Durk Pearson & Sandy Shaw’s®
Life Extension NewsTM
Volume 16 No. 5 • May 2013


Is Everything We Eat Associated With Cancer?

A paper with this actual title was published very recently.1 Noting that large and increasing numbers of different foods and food constituents are being investigated for positive or negative associations with cancer, scientists investigated the appearance of 50 common ingredients from random recipes in a cookbook, evaluating the conclusions, statistical significance, and reproducibility of published associations with cancer appearing in papers in PubMed.

The authors found that forty ingredients (80% of those that they examined in the literature) had articles published on cancer risk. They found that “[a]ssociations with cancer risk or benefits have been claimed for most food ingredients. Many single studies highlight implausibly large effects, even though evidence is weak. Effect sizes shrink in meta-analyses.”

The authors also state that “[s]tudies may spuriously highlight results that barely achieve statistical significance or report effect estimates that either are overblown or cannot be replicated in other studies.” Overall, they found that 39% of studies concluded that the examined ingredient conferred an increased risk of malignancy, 33% concluded that the ingredient reduced the risk of malignancy, 5% concluded that there was a borderline statistically significant effect, while 23% could not discern a clearly increased or decreased risk. Studied ingredients included veal, salt, pepper spice, egg, bread, pork, butter, tomato, lemon, duck, onion, celery, carrot, parsley, mace, olive, mushroom, tripe, milk, cheese, coffee, bacon, sugar, lobster, potato, beef, lamb, mustard, nuts, wine, peas, corn, cayenne, orange, tea, and rum. You can imagine some of the difficulties in, say, examining the effects of lamb on the risk of cancer. How do you hold everything constant (including subject genetic diversity) while varying only the intake of lamb?

Some of the problems that likely contributed to the inconsistency included variation in definitions of “serving size,” “often,” “never” as well as there being no standardized, consistent selection of exposure contrasts for the reported risks. The authors claim that the vast majority of these claims (increased or decreased malignancy) were based on weak statistical evidence. We would also like to point out that the variation in human responses to the analyzed ingredients imposed a very difficult hurdle to the isolation of effects that can be generalized to large human populations. That is why we like to see considerable research done on mechanistic (cell free or cell culture) studies and animal studies considered together with human studies. The analysis of studies of a particular ingredient’s effects on cancer risk, considered as a whole, still is as much of an art as a science, depending upon the ability to see connections between, say, chemical structure and function of an ingredient. You don’t get definitive answers. You get leads.

See quotes on the next page, on the dangers of scientific bias, which appeared in the editorial commentary on this paper, published in the same issue of The American Journal of Clinical Nutrition.

Reference

  1. Schoenfeld and Ioannidis. Is everything we eat associated with cancer? A systematic cookbook review. Am J Clin Nutr. 97:127-34 (2013).

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