Benefits of 'five a day' questioned
“Eating your five-a-day does little to cut cancer risk,” according to the Daily Mail.
The news is based on research that followed half-a-million Europeans for nearly nine years, comparing their diet to their risk of cancer. The results suggest that higher fruit and veg intake offered only a borderline reduction in risk of cancer. However, the research has some limitations. Diet, lifestyle and medical conditions were only assessed at the start of the study, which means that the factors measured may be subject to some inaccuracy and unrecorded changes over time.
The risk of cancer is usually governed by a complex relationship between many factors, such as genetics, lifestyle and medical history. While diet may be involved, the relationship needs further investigation. As the researchers say: “Given the small magnitude of the observed associations, caution should be applied in their interpretation.”
Importantly, the study did not specifically look at the effects of eating 'five-a-day' or examine diet’s effects on other important health outcomes, such as weight gain, diabetes, hypertension or cardiovascular disease.
Where did the story come from?
The study was carried out by Paolo Boffetta and colleagues from Mount Sinai School of Medicine and several other international research centres. The study was funded by the European Commission Directorate General for Health and Consumer Affairs and the International Agency for Research on Cancer. The study was published in the Journal of the National Cancer Institute, a peer-reviewed medical journal.
The newspapers have generally reflected the findings of this research in a balanced way. However, although the five-a-day dietary target has been called into question in all the news headlines, this study did not assess the number of pieces or portions of fruit and vegetables eaten, only the total mass. On this basis, the participants’ total fruit and vegetable intake could technically have been based on only one fruit or vegetable, rather than a variety of different types.
Also, the research and, in turn, newspaper reports have focused on protection against cancer. They did not examine the other types of health benefits that eating a diet rich in fruit and vegetables may provide.
What kind of research was this?
This was a cohort study that assessed the link between total intake of fruit and vegetables and the risk of cancer during an average 8.7 years of follow-up.
A cohort study is generally the best way to assess whether a risk factor is associated with a disease or health outcome. However, it needs to have a reliable way of assessing the exposure (dietary intake) and outcome (cancer development), and to take into account other possible confounding factors that may affect the risk relationship, such as smoking, alcohol or exercise. The cohort also needs to have sufficient duration of follow-up to allow for development of the outcome.
Ideally, this relationship would be assessed through a randomised controlled trial (RCT), where people are randomly assigned a set amount of fruit and vegetables to eat each day. However, such a trial is likely to be unethical, as it would limit how much fruit and vegetables a person could eat, and impractical due to the large number of years that would be required to observe cancer outcomes.
What did the research involve?
This study drew on data from a very large cohort study called the European Prospective Investigation into Cancer and Nutrition (EPIC). The EPIC study was conducted between 1992 and 2000 and recruited 521,448 men and women aged between 25 and 70 from across the UK and the rest of Europe. For the purposes of this subsequent study, the researchers examined 92% of the total cohort (142,605 men and 335,873 women) who did not have cancer at the start of the study and had complete follow-up information, including their dietary intake.
At the start of the study, a country-specific food questionnaire was used to assess food intake over the previous 12 months. Eight per cent of the participants also completed a 24-hour dietary recall assessment. For the purposes of this study, the researchers grouped people into different intake categories of total fruit, total vegetables and total combined fruit and vegetables (all in grams per day). Medical and reproductive history were also assessed, as were lifestyle factors including BMI, education, smoking, alcohol consumption and occupational and leisure physical activity.
Cancer incidence was assessed through population-based registries and health insurance records, with specific methods differing by country. When the researchers assessed the relationship between fruit and vegetable intake and cancer, they adjusted for the influence of the other medical and lifestyle variables that they had assessed.
What were the basic results?
The average intake of total fruit and vegetables across the cohort was 335 g/day, with a generally higher intake in southern European countries compared to northern Europe. Higher intake was also associated with other factors, including higher education and physical activity levels, lower alcohol intake and never smoking. Of their cohort, 9,604 men and 21,000 women were diagnosed with cancer during the follow-up period (incidence rates of 7.9 cases per 1,000 person years in men and 7.1 cases per 1,000 person years in women). Cancer incidence also varied by country.
The adjusted analyses found a borderline reduction in risk of cancer when consuming at least:
- 200 g/day of fruit and vegetables (hazard ratio [HR] 0.97, 95% confidence interval [CI] 0.96 to 0.99)
- 100 g/day of total vegetables (HR 0.98, 95% CI 0.97 to 0.99)
- 100 g/day total fruit (HR 0.99, 95% CI 0.98 to 1.00)
How did the researchers interpret the results?
The researchers concluded that there is a very small inverse association between intake of total fruits and vegetables and cancer risk (in other words, increasing intake weakly reduces cancer risk).
Conclusion
This well-conducted study collected data from a large population across 10 different countries and specifically assessed the effect of fruit and vegetable intake on overall risk of cancer. The authors say that the relationship between diet and incidence of total cancers is less frequently studied than that between diet and individual cancers, and that results in this area have been inconsistent. This particular study found only a borderline reduction in risk of cancer with increased consumption of fruit, vegetables and total fruit and vegetables.
There are several points to highlight when interpreting the results of this research:
- Accurate self-reporting of fruit and vegetable intake over the past 12 months is difficult, particularly when providing an estimate of the weight of food eaten. Intake may also vary over time, and the single measurement taken at the start of the study may not be representative of the participants’ diets in the years preceding the study or over the 8.7 years of follow-up.
- The study followed participants for an average of 8.7 years. This may not be long enough to capture the cancers that may develop, particularly among the younger majority of the cohort.
- The researchers made careful attempts to adjust for possible confounders, including lifestyle and medical factors, but their effects may be difficult to quantify or may vary over time. Other unmeasured factors may also have an effect on the results.
- Although the five-a-day dietary target has been called into question in all of the news headlines, this study did not assess the number of pieces or portions of fruit and vegetables eaten, only the total mass. On the basis of the study report, this could arguably have been made up of only a single fruit or vegetable. Therefore, the focus of this research is on increasing intake of fruit and vegetables and not reaching the five-a-day target, which was not studied here.
As the researchers aptly conclude: “Given the small magnitude of the observed associations, caution should be applied in their interpretation.”
Importantly, the purpose of this study was to specifically examine the effect of increased consumption of fruit and vegetables on cancer risk and not other health outcomes that a balanced diet may potentially provide. Further research will be needed to establish how a diet rich in fruit and vegetables may influence weight gain, diabetes, hypertension and cardiovascular disease.
