It is, perhaps, one of the most common health-care headlines: A new study linking a new food with a cancer risk. Search for "foods associated with cancer" and Google returns 196 million results, including new studies this month on salt, aspartame and high-carb diets.
Well, good news! You probably don't have to pay much attention to any of those studies: The vast majority of studies purporting to link foods to cancer have incredibly weak associations, often insignificant, according to new research in the American Journal of Clinical Nutrition.
Jonathan D. Schoenfeld and John P.A. Ioannidis recently combed through the body of research on 50 of the most common cooking ingredients. They found that a full 40 of them had been the subject of a study tying the ingredient to a different level of cancer risk.
Schoenfeld and Ioannidis started with the classic Boston Cooking School Cook Book, combing through its recipes for the most common ingredients. They then went to the body of cancer research, and found that 80 percent of those common ingredients had been the subject of a cancer risk study. The list, which you can see below, ranges from spices like salt and pepper to coffee to tripe.
At least one study was identiﬁed for 80% (n = 40) of the ingredients selected from random recipes that investigated the relation to cancer risk: 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.
The changes in cancer risk were all over the map: 39 percent found an increased risk, 33 percent found a decreased risk and 23 percent showed no clear evidence either way. Ingredients that had not yet been associated with cancer risk included baking soda and molasses.
Don't panic yet, though: The vast majority of those studies, Schoenfeld and Ioannidis found, showed really weak associations between the ingredient at hand and cancer risk. A full 80 percent of the studies had shown statistical relationships that were "weak or nominally significant," as measured by the study's P-values. Seventy-five percent of the studies purporting to show a higher cancer risk fell into this category, as did 76 percent of those showing a lower cancer risk.
"I was constantly amazed at how often claims about associations of specific foods with cancer were made, so I wanted to examine systematically the phenomenon," e-mails study author John Ioannidis "I suspected that much of this literature must be wrong. What we see is that almost everything is claimed to be associated with cancer, and a large portion of these claims seem to be wrong indeed."