Fruit and vegetables are good for general health but have little effect on cancer

Fruit and vegetables may not prevent cancer but are still good for overall health

Back in the 1990s, the evidence that fruit and veg played an important role in protecting against cancer seemed compelling. But as time has moved on, so has the science.

A new review published this week casts renewed doubt on the role of fruit and vegetables. Looking at the evidence overall, including the results of several long-term studies on fruit and veg, the review concludes that cutting down drinking and keeping a healthy bodyweight are far more important in protecting against the disease rather than eating more fruit and veg.

So what exactly does the new review show?

The author, Cancer Research UK’s Professor Tim Key, reviewed large, high-quality studies of the effect of fruit and vegetables on cancer risk published over the last 10 years.

He delved into the evidence about cancers of the mouth, throat, stomach, bowel, lung, breast, prostate, and overall cancer risk to assess the strength of the findings.

He focused particularly on four large, recent, good-quality studies. These studies all looked at how eating fruit and vegetables affects overall cancer risk – and yet only one found hard evidence of a link – and even then the effect was tiny. The team behind this study, from the European Prospective Investigation into Cancer (EPIC), concluded that it could have been a chance finding, caused by the way their research was conducted.  We blogged about this last April.

In the new review, Professor Key went on to examine the effect of fruit and vegetables on risk of different types of cancer. He concluded that for breast, prostate and stomach cancers there was little or no link. For lung and bowel cancer the evidence was weak and inconsistent.

The evidence was strongest in the case of cancers of the mouth and throat, but the effects were relatively small compared to the effects of smoking and drinking – the main causes of these cancers. But people who smoke and drink heavily generally eat fewer fruits and vegetables.

Professor Key argues that what these studies actually measured wasn’t the effects of a lack of fruit and veg, but the effects of too much alcohol and tobacco. Large studies like EPIC try to make allowances for these things, but when the effects of smoking and drinking are so big, it can be hard to fully adjust for them. This is known as “residual confounding”.

Overall, the new review suggests  that intake of fruit and vegetables does not have a significant impact on overall cancer risk. As Professor Key writes,

It is unlikely that fruit and vegetables have a ‘broad spectrum’ protective effect against cancer, and likely that some of the associations observed for particular cancer sites are simply due to confounding, particularly by smoking.

This conclusion implies that, at least in relatively well-nourished westernised populations, a general increase in total fruit and vegetable intake will not have a large impact on cancer rates.

How does the new study change things?

General recommendations for people to eat plenty of fruit and vegetables will not, and should not, change in response to this study. There is plenty of evidence that fruits and vegetables play an important role in reducing the risk of high blood pressure and coronary heart disease. Also, fruit and vegetables contain many of the nutrients needed for general health.

Even in terms of cancer, this study isn’t a big shift. Much of the original evidence linking fruit and vegetables to a lower cancer risk came from “case-control studies”. These studies ask people with cancer and healthy volunteers (controls) to remember their diets over previous years. They can all too easily be influenced by people’s beliefs about cancer affecting what they recall.

Over the last decade, these have been all but replaced by very large “cohort studies”, like those reviewed by Professor Key. These recruit healthy individuals and ask them to record their diet and lifestyle before cancer develops. These have found that fruit and vegetables are only associated with lower risks of mouth and throat cancers, and as Professor Key says, it’s not clear whether these protective effects are real.

But there are still many unanswered questions. It may be that certain nutrients found in fruits and vegetables do affect cancer risk, but that the effect is restricted to certain groups of people or certain types of fruits or vegetable. This could explain why these effects haven’t been seen in existing studies. Professor Key points out that more research into the basic nuts and bolts of how cancer develops could yield clues in this direction.

This won’t be the end of research into fruit and vegetables and cancer. But scientists may now need to consider taking a different tack to unravel any small but significant effects. In the meantime, efforts to address tobacco use, obesity and alcohol have a far greater potential to reduce cancer rates.  Or, as Professor Key concludes in the paper,

Current advice in relation to diet and cancer should include the recommendation to consume adequate amounts of fruit and vegetables, but should put most emphasis on the well-established adverse effects of obesity and high alcohol intakes.

Hazel

Further reading

From Healthy Living:

• Diet and healthy eating
• Obesity and weight
• Alcohol and cancer

From CancerStats:

• Statistics about diet, alcohol and cancer

Reference:

Key, T. (2010). Fruit and vegetables and cancer risk British Journal of Cancer DOI: 10.1038/sj.bjc.6606032