Guest Post: Paul Appleby on Fruits, Vegetables, & Cancer

On April 7, I linked to an NPR report about a European Prospective Investigation into Cancer (EPIC) paper that found that eating more fruits and vegetables cuts the risk of cancer by only 4%. After reading his comments elsewhere, I invited Paul Appleby, Senior Statistician at the Cancer Epidemiology Unit at the University of Oxford, to share his thoughts on the paper.

Paul writes…

Do fruits and vegetables prevent cancer? No. Does eating more fruits and vegetables reduce the risk of some cancers? Probably yes. To quote from the WCRF/AICR expert report on “Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective” (2007), “The strongest evidence, here corresponding to judgments of ‘probable’, shows that non-starchy vegetables and also fruits probably protect against cancers of the mouth, larynx, pharynx, oesophagus, and stomach, and that fruits also probably protect against lung cancer; and that allium vegetables, and garlic specifically, probably protect against stomach cancer.”

As I see it, there were two disappointing aspects to the EPIC paper in question (1). First, the authors examined the composite end point of all malignant cancers. Thus, the beneficial effects of fruit and vegetables at some cancer sites (see the WCRF/AICR list above) would be diluted by the inclusion of common cancer sites for which there appears to be no association with fruit and/or vegetable intake. As a result, the authors found only a small (but statistically significant) 3% reduction in overall cancer risk per 200 g/d increased intake of total fruits and vegetables. Although this would equate to the prevention of thousands of cancers every year, much of the media chose to put a rather negative spin on the story, overlooking the evidence for a beneficial effect of fruits and vegetables on other diseases such as cardiovascular disease (see, for example, ref 2 below). Interestingly, the latter study found that the “risk of coronary heart disease was decreased by 4% [RR (95% CI): 0.96 (0.93-0.99), P = 0.0027] for each additional portion per day of fruit and vegetable intake”, a benefit similar in magnitude to that found for cancer in the EPIC study. It is unrealistic to expect to see risk reductions of 10% or higher per extra portion of fruits and vegetables for such broad end points as all cancers or coronary heart disease.

The second disappointing aspect of the EPIC paper was the fact that the authors only considered total fruits and total vegetables (and the sum of the two) as their exposure variables, and did not look at sub-types of fruit and vegetables. As readers will be aware, different fruits and vegetables can have quite different nutritional characteristics (compare the vitamin C content of apples and oranges, for example) and it was a pity that the authors did not examine the associations between, say, citrus fruits or cruciferous vegetables and cancer risk. Such an analysis might have revealed stronger protective effects for certain types of fruit and vegetables that could have pointed the way to more nuanced public health advice to supplement the rather crude ’5-a-day of any sort of fruits and vegetables’ message.

References

1. Boffetta P, et al. Fruit and vegetable intake and overall cancer risk in the European Prospective investigation Into Cancer and Nutrition (EPIC). J Natl Cancer Inst. 2010;102:1-9 Advanced access published April 6, 2010.

2. Dauchet L, Amouyel P, Hercberg S, Dallongeville J. Fruit and vegetable consumption and risk of coronary heart disease: a meta-analysis of cohort studies. J Nutr. 2006 Oct;136(10):2588-93.

7 Responses to “Guest Post: Paul Appleby on Fruits, Vegetables, & Cancer”

  1. Re-Revisiting Fruits, Veggies, & Cancer Says:

    [...] Jack Norris has published a wonkish and super informative guest blog post evaluating the anti-cancer effects of fruits and vegetables. Impressively, Jack was able to get Paul Appleby, Senior Statistician at the Cancer Epidemiology Unit at the University of Oxford, to write this piece. Link. [...]

  2. Leslie Goldberg Says:

    It’s my understanding that this study asked participants to remember what they ate and generally people will over estimate their consumption of “good foods.” Also, I don’t think five a day is that much. You also don’t know — is the subject still eating a bunch of pesticide/hormone laden meat and dairy, as well as junk food? The study I give cred to is the China Study, where folks ate a lot of vegetables, no junk and little meat and dairy. Those folks were not getting cancer like Westerners.

  3. Jack Norris RD Says:

    Leslie,

    The EPIC study did not rely on participants to remember what they ate in the past; it measured current diets and tracked people afterwards.

    Jack

  4. Patrick Weix, MD, PhD Says:

    The EPIC study did require people to remember what they ate, but it did so prospectively (the diet information was collected and then the subjects observed), instead of looking for people with diseases and then tracking diet backwards. That does remove one sort of bias, but is still problematic.

    Although the study was large, the reported food intakes were totally by recall. Intake of legumes and tubers were excluded from being considered as vegetables, by what rationale I can’t imagine. Think of the reported health benefits of lentils; the fiber alone make excluding these remarkable. This survey was limited by the same biases that Willet describes for retrospective studies–for example, that people believe that they ought to eat vegetables so they say they do. The difficulty of summing up the past 12 months of food intake (quick, what did you have for breakfast yesterday?) is another. In some centers the surveys were collected by interviewers and in some by questionnaire. Also, although care was given to include the confounding affects of tobacco and alcohol (again, difficulties abound with self reporting intake of these), no mention was made of collecting data on animal protein and animal fat intake, both well associated with certain types of cancer. In addition, such information would have provided a cross check on the vegetable and fruit intake (total estimates on daily calorie intake, correlation with obesity and BF%, etc.). Perhaps this data is for future publication, but it is not even hinted at here other than that many of the UK participants were vegetarian, yet the UK fruit and vegetable intake is not even the highest reported. Maybe it is the missing legumes and tubers?

    What has been shown and not reversed is that studies looking at whole food intake (from the California Seventh Day Adventist studies to the China Study to the EPIC study comparing obesity rates) show consistently that as the percentage of calories from animal based sources decreases and the percentage of calories from vegetable sources increases, the study populations have lower cancer risk, less heart disease, lower rates of obesity, less risk of diabetes, etc. Does it matter if the vegetables are protective or the meat is carcinogenic or atherogenic? No. Once again, the reductionist diet approaches fail–you can’t separate out the vitamins and just take those, you can’t separate out the phytonutrients and just take a pill.

    I will still recommend to my patients those dietary changes that I myself have made. Reduce animal protein and animal fat consumption (I eliminated it; it is easier than you think), replace those calories with fruits, vegetables and whole grains (including legumes and tubers!). Avoid processed foods as much as possible. Did my health improve? It did. As for my cancer risk, who can tell? But I ate 500 gm of fruit for breakfast yesterday, so I’m already at the top of the chart before lunch.

    Some of my above comments were originally posted in response to another blog’s discussion of this EPIC study.

  5. Jack Norris RD Says:

    From Paul Appleby:

    In response to Patrick Weix I should point out that ‘potatoes and other tubers’ and ‘legumes’ (dried pulses here, as “grain and pod vegetables” such as garden peas are included under vegetables) are regarded as separate food groups to ‘fruit’ and ‘vegetables’ in the EPIC study, reflecting their different nutritional properties. It would, of course, be possible to combine some or all of the food groups, as indeed was done in this study to create ‘total fruit and vegetables’.

    Unfortunately, baseline data for ‘legumes’ is unavailable for 3 EPIC countries and one other study centre (and mean observed intakes are low anyway), so adding ‘legumes’ to ‘vegetables’ would mean excluding around one-third of the observations. In contrast, it would be easy to add ‘potatoes and other tubers’ to ‘vegetables’, but it should be remembered that whereas vegetables are usually eaten raw or boiled, potatoes are often eaten after deep frying in fat, which perhaps explains why they are usually treated as separate entities in diet/disease studies.

  6. Patrick Weix, MD, PhD Says:

    This is why it is always useful to have inside insight into the mechanisms of research. Of course, potatoes are often fried, but when thinking of tubers, I think mainly of sweet potatoes and yams. Another reason why stratification of the data (which fruits or which vegetables seemed protective) would have been ideal. I would also submit that although legume intake for the general populace would have been low, for those on vegan or vegetarian diets it would have been much higher. No doubt further studies await publication.

    Thanks for your insight.

  7. Keyur Shah Says:

    Hey Jack,

    Thanks for the information. There is also a good response to this study by Joel Fuhrman,

    http://www.diseaseproof.com/archives/cancer-fruits-and-vegetables-provide-only-modest-protection-from-cancer.html

    here’s a quote from his response:

    “Yes, 3% is a tiny reduction in risk – but 200 grams is also a tiny amount of fruits and vegetables! One medium apple is approximately 180 grams, one cup of blueberries is 150 grams, and 1 cup of chopped raw broccoli is 90 grams. So keep in mind all these people did is eat the standard cancer-causing diet and add one apple or two cups of vegetables with dinner, they did not follow a vegetable-centered diet. They were still eating all the cancer-causing processed foods and animal products as their major source of calories.”

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