Everything We Eat Cures Cancer! (or Causes it?)

Central to the nutritional and general healthcare philosophy of most alternative practitioners is that anything we eat is either good or bad. They often quote that cutting edge physician Hippocrates saying “Let food be thy medicine” (though they less commonly quote him saying “A physician without a knowledge of Astrology has no right to call himself a physician” or “What medicines do not heal, the lance will; what the lance does not heal, fire will.” for some reason). So-called “super foods” are lauded as having nearly magical healing properties, whereas foods like high fructose corn syrup, or even entire categories of foods such as grains are considered off-limits in a healthy diet. One of the most common scare tactics used to promote alternative approaches to pet nutrition is the claim that commercial pet food causes cancer.

So what is the evidence to support these claims that foods cause or cure cancer or other serious disease? Well, often there is little or no evidence at all, but occasionally there will be an observational study (one without any controls for chance, bias, or other sources of error). Usually these studies are done in humans, and they tend to drive food fads for both humans and, to a lesser extent, pets despite the dangers of extrapolating from human research to veterinary patients. However, as has often been pointed out, research studies are only as good as the quality of their design and conduct, and single studies, especially observational studies, are rarely solid enough evidence to justify major changes in behavior. I recently ran across a systematic review which has examined the human nutrition literature and has shed some light on why it sometimes seems like everything we causes or cures cancer.

Schoenfeld JD, Ioannidis JP. Is everything we eat associated with cancer? A systematic cookbook review. Am J Clin Nutr. 2013 Jan;97(1):127-34. doi: 10.3945/ajcn.112.047142. Epub 2012 Nov 28.

This clever little study selected recipes at random from a popular cookbook and then evaluated all the ingredients to see if there was any research literature suggesting they increased or decreased cancer risk. Here are the ingredients for which some research studies were found:

veal, salt, pepper spice, flour, egg, bread, pork, butter, tomato, lemon, duck, onion, celery, carrot, parsley, mace, sherry, olive, mushroom, tripe, milk, cheese, coffee, bacon, sugar, lobster, potato, beef, lamb, mustard, nuts, wine, peas, corn, cinnamon, cayenne, orange, tea, rum, and raisin.

Of these, there were more than 5 studies for 65% of the ingredients, with over 216 publications altogether. About 40% of the studies found an increased risk of cancer associated with one of these ingredients, 33% found a decreased cancer risk, and about 25% found no clear evidence either way. When a risk was identified, the statistical support was weak or not technically significant in 80% of the studies, so most individual did not show very robust results. About half of the meta-analyses included, however, had stronger statistical results, which is not surprising since the whole point of meta-analyses is that evaluations of multiple studies give stronger evidence than the results of individual studies. The distribution of effects reported in the meta-analyses centered around zero, suggesting random variation but no clear real effect.

The authors’ discussion summarizes very well not only the results of this study but the general problem with much of the observational and pre-clinical research often used to justify specific practices in the absence of clinical trials:

80% of ingredients from randomly selected recipes had been studied in relation to malignancy and the large majority of these studies were interpreted by their authors as offering evidence for increased or decreased risk of cancer. However, the vast majority of these claims were based on weak statistical evidence. Many statistically insignificant “negative” and weak results were relegated to the full text rather than to the study abstract. Individual studies reported larger effect sizes than did the meta-analyses.

…the credibility of studies in this and other fields is subject to publication and other selective outcome and analysis reporting biases, whenever the pressure to publish fosters a climate in which “negative” results are undervalued and not reported. Ingredients viewed as “unhealthy” may be demonized, leading to subsequent biases in the design, execution and reporting of studies. Some studies that narrowly meet criteria for statistical significance may represent spurious results, especially when there is large flexibility in analyses, selection of contrasts, and reporting. When results are overinterpreted, the emerging literature can skew perspectives and potentially obfuscate other truly significant findings. This issue may be especially problematic in areas such as cancer epidemiology, where randomized trials may be exceedingly difficult and expensive to conduct; therefore, more reliance is placed on observational studies, but with a considerable risk of trusting false-positive or inflated results.

Overinterpretation of individual studies with often small effects that are only marginally significant statistically and may be insignificant clinically is a major problem in all areas of medicine, and it manifests especially dramatically in alternative medicine, where any data is good data so long as it supports existing beliefs. To find out what is actually true and what will real improve health, we must be mindful of the limitations in the evidence and seek to improve the design, conduct, and reporting of clinical studies. That is a job for the researchers. However, the job for the rest of us is to know at least enough about research evidence to be wary of overinterpretation and placing excessive confidence in data that does not merit it. This will hopefully dampen the wild swings back and forth between claims like “Food X will kill you” and “Food X will make you live forever.”

 

 

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9 Responses to Everything We Eat Cures Cancer! (or Causes it?)

  1. rita says:

    Just wish people were more interested in foods not killing others – i.e., non-humans, and less in pandering to their own caprices, medical or otherwise.

  2. Beccy Higman says:

    This is an area that also seems to be especially prone to the misleading press release from the press relations departments of the institutions that have done the studies, so even when the scientists have not made exaggerated claims their work is spun into the ‘x cures cancer’ narrative.

  3. Sam Ivy says:

    Nutritional research has made a huge leap in recent years. Remember it’s other diseases too, not just cancer. According to recent research, people with diabetes 2 can reverse their symptoms with healthy nutrition.

  4. kitty says:

    @Sam Ivy – really? Don’t eat X,Y,Z, this is bad for you. Oh no, X isn’t bad for you after all, Y may be actually good, and Z is neutral… In all fairness, the official guidelines are like that too, what doctors told us was bad turns out to lack scientific evidence, and in the meantime people who followed the guidelines are actually harmed.

    It seems, our grandparents who told us to eat everything and in moderation weren’t so ignorant after all.

  5. Gail Torr says:

    My 8 yr old boxer was just diagnosed with lymphoma. I would keep him on the same food that he has been on, but even with prednisone he won’t eat the dry or wet version of his food. Of course I have read all the conflicting views on cancer diets. I don’t know where to begin to get him to eat something without upsetting his stomach further. Of course he loves people food but that can’t be good for him. Just looking for suggestions.

    Thank you

  6. skeptvet says:

    There is no simple answer for you, since every patient is different. If a dog who normally has a great appetite won’t eat when he or she has cancer, the problem really is not the food but the effects of the illness and/or the treatment on the dog’s appetite. Nausea can occur in some cases, and there are excellent medications to treat this. Sometimes, changes in diet may be helpful, but there is not strong evidence for specific nutritional therapies for cancer, and too much tinkering with diet can just further upset a dog who is not feeling well. The most important thing is that you talk with a cancer treatment specialist about your dog, since they have the most experience helping cancer patients.

    Here are a couple of resources that may have more information for you. Good luck!

    Nutritional requirements of dogs and cats with cancer
    Feeding the dog with cancer

  7. UrbanCollieChick says:

    Food will increase risk of cancer indirectly, when you eat too much of it such that you get FAT!

    Harder to get fat on broccoli than on mayonnaise or chocolate.

    That’s about the extent of it for me.

  8. Sheena says:

    Do the recent findings re processed meat affect this at all? (http://www.theguardian.com/lifeandstyle/2015/dec/27/processed-meat-carcinogenic-world-health-organisation?CMP=Share_iOSApp_Other) Is dog food processed in similar ways to meat for human consumption? And is it likely our bodies are affected by the same things?

  9. skeptvet says:

    Great question.

    The issue is what one considers “processed.” Preserved, salted lunch meat and freshly cooked steak are both “processed.” Anything not eaten raw directly from the recently killed animal is “processed.” Yet there are significant differences in the specific processing done, and the potential health implications. The article you linked to says this:

    “The majority of the scientists in Lyon agreed that there was enough evidence to say that processed meats – such as salted, cured, fermented or smoked meats – were indeed carcinogenic. For unprocessed red meat, the evidence was less compelling, but the group still thought it presented a possible risk.”

    Clearly, the meat in commercial dog food is not “salted, fermented, cured, or smoked,” so it isn’t going to be the same thing as these kinds of “processed meat” eaten by people. There is not yet any specific evidence that the cooking and other processing that does happen with meat used in dog food is associated with significant health risks. It is certainly possible, but this needs to be demonstrated directly, not inferred from research regarding very different kinds of processing employed for human foods.

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