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Fruit and vegetables have a small protective effect against cancer

Today’s headlines are loudly proclaiming that the message to eat five daily portions of fruit and vegetables isn’t all it’s cracked up to be. “Fruit and vegetables have little effect on cancer risk,” says the Guardian. “Five fruit and veg a day does not significantly reduce cancer risk,” says the Telegraph.

These claims are based on a new report from the largest ever study of diet and cancer. Here, we look at what the paper actually showed, how this fits in with the history of the five-a-day message and, most importantly, whether it means you should still try to eat plenty of fruit and veg.

What did the study show?

The latest paper comes from the EPIC study (the European Prospective Investigation into Cancer and Nutrition), the largest study ever on diet and cancer, part-funded by Cancer Research UK. EPIC looks at around half a million people from a variety of European countries. With its large size and the varied diet of its volunteers, it’s well-placed to look at potential links between the things we eat and our risk of cancer.

The study found that eating 200g of fruit and vegetables a day (2 and a half portions) was associated with a 3 per cent lower risk of cancer. Compared to people who ate less than 2.5 portions a day, those who got their five-a-day had 9 per cent lower risk of cancer. And the heaviest fruit-and-veg-eaters, who were eating more than 8 portions a day, had 11 per cent lower risk.

All of these results were adjusted for other things that can affect the risk of cancer, including smoking, alcohol, use of HRT, physical activity, social status, body weight, meat, fibre and more. These statistical adjustments are done so that, as far as possible, the statistics should reflect the effects of fruit and vegetables alone.

The study also found a statistically significant “dose-response effect”. This means that the more portions people ate, the lower their risk of cancer.

If the study’s results are to be believed, the authors calculate that if everyone ate two more portions a day on average, then 2.6 per cent of cancers in men could be avoided, as could 2.3 per cent of cancers in women. In the UK, this equates to around 7,200 cases of cancer every year.

Does this mean that experts were wrong?

Not quite. It actually fits very well with our current understanding of diet and cancer and how that understanding has changed over the last decade. Unfortunately, the headlines haven’t quite caught up with the science.

If you look back to the late 90s, fruit and vegetables were often touted as the big thing in cancer prevention. In 1997, the World Cancer Research Fund claimed that there was “convincing” evidence that fruit and vegetables reduce the risk of mouth, oesophageal, lung, stomach and bowel cancers.

In 2003, the World Health Organisation’s World Cancer Report said that the protective effect of fruit and vegetables against cancer was “the most consistent finding on diet as a determinant of cancer risk”. At that stage, around 80 per cent of the 250 or so existing studies found that eating more fruit and vegetables was associated with a significantly lower risk of cancer.

However, most of this evidence came from ‘case-control studies’. In these studies, cancer patients (cases) are compared to healthy people (controls) and asked to remember their choices and habits many years back to see if these affected their odds of developing their disease.

These studies all have basic problems. People might give inaccurate answers. There may be important differences between the cases and controls, especially in terms of other aspects of their health. And the studies might not adjust for other things that could cause cancer, such as whether the volunteers smoked or drank alcohol.

Since then, far larger ‘cohort studies’ have been published. These are generally a more convincing source of evidence. They follow a large group of healthy people, ask them questions about their lifestyles and wait to see which of them develop cancer or other diseases. In collecting information here and now, these studies are less vulnerable to the distorting effects of faulty memories. EPIC is one of these studies, and one of the largest ever done.

The results from cohort studies have been less charitable to the protective effects of fruit and vegetables. The long list of cancers that these foods supposedly protect against has shrunk. However, even cohort studies have found that people who eat the most fruit and vegetables have a lower risk of certain cancers, especially those of the digestive system – mouth, oesophageal and stomach cancers.

These changes are reflected in the latest assessments of the evidence. The WCRF’s second expert report, published a decade after the first, downgraded the evidence for mouth, oesophageal and lung cancers to “probable” and that for bowel cancer to “limited”. The report claimed that recent studies have made the overall evidence “less impressive” and “less compelling”.

So the results from the latest EPIC paper aren’t all that surprising. If fruit and vegetables only really reduce the risk of a handful of cancer types, then you wouldn’t expect to see much of an impact against cancer overall (remember that cancer is a collection of over 200 diseases).

Indeed, previous EPIC publications have reported that fruit and vegetables could reduce the risk of cancers of the mouth, larynx, oesophagus, bowel and lung, but they have no effect against breast, prostate or ovarian cancers, or lymphoma. And the similarly large NIH-AARP study found a link between eating lots of fruit and veg and a lower risk of head and neck cancers.

So should we still get our five-a-day?

Yes. Even if fruit and vegetables have a small protective effect against cancer, that’s still an important contribution. Remember that this study found a small benefit against cancer overall, but EPIC has previously documented more substantial benefits against specific types of cancer.

The problem comes when five-a-day is sold as the be-all-and-end-all of healthy eating. It’s not – it’s just part of a larger package of healthy lifestyle choices. Certainly, in terms of your diet, there are very few things that make a big impact on cancer risk on their own. It’s far more important to eat a healthy, balanced diet, where the impact of each individual food, while small, can add up.

This is another reason why superfood stories in the press are so misleading. They paint a picture of individual fruits and vegetables as the ultimate answer to the tricky issue of cancer prevention. However, as we’ve seen above, this evidence often hails from weak studies.

There are also other reasons to eat fruit and vegetables other than to protect yourself against cancer. For a start, they’re not harmful; there’s no real flipside to consider.

They could indirectly help to reduce the risk of cancer. They could help people to maintain a healthy body weight by taking the place of more energy-dense foods in our diet. They’re also a good source of fibre, and the EPIC study has shown that eating lots of fibre is associated with a lower risk of bowel cancer. The link between bodyweight and cancer is firmly established and based on a large body of consistent evidence.

Finally, they could help to reduce the risk of other diseases, such as heart disease. Indeed, the Spanish arm of the EPIC study reported that that people who ate the most fruit and vegetables were around a quarter less likely to die of chronic diseases at a given age, than people who ate very little.

-Ed

Reference: Boffetta, P., Couto, E., Wichmann, J., Ferrari, P., Trichopoulos, D., Bueno-de-Mesquita, H., van Duijnhoven, F., Buchner, F., Key, T., Boeing, H., Nothlings, U., Linseisen, J., Gonzalez, C., Overvad, K., Nielsen, M., Tjonneland, A., Olsen, A., Clavel-Chapelon, F., Boutron-Ruault, M., Morois, S., Lagiou, P., Naska, A., Benetou, V., Kaaks, R., Rohrmann, S., Panico, S., Sieri, S., Vineis, P., Palli, D., van Gils, C., Peeters, P., Lund, E., Brustad, M., Engeset, D., Huerta, J., Rodriguez, L., Sanchez, M., Dorronsoro, M., Barricarte, A., Hallmans, G., Johansson, I., Manjer, J., Sonestedt, E., Allen, N., Bingham, S., Khaw, K., Slimani, N., Jenab, M., Mouw, T., Norat, T., Riboli, E., & Trichopoulou, A. (2010). Fruit and Vegetable Intake and Overall Cancer Risk in the European Prospective Investigation Into Cancer and Nutrition (EPIC) JNCI Journal of the National Cancer Institute DOI: 10.1093/jnci/djq072

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Comments

Keith Dancey June 1, 2010

My wife has not smoked or drunk alcohol for over thirty years, dislikes sweet food, loves eating lots of vegetables, keeps slim and active, yet five years ago she developed breast cancer, which she beat. Now she has just been diagnosed with incurable advanced kidney cancer.

People who think eating “five-a-day” will prevent cancer are just kidding themselves: the evidence is non-existent for MOST cancers, and – at best – slim for a few others.