The Guardian is reporting a fascinating new study in The American Journal of Clinical Nutrition by Stanford University epidemiologist extraordinaire John Ioannidis and Harvard University oncologist Jonathan Schoenfeld. The two researchers randomly chose of list of ingredients from the Boston Cooking-School Cook Book and then scoured the scientific literature to find studies linking those ingredients to cancer. They found that 40 of the 50 ingredients had been associated with an increased risk of cancer. As The Guardian reports:
Among the 40 foods that had been linked to cancer risks were flour, coffee, butter, olives, sugar, bread and salt, as well as peas, duck, tomatoes, lemon, onion, celery, carrot, parsley and lamb, together with more unusual ingredients, including lobster, tripe, veal, mace, cinnamon and mustard.
Schoenfeld and Ioannidis then analysed the scientific papers produced after initial investigations into these foods. They also looked at how many times an ingredient was supposed to increase cancer risk and the statistical significance of the studies.
"Statistical significance" is the term used for an assessment of whether a set of observations reflects a real pattern or one thrown up by chance. The two researchers' work suggests that many reports linking foodstuffs to cancer revealed no valid medical pattern at all.
"We found that, if we took one individual study that finds a link with cancer, it was very often difficult to repeat that in other studies," said Schoenfeld. "People need to know whether a study linking a food to cancer risk is backed up before jumping to conclusions." ...
[Yet] these initial studies have often triggered public debates "rife with emotional and sensational rhetoric that can subject the general public to increased anxiety and contradictory advice".
"When we examined the reports, we found many had borderline or no statistical significance," said Dr Jonathan Schoenfeld of the Harvard School of Public Health in Boston.
For more background, see Ioannidis' devastating, "Why Most Published Research Finds Are False," published in 2005 in PLoS Medicine. Epidemiology is not the only field clotted with low-quality studies; see also my column, "Can Most Cancer Research Be Trusted?," that reported that researchers could reproduce the results from only six out of 53 landmark papers in preclinical cancer research.