Evidence that fruits and vegetables prevent cancer has fallen away over the last decade but our national nutrition authorities continue to perpetuate the myth. Why don’t they just review the science and give us the facts?
In 1981 the eminent epidemiologists Richard Doll and Richard Peto wrote a famous paper in which they argued that many cases of cancer were due to environmental factors and were therefore potentially preventable. It was suggested that poor diet was second only to smoking as a cause of cancer and could account for 35% of cases. Based on the evidence available at the time, Doll and Peto suggested that increased intakes of fruits and vegetables may prevent cancer. A generation of nutritionists and dietitians embraced the idea with a passion and started communicating the good news.
Better evidence now available
The early evidence suggesting protective effects of fruits and vegetables against cancer came primarily from case-control studies, with all their well-documented bias. Over the last two decades much better evidence in relation to fruits, vegetables and cancer has become available as large prospective studies and in some cases pooled analyses of these studies have been conducted. Most of the results are negative.
The effect of fruits and vegetables on colon cancer risk was assessed in a pooled analysis of 14 cohort studies, with three quarters of a million subjects. The results – no statistically significant effect for fruits alone, vegetables alone, or fruits and vegetables combined. It was the same story in a pooled analysis of eight cohort studies of fruits, vegetables and breast cancer. And no association was found with prostate cancer or lung cancer.
Image: source
Dietary fibre and colon cancer
A recent meta-analysis appeared to provide a glimmer of hope finding that dietary fibre was protective against colorectal cancer. However, when individual sources of dietary fibre were considered only cereal fibre was associated with protection. No significant associations were observed with intake of fibre from fruit, vegetables or legumes. Wholegrains were found to be protective in the same study suggesting that the emphasis of food-based recommendations for the prevention of colorectal cancer should be on grains rather than fruits and vegetables.
Overall cancer risk: the EPIC Study
The European Prospective Investigation into Cancer and Nutrition (EPIC) is the largest study of diet and health ever undertaken, with over half a million subjects from ten countries. The very different dietary patterns between these European countries provided a good opportunity to assess the role of various foods in cancer prevention. A paper from EPIC on the role of fruit and vegetable consumption in relation to overall cancer risk was recently published. Inverse associations between intake of fruits, vegetables, and fruits plus vegetables combined and cancer risk were observed in this study but in each case the magnitude was tiny, just 1-3% reductions. Both the researchers and commentators noted that these results may be due to residual confounding rather than any real effect of fruits and vegetables.
In the accompanying editorial Harvard researcher Walter Willett concluded In summary, the findings from the EPIC cohort add further evidence that a broad effort to increase consumption of fruits and vegetables will not have a major effect on cancer incidence. Willett’s view is particularly relevant as not only he is eminent in the field, he is also a vegetarian.
In a recent review another prominent vegetarian cancer researcher, Tim Key from Oxford University, concluded For other common cancers, including colorectal, breast and prostate cancer, epidemiological studies suggest little or no association between total fruit and vegetable consumption and risk.
Tim Key and Walter Willett agree that obesity and high alcohol intakes are associated with increased cancer risk and that these should be the focus of efforts to prevent cancer by dietary means.
Response by health authorities
It is interesting to compare and contrast how various health authorities have responded to these developments. In its extensive 2007 report the World Cancer Research Fund downgraded the evidence of a link between the consumption of fruits and vegetables and the risk for cancer. In the Dietary Guidelines for Americans (2010) the whole section on fruits, vegetables and cancer was removed and the topic is now addressed in a single sentence. This is science in action – the evidence changes, so the advice to the general public changes.
And in Australia?
We do things differently in Australia: witness the draft Australian Dietary Guidelines. There was obviously plenty of interest in whether fruits and vegetables may protect against cancer – no fewer than 21 systematic reviews were commissioned. (You may recall that no systematic reviews were conducted in relation to saturated fat, trans fat, glycaemic index or vitamin D). None of the 21 reviews found more than C Grade evidence of a link between fruits, vegetables and cancer.
However, why let a lack of evidence stand in the way of a good yarn? In the draft Australian Dietary Guidelines the section devoted to fruit, vegetables and cancer has doubled in size since the previous edition. Using terms such as ‘evidence suggests’ and ‘emerging evidence’ the Guidelines paint a misleading picture suggesting that scientific support for a protective effect of fruits and vegetables against cancer is actually building, contrary to the findings of the Guidelines’ own literature reviews. Over a page of text is dedicated to mechanisms explaining the protective effect of vegetables, legumes/beans and fruit for some cancers. What protective effect do you imagine the authors are talking about? Truly bizarre.
So in Australia the science may have changed but the message stays the same. Why are our national nutrition authorities attempting to perpetuate a myth rather than distilling the evidence and giving us the facts? If this section is not aligned with the evidence in the final version of the Dietary Guidelines it will mislead all who read it.
Dear Bill et al….I REALLY THANK YOU SINCERELY FOR YOUR LATEST POSTING as it is so ‘close to home’ for myself. I lost my Father and his Mother [my Grandmother]…both to BOWEL CANCER at a young age…..and is why I became a dietitian myself. Your posting helps to solve some of the genetic and diet link as both grew and consumed large amounts of fruit and veg [although rarely raw and usually cooked to death....the fruit peeled [with lots of sugar and fat added]. Both loved and ate only white bread and refined pastries, biscuits and cakes. Both were overweight before being diagnosed. My Father was NEVER SICK…apart from suffering from malaria and gastro bouts that we always put down his war days fighting in New Guinea….but he did suffer from constipation and gastro bouts continually. His only medication ever was Epsom salts and Eno….the favoured medicaiton for his day. Thus your posting is GREAT FOOD FOR THOUGHT AND STIMULATION OF MY OWN BRAIN CELLS. THANK YOU AGAIN!….Pamela
Hello Pamela. Thanks for your comments. I think most of us who have become dietitians and nutritionists did so because we wanted to improve people’s health. But we don’t do anything worthwhile if we tell them the wrong message. In fact, we could do harm – raising false hopes.
Preventing cancer is important and the best way for dietitians to help is by preventing and managing obesity and discouraging high alcohol intake. We thought that fruits and vegetables were part of the story but it turns out that’s just not the case. There are still plenty of good reasons to encourage increased intakes of fruits and vegetables. Regards, Bill
Such as…? (I’m not being sarcastic…I really would like to hear)
Hi Jenna. Fruits and vegetables are rich sources of micronutrients and their intake is associated with lower risk for cardiovascular disease. Do you disagree?
No, I don’t disagree. You’re much better read than I am so I’ll take your word for that one. I just wonder where we place our value on food. If you look at the nutrient profile for the poor fuji apple compared to the NRV’s, it really doesn’t have much to offer. I asked a colleague what’s so healthy about an apple. She said “it’s fresh”. Well, that’s not a good enough reason! “…it’s got a lot of nutrients”. Hardly. “Doesn’t it protect against cancer?”. Apparently not! Even though we suspect (know?) that fibre can protect against colorectal cancer, we can’t show in a large study that more fruit and vegtables assist us to do this. Does that mean we stop telling people about it? When we’re asked, do we say ‘there’s really no evidence for that’?
Hi Jenna. Yes, apples, pears, grapes – full of sugar but low in nutrients. Sounds like a definition of junk food doesn’t it? I guess this is why I argue that we need to be careful about the criteria by which we make judgments about the nutritional quality of carbohydrate-rich foods. There may be various dimensions of goodness that we don’t fully understand yet.
The EPIC study showed fruits and vegetables were associated with protection against heart disease (Crowe FL et al. Eur Heart J 2011;32:1235) but we just don’t know what mechanisms are at play. It might be fibre or polyphenols or potassium or glycaemic index – many fruits have low GIs, or a combination of these. Nutrition doesn’t have much black and white but there are many shades of grey.
In relation to fruits I don’t think their benefits should be overstated – fruits are not ‘superfoods’, just a part of a healthy diet.
My advice to people: ‘No day without fruit’.
I know one person doesn’t evidence maketh!!………..but…………… Just to say I’m also a dietitian. Last year, at 43 I was diagnosed with breast cancer. I lead a blameless existence.loads of fruit and veg, no alcohol, no smoking, slim and do lots of sport. Genes obviously win out with some diseases and whilst I keep an eye on research on different dietary components and recurrence etc, I’ll place 99% of my faith in herceptin and tamoxifen. I agree info should be evidence based. Apart from anything else, it prevents unnecessary blame on the disease ‘afflicted, and keeps peoples minds open to other factors that really can have an effect.
Yes I agree….No matter how good our diet and lifestyle is…our genetics can be our downfall. I guess the upside is that a really good diet/lifstyle that we preach….may give us a ‘few more healthy years’ in the long run. At least we can hope. I really admire your postings Bill and again THANK YOU for sharing your wisdom and research with us….Pamela
Taking on board your comments, Bill, there is another way of looking at things.
We know that certain foodstuffs/drinks may very well contribute to the risk of cancer.
Surely a person who eats a lot of fruit and veggies is on average less likely to ingest/imbibe unhealthy things?
Just my two cents’ worth.
Robin (not a vegetarian!)
Hi Robin
I totally agree with your comment ‘Surely a person who eats a lot of fruit and veggies is on average less likely to ingest/imbibe unhealthy things’.
I’m not arguing that fruit and veg are not important. However, many nutritionists’ views on fruit, veg and cancer were locked in a more decade ago on the basis of low quality studies. Now that better studies are available, we should base our views and advice on these. And they suggest that fruit and veg intake has little effect on cancer risk.
More generally, the links between diet and cancer look pretty weak, with two notable exceptions – alcohol and excessive energy intake leading to obesity. Regards, Bill
I’ve long been annoyed about about the unfounded advice that people get from various sources, including RD’s about the value of fruits and vegetables. Aside from outdated claims about reducing cancer risk, we get the line about “rich in phytonutrients”. True enough. Plants make a lot of chemicals. But many of them have defensive purposes.
Plants do not like getting eaten any more than animals do. We can run from predators. They can’t, but still have to protect themselves. To that end they long ago declared chemical warfare upon animals who would eat them. They’re literally trying to kill us (with toxins & antinutrients), or at least interfere with our reproduction (with phyto-hormones). We evolve defenses against their toxins/hormones. They evolve new ones. It’s the usual evolutionary arms race.
Overall, plants in our diets may (or may not) be beneficial to us. It’s not a given. It’s a question which needs to be studied. But how do you get a research grant to study this when it’s “common knowledge” that fruits and vegetables are the very foundation of health?
Hello Steve. Very valid comments. On your research question, it’s very difficult to get a grant to research something that might contradict accepted wisdom. A few years ago a high ranking US nutrition researcher was addressing the Australian Atheroscerosis Society arguing that low fat diets weren’t beneficial to health and that the focus should be on optimising fat quality. When I spoke to him later he said he had struggled for two decades to get research funding from the NIH: “if it wasn’t about low fat, they just weren’t interested”. Dogma is an big issue, even at these heights. Regards, Bill