Does eating organic food reduce cancer risk?

Not so much, according to this paper published a few days ago in the British Journal of Cancer. Here is what the authors did:

We examined the hypothesis that eating organic food may reduce the risk of soft tissue sarcoma, breast cancer, non-Hodgkin lymphoma and other common cancers in a large prospective study of 623 080 middle-aged UK women. Women reported their consumption of organic food and were followed for cancer incidence over the next 9.3 years.

Here is what they found:

At baseline, 30%, 63% and 7% of women reported never, sometimes, or usually/always eating organic food, respectively. Consumption of organic food was not associated with a reduction in the incidence of all cancer (n=53 769 cases in total) (RR for usually/always vs never=1.03, 95% confidence interval (CI): 0.99–1.07), soft tissue sarcoma (RR=1.37, 95% CI: 0.82–2.27), or breast cancer (RR=1.09, 95% CI: 1.02–1.15), but was associated for non-Hodgkin lymphoma (RR=0.79, 95% CI: 0.65–0.96).

Conclusions: In this large prospective study there was little or no decrease in the incidence of cancer associated with consumption of organic food, except possibly for non-Hodgkin lymphoma.

These findings mesh well with other research I've pointed to in the past noting that food pesticide are a relatively small risk in the grand scheme of things. If the results had been the other way around (that eating organic food reduced cancer risk), I would have pointed out that this is an observational study and that it is really hard to identify causation. For example, maybe people who eat organic engage in all kinds of other healthy activities that reduce cancer risks. Organic consumption is likely correlated with income (given the higher price of organic), and higher income folks are likely to be able to better protect against all kinds of illnesses than poorer folks. That's what I would have said had this study shown a correlation between organic consumption and reduced cancer risk.

Thus, it is only fair play to apply the same thinking to this study which generates a result consistent with arguments I've made in the past. The article finds little to no correlation among people who choose to eat organic and cancer risk. But, maybe people at greater risk for cancer in the first place choose to eat more organics, hoping it will reduce the odds? Maybe people who can't afford to eat organic self-protect in other ways, such as more exercise or eating more fruits and veggies? I don't personally find such explanations for the null result very plausible, but these are the sorts of things one must worry about in observational studies.

To really provide a definitive answer to this question, one needs to do a randomized controlled trial. Or, at a minimum, apply some of the more advanced identification methods and sensitivity analyses that are today being used in the best economics papers (eg, regression discontinuity designs, propensity score matching, model specification comparison, etc). In many ways, it seems to me that much of what I read in epidemiology and nutrition reminds me of the state of the econometrics literature in economics 20 years ago.