  Background: Risk ratios or p-values from multiple, independent studies,
observational or randomized, can be computationally combined to provide an
overall assessment of a research question in meta-analysis. However, an
irreproducibility crisis currently afflicts a wide range of scientific
disciplines, including nutritional epidemiology. An evaluation was undertaken
to assess the reliability of a meta-analysis examining the association between
red and processed meat and selected human health effects (all-cause mortality,
cardiovascular mortality, overall cancer mortality, breast cancer incidence,
colorectal cancer incidence, type 2 diabetes incidence).
  Methods: The number of statistical tests and models were counted in 15
randomly selected base papers (14%) from 105 used in the meta-analysis.
Relative risk with 95% confidence limits for 125 risk results were converted to
p-values and p-value plots were constructed to evaluate the effect
heterogeneity of the p-values.
  Results: The number of statistical tests possible in the 15 randomly selected
base papers was large, median = 20,736 (interquartile range = 1,728 to
331,776). Each p-value plot for the six selected health effects showed either a
random pattern (p-values > 0.05), or a two-component mixture with small
p-values < 0.001 while other p-values appeared random. Given potentially large
numbers of statistical tests conducted in the 15 selected base papers,
questionable research practices cannot be ruled out as explanations for small
p-values.
  Conclusions: This independent analysis, which complements the findings of the
original meta-analysis, finds that the base papers used in the red and
resulting processed meat meta-analysis do not provide evidence for the claimed
health effects.
