Fruit and vegetable intake was not associated with risk of total cancer in this prospective cohort of women in Sweden. This finding is not consistent with the recent multicentre analysis within the EPIC cohort study [8
] that reported a weak, but statistically significant, inverse association between fruit and vegetable intake and overall cancer risk [8
]. However, their analysis included nearly 30,000 cases (21,000 female ones), and it is the second largest study that has evaluated fruit and vegetable intake in relation to total cancer incidence [8
]. Thus, they had a substantially higher power than this cohort. If the true magnitude of the overall cancer protective effect of fruits and vegetables is modest, our study may not have had the statistical power to detect this; however, our results are in line with analyses within four other cohorts [4
The Greek component of the EPIC cohort study reported an inverse association between higher intake of fruits and vegetables and cancer incidence in women. That analysis relied on only 430 cases of cancer among women [3
]. In the Greek cohort, the median intake of fruits and vegetables (including legumes) was 837 g/day [3
]. This figure is much higher than in our cohort (median intake 204 g/day), as well as in earlier prospective cohorts [4
]. Hence, there may be a biological threshold for the cancer protective effect of fruits and vegetables. Furthermore, anticancer effects may also differ between types of fruits and vegetables, and the distribution of intake likely varies between the populations. The Greek diet is characterised by higher intake of legumes than US and Northern European diets, and it also likely consists of more fresh seasonal vegetables and fruits. In this study, we did not find any evidence for that the low fruit and vegetable intake in this population were harmful in terms of increased overall cancer risk. In this context, it is relevant to note that the multicentre EPIC analysis could not detect statistically significant heterogeneity among countries, although absolute intake between the investigated countries varied greatly (with higher intake in southern European countries such as Greece and lower intake in northern European countries such as Sweden) [8
In our study, we assessed fruit and vegetable consumption only once, entailing misclassification among those who changed their dietary pattern during follow-up. Because it is likely to be non-differential, such misclassification attenuates the strength of any true association. However, there is a potential risk that the degree of this misclassification increases over time, because women change their dietary habits. Thus, the longer the follow-up, the more attenuated the association might be. However, for 0–8 years follow-up, the results were similar as for the complete follow-up (0–14 years), and thus no evidence for such an attenuation was observed.
Both EPIC and the National Institutes of Health-AARP cohorts assessed residual confounding due to smoking [4
]. In this study, the results of the restricted analysis to never-smokers argue against substantial residual confounding by smoking.
Our study has some limitations. Consumption of fruits and vegetables might be underestimated in our study, because intake of only 10 vegetables and four fruits was assessed in our FFQ. However, the estimated fruit and vegetable intake in our cohort is similar to that in the Swedish component of the EPIC cohort [8
], another Swedish cohort [20
] as well as in a nationwide survey from 1997 [18
]. The latter used 7-day food diaries in order to assess dietary intake. These diaries are considered to be superior to FFQs in assessing actual current levels of intake, and they often produce slightly higher estimates. There is also a risk that fruit and vegetable intake may be overestimated since these foods are considered healthy, but considering the low intake overall and that the intake agreed with other dietary data in Swedish women [8
], it is less likely. Misclassification of fruit and vegetable intake due to measurement error (whether underestimation or overestimation) associated with the food frequency questionnaire is unavoidable, but, given the prospective study design, it is likely to be non-differential and thus could attenuate any true association. Another limitation is that our study was too small to separately examine the relationship between fruit and vegetable intake and cancer at specific sites. Strengths of our study include its prospective design, relatively large size and complete follow-up. Cancer registration in Sweden is mandatory, making ascertainment of incident cases virtually complete. Furthermore, we were able to adjust for possible confounding variables such as smoking, alcohol and multivitamin supplement use.
Almost half of all cancers in our cohort were breast cancer, while the prevalence of colorectal cancer was low (5%). Our results are reasonable considering the conclusions in the recent World Cancer Research Fund (WCRF) report [2
] that the evidence for a protective effect of fruits and vegetables is probable for colorectal cancer and limited for breast cancer. Hence, with the prevailing distribution of cancer sites and types among young and middle-aged women in Sweden and many other western countries, the results from this and most other prospective cohorts do not suggest that increased fruit and vegetable intake reduces overall cancer risk in this segment of the population. Still, our results are relevant in the context of understanding cancer aetiology. Moreover, they do not contradict current public health recommendations, because high intake of fruits and vegetables may protect from specific types of cancer, mostly in the gastrointestinal tract [2
], as well as from cardiovascular disease [5
]. Furthermore, fruits and vegetables are important sources of dietary fibre, essential vitamins and minerals and facilitate, due to their low energy content, the maintenance of a healthy body weight which is conducive to a lower overall cancer incidence [21
In conclusion, fruit and vegetable intake was not associated with risk of total cancer in this prospective cohort of Swedish women.