gree to which a food or behavior caused cancer. WCRF/AICR established its own criteria for determining the threshold for causation and primarily relied upon human
epidemiological studies, although they state that “evidence of plausible biological
mechanisms” is needed to support the epidemiological link. According to the
WCRF/AICR summary report, some of the methods used by this panel are “new”
and, as such, have not been subjected to broad scientific peer-review and concurrence.
The criteria the group established indicate a “convincing” declaration of causation, requiring evidence from two scientifically sound cohort studies along with evidence of
dose response and plausible experimental evidence. The other classifications that require less stringent evidence were “probable,” “limited-suggestive,” “limited-no conclusion” or “substantial effect on risk unlikely.”
“But we must think when it comes to
the controversial subject of meat
and cancer because unconventional
new wisdom is emerging.”
WCRF/AICR declared that the evidence of a link between red and processed meat
and the development of colon cancer was “convincing.” The finding was supported
primarily by weak to modest associations that may have been influenced by bias or
confounding. Still, they recommended a public health goal that no more than 11 oz.
of red meat per week be consumed, very little if any of which should be processed
meat because they claimed there is no safe level of processed meat. In the discussion,
they said nitrite in processed meats or other factors may be to blame.
Studies showing the health benefits of red and processed meat, and studies documenting sodium nitrite’s lack of a cancer effect in major rat and mouse lifetime bioassays (like those done by the National Toxicology Program) were not considered. In
addition, epidemiological studies showing no relationship between meat and cancer—
and there are many—were apparently insufficient to sway the panel opinion about
their “convincing” judgment.
Perhaps most critically, one of the largest epidemiological study ever done of the
relationship between red and processed meat and colon cancer was not considered.
The Pooling Project, led by Harvard University, used a more robust form of meta-analysis called “pooling” whereby source data from international cohort studies are
pooled together and re-analyzed using uniform and standardized exposure and analytical methods. This quantitative assessment included 14 cancer centers and involved
over 725,000 people.
The study authors concluded that there was no association between red and
processed meat consumption and colorectal cancer. Although the results were presented at the 2004 American Association for Cancer Research conference and published in the conference proceedings, the study has never appeared in a journal and,
therefore, its landmark findings were not factored into the WCRF report. Had the
findings been considered, the WCRF/AICR panel would have had trouble reaching
the conclusion that it did.
Key senators have asked the Health and Human Services Secretary for an explanation about why this federally funded study remains unpublished. AICR says that
many of the individual publications representing studies that were comprised in The
Pooling Project were considered in its analysis. That may be true, but the WCRF report clearly underscores the value of pooled data: “the combination (and re-analysis)
of data from multiple studies creates a larger data set and increased statistical power.”
An Intellectual Leap over the
While AICR declared in a press release that the report was “the most comprehensive ever done” and claims to
have “convinced the scientific community” about cancer prevention strategies,
many scientists are not convinced.
David Klurfeld, Ph.D., national program leader in human nutrition at the
USDA Agricultural Research Service,
provided an extensive critique of the
2007 report at a 2008 International Association of Food Protection (IAFP)
symposium. Depsite WCRF/AICR’s
claims of a “convincing link” between
red and processed meat and colorectal
cancer, Klurfeld noted that a careful read
of the 517-page report and its companion 2,334-page systematic literature review (SLR) on colorectal cancer shows
that the SLR’s conclusions do not support the warnings issued in the summary
report or the press release.
According to Klurfeld, the SLR of
colorectal cancer said, “Overall, mechanisms explaining the data linking meat
(processed and red) intake and colorectal
cancer are far from plausible biological
mechanisms.” But in stark contrast, the
final summary report declared “There is
strong evidence for plausible mechanisms operating in humans. Processed
meat is a convincing cause of colorectal
cancer.” The criteria for “convincing” established by the expert panel states that
there must be plausible biological mechanisms, but apparently the panel chose
to ignore the findings of the research
team it had hired to conduct the SLR.
“While few people likely will tackle
the 2,334-page literature review after
reading a 500-page summary of findings,
those who do will find some critical information that was disregarded and contradicted in the report’s summary,”
Klurfeld said. He expressed frustration
that the group’s press release reflected so
poorly what was actually in the report
Cancer Research: Approaches,
Strengths and Weaknesses
Assessing whether a factor is a
“cause” of disease involves a multi-disci-plinary approach that involves epidemi-