Abstract

BACKGROUND: Red meat and processed meat consumption may increase colorectal (Alexander DD. 2010; Larsson SC. 2006) and stomach (Larsson SC. 2006) cancer risk. But less is known about total meats consumption, and no systematic review was published about the relation with total cancer until now.
OBJECTIVES:

• To review all prospective studies which published information about total meat, and meat products in relationship with total cancer risk, disease progression and mortality/survival.
• To define the amount of consumption found to be related with possible effects on total cancer.
• To define possible effect modification by confounders.

DATA SOURCE: The Pubmed database was searched (No start date - May 25th, 2010) for relevant articles using the keywords "meat, meats, vegetarian, vegetarianism, or vegetarians", and a fair amount of other keywords" combined with "prospective, cohort, follow-up, or longitudinal". The exact search term is described Here.
Prospective studies published in the English language were included. Reference lists were searched for additional articles.
RESULTS: 20 articles were found which provided information about 14 different cohorts. Of these, 0 articles were excluded.
Results are described when any evidence for an association - as defined in the Methods - was found. In addition, data about total meats, red meat, and processed meat is described.

Total disease risk.

• Total meats: Data was provided about 4 cohorts, including 6,398 cases. Suggestive evidence was found for an increased total cancer risk of total meats consumption (RR = 1.10).
• Red meat: Data was provided by 2 cohorts, including 5,125 cases. No evidence was found for an association.
• Processed meat: No data was found.

Advanced stage/metastatic disease risk or disease progression.
No data was found.

Mortality risk.

• Total meats: Data was provided about 5 cohorts, including 2,176 cases. No evidence was found for an association (RR = 1.02).
• Red meat: Data was provided by 4 cohorts, including 27,501 cases. Inconclusive evidence was found for an association (RR = 1.20).
• Processed meat: Data was provided by 4 cohorts, including 30,931 cases. Inconclusive evidence was found for an association (RR = 1.09).

CONCLUSION: Overall, little data was found about the different meat consumption variables in relation to cancer risk and mortality. Suggestive evidence was found for a weakly increased total cancer risk of total meats consumption (+ 10%). This evidence was found among primarily vegetarian cohorts in which no adjustments were made for any dietary items, except for alcohol. No other evidence was found for any other association.
LIMITATIONS: Data about meat consumption and vegetarianism was combined. Vegetarianism may reflect another lifestyle instead of just a lack of meat consumption, and therefore a separate analysis of vegetarianism and meat consumption could be justified. But the amount of data did not allow for such an analysis. Most data from vegetarian cohorts was adjusted for age, sex, and smoking only, further limiting the possiblity of proving any causality.
PERSPECTIVE: When all data is taken into account, the possibility exists for an increased total cancer risk or mortality risk of some types of meat consumption. But consistency is low for all meat variables, and any possibly increased risk is probably caused by certain types of red-, or processed meats, and not by poultry or total white meats.

|References: Alexander DD. Processed meat and colorectal cancer: a quantitative review of prospective epidemiologic studies. Eur J Cancer Prev. 2010 Sep;19(5):328-41. Link.
Larsson SC. Meat consumption and risk of colorectal cancer: a meta-analysis of prospective studies. Int J Cancer. 2006 Dec 1;119(11):2657-64. Link.
Larsson SC. Processed meat consumption and stomach cancer risk: a meta-analysis. J Natl Cancer Inst. 2006 Aug 2;98(15):1078-87. Link. |

Total meats and total cancer.

Background: Dr. Hirayama examined the effects of a small amount of food groups in relation to a large amount of mortality end points in a Japanese cohort of very large size. An extended review of his work was published as a book in 1990 (Hirayama T [3]). Data about this cohort is seldom included in current systematic reviews about the relations mentioned. Dr. Hirayama published a lot of articles stating that vegetables and meats were related to several disease end points, adjusted for age and sex. However, the book included one page showing effects after multivariate analysis including cigarette smoking, meat, green-yellow vegetables, and alcohol. And this analysis showed that a large amount of previously published effects completely changed when these variables were taken into account.
Since a) Dr. Hirayama himself only published sex, and age-adjusted results in the English language, b) the results were published as a book and not in a peer-reviewed journal, and c) Dr. Hirayama was the only researcher examining this cohort, results from his cohort are debatable. Results will be presented including effects from his work, but his work will not be included in the evidence for a possible effect.

Total cancer risk: Data was provided by 5 articles about 4 cohorts, including a total of 6,398 cases. A significant protective effect was found of vegetarianism vs eating meat from pooled results of 2 cohorts (Key TJ [10]), including 3,033 cases (47% of all cases). No other associations were found. The average RR = 1.10 for total meat consumption (excluding incomplete data from McCullough ML [9]).
Total cancer mortality: Data was provided by 9 articles about 6 cohorts, including a total of 16,916 cases. A strong significantly increased risk was found in one cohort of very large size among men only. No other associations were found. The average RR = 1.25 for total meat consumption.
When results from one cohort with debatable results were excluded (Hirayama T [3]), effects were restricted to findings from 5 primarily vegetarian cohorts, including 2,176 cases. No significant effects were found. RR's for meat consumption were < 1 in 3 cohorts (Chang-Claude J [2]), Key TJ [6], Key TJ [10]), and > 1 in the remaining 2 cohorts (Phillips RL [1], Appleby PN [5]). The average RR = 1.02 for total meat consumption.

Conclusion: A significant protective effect of vegetarianism against total cancer risk was found from pooled results of 2 cohorts, including 47% of all cases. Suggestive evidence was found for an increased total cancer risk of total meat consumption (+ 10%). No associations were found with total cancer mortality.
Perspective: A significant protective effect (RR = 0.88) of vegetarians vs meat eaters was found from pooled results of 2 cohorts (Key TJ [10]). But the effects size was stronger among fish eaters vs meat eaters (RR = 0.82; See extended table).

Red meat and total cancer.

Total cancer risk: Data was provided by 2 cohorts, including 5,125 cases. No associations were found.
Total cancer mortality: Data was provided by 4 cohorts, including 27,501 cases. A significantly increased risk was found among both men and women in one cohort of very large size (Sinha R [14]). Noticeable is the fact that the risk increased gradually with every quintile of consumption in this cohort. No other associations were found. The average RR = 1.20. This effect size is primarily driven by one single cohort (Sinha R [14]).
Conclusion: A significantly increased cancer mortality risk was found in one cohort of very large size. No other associations were found. Even though the effect size suggests an increased mortality risk of red meat consumption, consistency is low. Inconclusive evidence was found for an association between red meat consumption and total cancer mortality. No associations were found with total cancer risk.

Processed meat and total cancer.

Total cancer risk: No data was found.
Total cancer mortality: Data was provided by 4 cohorts, including 30,931 cases. A significantly increased risk was found among both men and women in one cohort of very large size (Sinha R [14]). The average RR = 1.09. This effect size is primarily driven by one single cohort (Sinha R [14]).
Conclusion: A significantly increased cancer mortality risk was found in one cohort of very large size. No other associations were found. Inconclusive evidence was found for an association between processed meat consumption and total cancer mortality. No data was found about the relation with total cancer risk.