It isn鈥檛 surprising that a paper published in the prestigious British Medical Journal linking food preservatives to cancer roused much media attention. After all, preservatives in food are not rare and cancer is frightening. But just how much should our eyebrows furrow? That depends on whether you look at the headlines that scream about preservatives increasing cancer rates by 16% or you delve deeply into the data that gave rise to these numbers. Let鈥檚 delve!
The data described in the paper are derived from the 鈥淣utriNet-Sante鈥� study in France that since 2009 has followed some 105,000 people of whom 4226 were eventually diagnosed with cancer. On three days every six months, participants filled out detailed dietary questionnaires which were used to estimate the preservative content of the foods they consumed from published food composition data bases. Significant uncertainty is inherent in such calculations. For example, different producers may use different amounts of sorbic acid in their jams, and the same goes for other preservatives such as nitrites in ham or sulfites in wine.
People are also notoriously unreliable when it comes to accurately reporting the foods they consume and especially their amounts. And needless to say, such observational studies may establish a 鈥渓ink,鈥� but can never prove a cause-and-effect relationship. Attempts can be made to correct for confounders such as smoking, activity level and body weight, but there are other factors that can skew the results. For example, foods that contain preservatives may also contain dyes, emulsifiers, sweeteners, flavour enhancers and chemicals such as acrylamide and polycyclic hydrocarbons produced during cooking. The presence of these can cloud the preservative-cancer relationship.
Let鈥檚 set those uncertainties aside and take a look at look at how the study鈥檚 authors came up with the 16% increase in overall cancer. The subjects were divided into three groups, described as 鈥渓ower consumers,鈥� 鈥渕edium consumers鈥� and 鈥渉igher consumers鈥� according to the amount of preservatives they were believed to be consuming. Then the percentage of people diagnosed with cancer in each group was calculated to see if it was related to preservative intake.
Of 43,586 participants in the low consumer group, 1288, or 3%, were diagnosed with cancer. In the higher group, it was 1339 out of 38,503 or 3.5%. So where does the 16% increase in cancer come from? The number 3.5 is 16% greater than 3! But what a 0.5% difference in the cancer rate really means is that for every 200 people who switch from a high to a low preservative diet, one case of cancer may be avoided. Similarly, for all the specific cancers for which the increases were calculated, such as聽potassium sorbate increasing breast cancer by 26% or sodium nitrite increasing prostate cancer by 32%,聽anywhere from 0 to 3 cancer cases could be avoided if a hundred people switched to a low preservative diet. But are preservatives really the culprits or is it possible that a high preservative diet is just a marker for a diet that is higher in fat, sugar and salt? Such diets have been linked with an increased risk of cancer. 聽
That possibility, though, does not let preservatives off the hook because a couple, namely nitrites and benzoates can produce a carcinogenic effect. Nitrites, widely used in processed meats, can be converted in the body to nitrosamines that are recognized as carcinogens. Benzoates under certain conditions can release benzene, another known carcinogen. However, there is no independent evidence from laboratory or animal studies that any other preservative can spark cancer. That makes the findings of the NutriNet study in which the researchers linked 11 preservatives to cancer very curious. That curiosity extends to their calculation that acetic acid, the main ingredient in vinegar, increases cancer rates by 12%. If acetic acid were really carcinogenic, then the consumption of vinegar and pickled foods would long ago have revealed an association with cancer epidemiologically.聽
Another issue in this publication is the reliability of the data as judged by the reported 鈥渃onfidence interval鈥� which is an estimate of the range of values that contains the value of interest. For acetic acid, it is reported to be 1.01 to 1.25 meaning that the increased risk could be as little as 1% or as much as 25%. Such a large confidence interval means that the result has to be taken with a very large grain of salt.
Furthermore, for many of the preservatives in the study, the confidence interval of the link to cancer spans the number 鈥�1鈥� as in 0.91 to 1.10 for calcium propionate, This essentially makes the result meaningless because it says that consuming calcium propionate could reduce the cancer risk by as much as 9% or increase it by as much as 10%.
There is one final and critical point. Concentrating on risk without taking benefit into account is unrealistic. Food producers do not add preservatives on a whim. They add them to prevent disease. Nitrites prevent potentially deadly botulism, benzoates inhibit yeasts and bacteria, sorbates guard against molds, fungi and yeasts and propionates prevent mold from taking root in bread. It is impossible to know the exact benefit of preservatives because 鈥渄iseases you prevent鈥� cannot be calculated. What is clear is that there has been a dramatic drop in food-borne ailments since preservatives were introduced. You no longer have to worry about your bread going moldy after a day or being afflicted with botulism by eating a hot dog.
What then do we take away from this paper that links food preservatives to cancer? Reporting risks in terms of percentages without alluding to absolute numbers is misleading, and ignoring the potential benefits of preservatives is negligent. In many cases, the size of the confidence interval makes the data questionable.
On the positive side, while this paper in no way proves that preservatives cause cancer, the data gathered does suggest a small increase in cancer from eating foods with preservatives. Of course, the presence of preservatives may just signal foods that are high in salt, fats and sugar. Nevertheless, the authors鈥� advice that public health policies should be strengthened to promote and make accessible and affordable fresh, seasonal, homemade products to consumers is sound. However, did we need 20 authors spending a great deal of time and money to arrive at that conclusion. My eyebrows deeply furrow at that thought given the enormous number of variables in this study.
