PLEASE NOTE: Since sending the below letter I have discovered an error in my original figures, so I am amending this post accordingly. I apologise for any misleading information I have given.
I have this morning responded to Michael Griffin of the Department of Health on the Scientific Advisory Committee on Nutrition (SACN) report on red meat and cancer. I wrote as follows:
As the SACN/Government/WCRFUK health warnings on red meat tell the public that "eating red meat may cause cancer" rather than "eating red meat may cause cancer if you're a smoker or between the ages of 50 and 71, or taller than average, or have/don’t have a family history of cancer" I have also chosen to ignore variables in the 700 (approx.) studies referenced in the SACN report.
After filtering out all studies concerning iron deficiency, iron fortification in flour, iron supplementation in cereal, iron supplementation in babies' formula, iron overload, pregnancy, birth, breastfeeding, rheumatic disease, tuberculosis, alcoholism, and other studies unrelated to either diet in general, or red meat in particular, and meta-analyses of detailed studies already included, I was left with fewer than sixty.
I then discarded those produced by NDNS (whose data are intrinsically flawed, in that they do not differentiate between fresh farm/butcher meat and that from unknown sources which has then been processed in a factory with numerous additives, then canned or frozen); those produced by WCRFUK (as their data are based wholly on NDNS); Department of Health (again based on NDNS); and SACN (based on NDNS and other unrelated studies).
One major problem with the studies is the Food Frequency Questionnaires used, which ranged from “a 24-hour food recall” through “during the last 12 months” to, in one case, an incredible “during the two years prior to being diagnosed”!
From personal experience of monitoring my own daily nutrition for extended periods (up to six months at a time) over the last four years, and from the accounts of colleagues attempting to do likewise, I believe it is impossible for anyone to accurately recall and estimate precise amounts of any food eaten even the previous day. The only accurate method is to weigh and record every item before it is eaten. This flaw is acknowledged in most studies, using terms such as “limitations”, “estimated”, etc.
This cull left just 21 studies, most of which (but not all) looked at cancer risks and red meat. However, there is one factor common to all the studies that is not taken into account - the fact that they all contained carbohydrates such as grains, starches, and many different sugars, and such ‘foods’ as chocolate bars.
The remaining studies fall into two general categories:
1) Those which showed no association, an inverse association, or were inconsistent in methodology or findings (12)
Meat consumption and risk of colorectal cancer in Japan: the Miyagi Cohort Study
We found no significant association between total meat consumption and the risk of sub-site of colorectal cancer. In conclusion, our data do not support the hypothesis that meat consumption is a risk factor for colorectal cancer.
Meat consumption in a varied diet marginally influences nonheme iron absorption in normal individuals
No mention of cancer of any type
Nutrition and dietary carcinogens
Relates only to cooking methods of meats
The relationship between the consumption of meat, fat, and coffee and the risk of colon cancer: a prospective study in Japan
Unable to locate full text, but abstract covers only processed meat and coffee
Comparison of risk factors for colon and rectal cancer
” our findings support the hypothesis that some risk factors, including family history, physical activity and possibly height, differ in their association with colon and rectal cancer.”
A cohort study of dietary iron and heme iron intake and risk of colorectal cancer in women
” ... heme iron intake, and iron intake from meat showed no association with colon cancer, rectal, or colorectal cancer”
”The present analysis showed no association between dietary intake of iron, heme iron, iron from meat sources, or non-heme iron and risk of cancer of the colon, rectum, or proximal or distal colon.
Effect of white versus red meat on endogenous N-nitrosation in the human colon and further evidence of a dose response
Conclusion: “Despite the consistent response to meat, there is substantial individual variation in the extent of response. This individual variation remains despite the highly controlled conditions under which studies are carried out.”
Paleolithic nutrition. A consideration of its nature and current implications
Unable to access full text but unlikely to associate red meat with cancer.
Meat consumption and colorectal cancer risk: dose response meta-analysis of epidemiological studies
Unable to access full text, but it appears to only be a meta-analysis of articles already covered.
Red meat, chicken, and fish consumption and risk of colorectal cancer
Data in Table 3 (hazard ratios) do not make sense. Those for fresh red meat do not show the increased risk with increased consumption in either colon or colorectal cancers which those for processed meats do. Interestingly, higher fish consumption shows an increased risk of colon cancer, but I’ve yet to read that Government health warning!
Haem, not protein or inorganic iron, is responsible for endogenous intestinal N-nitrosation arising from red meat
Studies involved 11 males and 17 females, but “Only male diets were analysed” for NOC and haem.
Iron and colorectal cancer risk in the lpha-tocopherol, beta-carotene cancer prevention study
” Comparing the highest to the lowest quartiles, there was an inverse association between serum ferritin and colorectal cancer risk and a suggestion of an inverse association between dietary iron and colorectal cancer risk. In addition, serum ferritin, serum iron and transferrin saturation were all inversely associated with colon cancer risk specifically whereas serum unsaturated iron binding capacity was positively associated with colon cancer risk. In summary, we found a significant inverse association between several serum iron indices and colon cancer risk.”
” We found no correlation between dietary iron (or intake of meat) and iron stores”
2) Those showing some association between meat and colorectal cancer (9)
Meat consumption and risk of colorectal cancer
Results: 1667 out of 148,610 participants (1.12%) developed colorectal cancer
Heme and chlorophyll intake and risk of colorectal cancer in the Netherlands cohort study
Results: 1535 out of 120,852 participants (1.27%) developed colorectal cancer
Red meat consumption and risk of cancers of the proximal colon, distal colon and rectum:
Result: 733 out of 61,433 participants (1.19%) developed colorectal cancer
Iron intake and the risk of colorectal cancer
Result: 156 out of 11,317 participants (1.34%) developed colorectal cancer
A prospective study of red and processed meat intake in relation to cancer risk
Result: 53,396 out of 494,036 participants (10.8%) developed a cancer of some type.
PLEASE NOTE: the section which follows has been amended as there was a mathematical error in the original.
Cancer incidence in British vegetarians.Conclusion: The incidence of some cancers may be lower in fish eaters and vegetarians than in meat eaters.
Meat, fish, and colorectal cancer risk: the European Prospective Investigation into cancer and nutrition
There was no significant increased risk of colorectal cancer with high intake of red or processed meat in the presence of high intake or fish or fibre.
Conclusions: The overall cancer incidence rates of both the vegetarians and the nonvegetarians in this study are low compared with national rates. Within the study, the incidence of all cancers combined was lower among vegetarians than among meat eaters, but the incidence of colorectal cancer was higher in vegetarians than in meat eaters.
