible to the cancers most likely to be prevented by an organic-only diet – instead, men of a different age may be at greater risk, or children of both sexes, and so forth. Sub-groups in the study, such as vegetarians, may see a particular benefit in an organic diet (the study did not look at this).

Cancer is also only one health end-point among many which could be influenced by pesticide exposure; of these, the vulnerability of children to neurotoxic effects of pesticides is of particular interest (Young 2014). These considerations do not invalidate the study but caution should be exercised in drawing general about health lack of health benefits of an organic diet from the findings described here.

What do we conclude?

From a research perspective, this is a well-conducted study which is unique for looking at cancer risk in relation to consumption of organic food in the diet. It did not really find anything. It is difficult to judge if this is either because there was no significant effect to be found, or because the effect of eating organic food is too small to be detected, or instead because there was a lack of a real difference between the two groups being compared.

Therefore, it is difficult to draw a conclusion which goes further than saying that women who eat an unclear amount more of organic food for an unclear period of time (potentially starting quite late in life) see no benefit in terms of reduced cancer risk. As such, this study adds a little but not much to the evidence that there is no benefit to eating organic.

If there really is no actual benefit to eating organic in this group of women, it has to be remembered that a beneficial effect may still exist for other population groups or sub-groups in the study. Perhaps more importantly, dietary pesticides may affect other health end-points to a greater degree than they do cancer, with middle-aged women not necessarily being the part of the population most at risk of this harm.

The major limitation of the study, at least in this author’s opinion, is the uncertainty around difference in diet between the never-eats and usually/always-eats groups. Even if this was obscuring only a small beneficial effect, for a disease as common as cancer this could have important consequences for cancer prevention strategy. If, as may be the case, there is no significant exposure difference between the groups at all, even a fairly large beneficial effect could be obscured in this study design.


Future epidemiological studies should include biological sampling in order to give a clearer picture of chemical exposures in relation to diet and disease risks. If we are interested specifically in quantifying the health benefits of an organic diet, then epidemiology may well be ruled out as a useful tool: the authors state that confirming whether or not an organic diet reduces risk of non-Hodgkin’s lymphoma could require a 10-year study with 500,000 participants. It might be more realistic to study this in an animal model instead, if it was felt that the resulting data would provide sufficient ethical justification for conducting such research.

Even if that data were still to be inconclusive or show an absence of benefit, there are still other reasons for eating organic. The possibility that pesticides may play a role in increased incidence of neurodegenerative disorders in farm workers (van Maele-Fabry et al. 2012, Allen & Levy 2013) along with the acute hazards their use presents, means that pesticides pose some risk even if that is not transmitted down to the consumer. There is also the argument that organic farming practices are more sustainable, yielding more robust and varied agricultural ecosystems. It would therefore be a mistake to put too much weight on this particular study in building a case against the consumption of organic foods.

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  1. […] 1. Organic food: does eating it cut cancer risk? May 28, 2014 at 1:16 pm | Posted in H&E Features, […]

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