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Kids, Fries and Cancer: Is There a Connection?

By John Luik - August 30, 2005 12:00 AM

It's been a tough time for French fries lately. First there was the unpleasant association with, well, all things French that led to one of America's favourite foods being renamed, at least within the Beltway, Freedom Fries. Then there was the New York City fat police "request" that restaurants stop using trans fats for frying fries, a move that many restaurant owners say will adversely affect the taste of fries. And finally there was the new scientific study from the Harvard School of Public Health that suggests that eating French fries as a child might lead to breast cancer later in life.

The study, by a team of researchers led by Karin Michels, looked at children's preschool diets and the subsequent risk of breast cancer using the long-running Nurses' Health Study which followed 115,195 nurses for sixteen years. Michels' group studied 582 women with breast cancer and 1569 breast cancer free women in 1993. Their interest was the childhood diet of these women and its possible connection with their risk for breast cancer.

In order to find out about their diets, the subjects' mothers were asked to complete a 30 item food frequency questionnaire about their daughters' eating habits when they were aged 3-5. After correlating the answers to the questionnaire with the women's health status they discovered that women who had had as children one extra serving of French fries per week had a 27% increased risk for breast cancer in later life. Interestingly, foods like ice cream and hot dogs were not associated with an increased risk for breast cancer. Commenting on her study, Michels noted that "This study provides additional evidence that breast cancer may originate during the early phases of a woman's life and that eating habits during that phase may be particularly important to reduce future risk of breast cancer."

Correlation and Causation

But before we have mass hysteria about the possible role of French fries in breast cancer, there are several things about this study which suggest it probably merits little serious attention. The first of these is the confusion that appears to surround so much science (and even more science reporting) between statistical associations and causal connections. Doctors, for instance, have stethoscopes, but having a stethoscope doesn't make one a doctor. In certain parts of the world there is a strong statistical association between having malaria and not wearing shoes, but failing to wear shoes is not a cause of malaria.

One of the more famous of these confusions between association and causation was a study that appeared in the scientific journal Nature in May, 1999. The research reported that babies, aged two and younger, who slept in rooms with a nightlight, were more likely to be myopic as adults than babies who did not. The scientists who reported the finding were unable to provide any explanation for this strange connection between nightlights and adult myopia. A second study, however, which appeared the following year was able to explain the "connection" between nightlights and myopia. According to the authors of this report, it was due to the fact that nearsighted parents were more likely to use nightlights in their babies' rooms than normal sighted parents. In other words it was true that near-sighted adults had slept as babies in rooms with nightlights, but their myopia was not due to the nightlights but rather to the fact that their parents were myopic. There was a causal connection at work, but it was not the one assumed by the researchers. The first worry about the French fries/breast cancer connection then is that it is an instance of the nightlight-myopia confusion where it is true that the women with breast cancer ate French fries but it is not true that eating French fries was the cause of their breast cancer.

The Mismeasurement Problem and Recall Bias

This worry is strengthened by a second problem about this research, namely its methodology, in this case using a questionnaire to find out what someone ate decades before. One of epidemiology's fundamental problems is what might be called its measurement problem. Part of this revolves around the fact that its data is compiled from a subject's or a surrogate's memories, often from decades before. These memories, however, may be extremely unreliable. Many of us have difficulty remembering what we had for lunch yesterday, let alone remembering what we gave our children to eat years before. The mothers in this study were asked to recall with considerable precision not only what kinds but how much food their daughters ate some 30-35 years previously.

But beyond memory itself, there is also the problem of recall bias, where memory is biased by something else. The classic instance is a study of diesel exhaust and cancer in which the researchers asked elderly subjects who were dying from cancer about their exposure to exhaust and whether they thought it was connected with their cancer. Similar recall problems have plagued studies on the alleged connection between second-hand smoke and lung cancer, which are based on non-smokers -- or in many instances on the relatives of deceased non-smokers' -- memories of "how much" second-hand smoke they were exposed to. In the fries/breast cancer study, the mothers were asked to remember what their daughters ate AFTER they knew that their daughters had breast cancer, something which could have easily biased their recollections.

These sorts of problems suggest how weak the foundations of much epidemiology are. Science obtains its authority from the fact that its measurements and its data are precise. Imprecise measurements are measurements that cannot support scientific conclusions. The difficulty with recall epidemiology is that one's statistics will be based not on precise measurements but on years old memories. These memories may be accurate or they may be inaccurate but there is no way to know.

Low Statistical Power

But there is another problem with the fries/breast cancer study and that is its terribly low statistical power. The OR (odds ratio) of increased breast cancer risk from early childhood consumption of French fries was only 1.27. In other words it was barely statistically significant. The press, however, reported the increased risk as 27%, which made the increased risk look enormous to readers who knew little or nothing about statistics. But if the number -- 1.27 -- is placed in perspective, its real lack of significance is apparent. Risk ratios (RR's and OR's) are reported as numbers typically ranging from 1.00 to 15 or 20. RRs under 2 are considered very weak; the statistical association is likely due to chance rather than to their being a real link between, for instance, fries and breast cancer.

Still another problem stems from the fact that the study's authors were unable to provide any explanation as to how French fries would lead to breast cancer some year years latter, just as the authors of that Nature study were unable to explain how nightlights lead to adult myopia. Indeed, lead author Michels, appearing on the Today Show, was unable to provide any answer, when asked, as to how childhood consumption of fries might lead to breast cancer. Epidemiologists refer to this as the biological plausibility test. Epidemiological studies are, after all, simply statistical reports. For them to have any scientific authority they must be corroborated by a biological explanation as to how something happens. The fact that this study can provide no biological plausibility significantly weakens its credibility.

Finally, the fries/breast cancer connection is contradicted by other studies that have examined the link between breast cancer and French fries. In a study published in March of this year, researchers at the Karonlinska Institute in Stockholm looked at the diets of 43,000 women in the Swedish Women's Lifestyle and Health Cohort. The study found no statistically significant connection between the intake of fries and increased risk of breast cancer.

The world is full of statistical associations like those found between nightlights and myopia and malaria and being shoeless. No one, however, really believes that not wearing shoes is the cause of malaria. The numerous problems plaguing this study suggest that we should be equally skeptical about French fries and breast cancer.

John C. Luik is writing a book about health care policy. He lives in Canada.

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