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The Weight of Evidence

by Lisa Saffron(more info)

listed in cancer, originally published in issue 24 - January 1998

During the last year, I have been looking closely at the evidence behind the theory that food produced by intensive agriculture increases the risk of cancer and that people can reduce their risk by eating organic food. My conclusion is that the evidence clearly does not support this theory. Hence, I have other reasons for buying and growing organic food and other means, based on truly convincing evidence, of reducing my risk of cancer.

My conclusion is based on interpretation of a large body of evidence, in which I took to heart the advice given in an article about the toxicology of pollutants -- "the first steps are to separate fact from hypothesis and conclusion from speculation". Other people, particularly those with a strong belief in the hazards of modern agriculture, have a different approach to the evidence.

I'm often told that I must have left out, either by accident or by design, those studies which would have led me to the opposite conclusion. Initially I asked the British Library to search their databases for all the studies on the nutritional value of organic food.

They only produced 31 articles and assured me that hardly any studies have been done in this area. I went on to search many other on-line databases, using many different search terms, to search the references in papers, to talk to researchers in the field and to follow up all possible leads. Eventually, I found more than 380 relevant references.

There could be more studies out there but I have enough ammunition now to "prove" either argument. If I really wanted to convince you that organic food prevents cancer, I would have no difficulty selecting appropriate studies. If I wanted to argue that organic food is irrelevant to cancer risk, I would just as easily make another selection.

This is fine if the aim of the exercise is to score points but it won't lead to a better understanding of the causes of cancer. Studies with unpopular results cannot be ignored. More to the point, no single study on its own will give a conclusive result that can be quoted as once-and-for-all proof. What we must look for are trends. The evidence becomes convincing when the result is consistently reproduced in study after study and not contradicted too often. An example of a satisfyingly consistent trend is the research showing that eating vegetables and fruit reduces the risk of many kinds of cancer. Out of 196 case-control studies, 144 (nearly 80%) show a statistically significant protective effect.

No consistent trends have emerged from the studies on the nutritional value of organic food. The results are a mass of contradictory results.

On the other hand, there is a fairly consistent picture about nitrates in vegetables and digestive tract cancers – eating vegetables protects against these cancers regardless of the presence of nitrates. For organochlorine pesticides and breast cancer, more of the studies show no effect than a cancer-causing effect.

Another line of argument I've come across is selective criticism of studies. Get the wrong result, and if not completely ignored, the design of the study will be heavily criticised as will the motives of the researchers. Get the right result and even the flimsiest study is elevated to the status of conclusive proof. Criticism should be fairly and evenly distributed but in fact, some kinds of evidence carry more weight than others. Evidence that pesticides are linked to cancer in people is more persuasive than evidence that they cause cancer in laboratory animals fed extremely high amounts. Even less weight should be placed on experimental tests on human cells and bacteria.

Least weight should be given to evidence of cancer from other chemicals or to evidence of other ill-effects from pesticides.

Whether or not the evidence that pesticides cause fertility problems is convincing, it does not strengthen the evidence that pesticides cause breast cancer. Of course, consistent evidence from all of these kinds of studies is more convincing than evidence from one kind of study alone. Among studies of cancer in people, there is also a hierarchy of evidence. Studies showing changes in cancer rate over time, between countries and among migrants are not strong evidence of cause and effect. They are useful for producing hypotheses but if those theories are not confirmed by more powerful studies (such as case-control, cohort or controlled intervention trials), they remain nothing more than interesting theories. People often quote an Israeli study as proof that the 1978 ban on lindane and DDT caused a drop in breast cancer death rates ten years later. The study has been criticised on several grounds, not least of which is that new cases of breast cancer continued to increase over that period. But the main criticism is that there could be other equally plausible explanations for the drop in death rates. During that period there was significant immigration from countries with a low risk for breast cancer which may completely explain the change. The principle that "the dose makes the poison" is well accepted by toxicologists but not always by the public.

In a US survey, members of the public responding to a questionnaire about risk were more likely than toxicologists to agree with the following:

  • the fact of exposure to a pesticide is the critical concern, rather than the amount of exposure,
  • reducing the concentration of a possibly harmful chemical in a city's drinking water would not reduce the danger associated with drinking that water,
  • there is no safe level of exposure to a cancer-causing agent.

In theory, exposure to a single cancer-causing molecule could lead to cancer. However, we have effective defence mechanisms to cope with low level exposure, there to protect us from the daily bombardment by carcinogenic hormones and by-products of metabolism produced within our bodies. We would have to be exposed to massively higher levels of pesticides and nitrates than we ever have been exposed to on food in order to top our own carcinogens.

The most extreme position I've come across is that the evidence is irrelevant. To some people, there is no argument that pesticides and nitrates cause cancer and that organic food is nutritionally superior.

They distrust any scientific study showing the opposite but they uncritically trust mental and consumer organisations, scientists, newspaper and magazine journalists and book authors who claim speculation as fact.

I'll end with a quote by Richard North, an expert on food safety, who voiced my views, "A suspicion, however strong, may not without direct support from real evidence be made into a cause."


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About Lisa Saffron

Lisa Saffron is a health researcher and writer with a particular interest in the effect of environmental pollution on health. She has a Masters in Environmental Technology and a first degree in microbiology. She is committed to providing accurate and accessible information. Lisa also wrote a regular column in Positive Health magazine.

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