The short answer is: Most of them.
That’s the conclusion from a meta-analysis by Jonathan Schoenfeld and John Ioannidis (yes, that John Ioannidis) in the American Journal of Clinical Nutrition.
From James Choi’s blog:
Objective: We aimed to examine the conclusions, statistical significance, and reproducibility in the literature on associations between specific foods and cancer risk.
Design: We selected 50 common ingredients from random recipes in a cookbook. PubMed queries identified recent studies that evaluated the relation of each ingredient to cancer risk. …
Results: Forty ingredients (80%) had articles reporting on their cancer risk. Of 264 single-study assessments, 191 (72%) concluded that the tested food was associated with an increased (n = 103) or a decreased (n = 88) risk; 75% of the risk estimates had weak (0.05 > P ≥ 0.001) or no statistical (P > 0.05) significance. Statistically significant results were more likely than nonsignificant findings to be published in the study abstract than in only the full text (P < 0.0001). Meta-analyses (n = 36) presented more conservative results… The [relative risks] from the meta-analyses were on average null (median: 0.96; IQR: 0.85, 1.10).
Conclusions: Associations 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.
Or, as Chuck Palahniuk would have it: On a long enough timeline, the survival rate drops to zero.
(HT: Cyrus Samii, via Twitter.)