Some nutritionist and epidemiologists believe that eating red meat and processed meats may increase the risk of cancer. They are not sure because many of the studies of this subject have been contradictory. However, the belief “that eating anything that tastes that good has to be bad” is always bouncing around in their heads. So they keep looking.
This topic is important because the news media love to tell their audience what they should and should not eat. Especially when the scientists say it could cause cancer. Important note: Often the word could gets left out and the headline reads, “EATING STEAK CAUSES BREAST CANCER”.
Cho and co-workers in 2006 set out to see if the occurrence of breast cancer in pre-menopausal women ages 26 to 46 was increased by eating red meat. This was done even though this same group had concluded from their previous analysis of 20 different studies that eating red meat was not associated with increased risk of breast cancer in either pre-menopausal or post-menopausal women (Missmer et al 2002).
But, they might have overlooked something. So they examined the data from 90,659 women over a period of 12 year and documented 1021 cases of invasive breast cancer (Cho et al 2006). Once again they found no association between red meat intake and the relative risk of breast cancer when all cancer were grouped together in agreement with their earlier analysis (Missmer et al 2002; Fig 1-A). Not giving up that easily they decided to divide the women with breast cancer into two groups: Those whose cancers did not contain estrogen and progesterone receptors (Fig 1-B) and those whose cancers did contain these hormone receptors (Fig 1-C). No association between intake of red meat and breast cancer in the women with receptor negative cancers was observed.
The authors then examined the possibility of any association in receptor positive group. They claim their data show the possibility of an increased risk of breast cancer in the women with the greatest intake of red meat (category 5 in Figure 1-C). However the lack of any upward trend in all of the groups with lower intake (2,3 & 4) makes it likely that the value for category 5 is a statistical fluke and is not significant. Certainly the over all evidence for any increased risk is marginal and as Shapiro (1997) has pointed out, when relative risks (RR) are 2 or lower they do not have the resolving power to distinguish between confounding factors and causation. In other words the RR value of 1.97 with a 95% confidence interval of 1.35 to 2.88 is of no real significance and certainly does not say anything about effects of eating red meat.
It seems obvious to me that when the RRs for groups 2 – 4 show no upward trend and the value for group 4 is lower than group 3 and sudden increase as in group 5 is likely to be due to chance.
Figure 1 The relative risks of breast cancer as a function of red meat intake. These data are taken from Table 2 in Cho et al 2006. Category 1 is the lowest intake and 5 is the highest. Error bars represent 95% confidence intervals and dashed line is the no effect level.
Intake categories are the following:
Servings of red meat in each category
1. 3/wk or less
2. > 3/wk to 5/week or less
3. > 5/week to 1/day or less
4. > 1/day to 1.5/day or less
5. > 1.5/day
You might be wondering why anyone would even think that red meat causes breast cancer other than the belief cited above that “anything that good, must be bad”. One is the possibility that red meat might contain sufficient estrogenic hormones to stimulate breast cancer growth. Most scientist believe the exceedingly low amounts of estrogen in red meat would not make any significant contribution to the amount of estrogen present in body of a pre-menopausal women. The ovaries of these women produce amounts of estrogen which far exceed any small amount of some estrogenic substance found in the diet.
Another potential culprit in the epidemiologist’s arsenal are heterocyclic amines (HCA). These are formed when meat is cooked and over cooked and they have been shown to be carcinogenic in some animal models. However when put to the test in human studies they fall short. For example, one of the studies quoted in their paper which reportedly shows the HCAs in red meat are associated with breast cancer was done by Shina et al 2000. The results of this study do not show any association between HCAs and breast cancer for two of the three amines studied (Figure 2A and 2B) and only a marginal, and I believe insignificant, association between 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and cancer (Figure 2C).
Figure 2. Relative risk of breast cancer as a function of heterocyclic amines (HCA) in the diet. The intake categories represent the estimated amount of HCA in the diet. Category 1 is the lowest and 5 is the highest. Error bars represent 95% confidence intervals and dashed line is the no effect level.
Another possible dietary substance which might be blamed is iron. Iron is a very important constituent of hemoglobin and myoglobin and plays a role in other necessary physiological functions. However, iron has been shown in animal studies to increase mammary cancers when the animals are fed large amounts of iron in the diet and when an iron compound is injected under the skin in rats (Liehr & Jones 2001). Subcutaneous injections and dietary levels of iron which are 10 or more times the normal levels are non-physiological and would be very likely to ever occur in humans. Certainly it is difficult to imagine that 1.5 serving/day of red meat could contain a sufficient quantity of iron to make any difference in the amount of iron present in a woman’s body.
CONCLUSION
I believe the evidence does not support the idea that eating red meat is associated with an increased risk of breast cancer.
REFERENCES
Cho E et al 2006 Red meat intake and risk of breast cancer among premenopausal women. Arch Intern Med 166 2253-2259.
Liehr JG, Jones JS 2001 Role of iron in estrogen-induced cancer. Curr Med Chem 7:839-49.
Shapiro S 1997 Do trans fatty acids increase the risk of coronary artery disease? A critique of the epidemiologic evidence Am J Clin Nutr 66(suppl):1011S-7S
Sinha R et al 2000 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, a carcinogen in high-temperature-cooked meat, and breast cancer risk. J Natl Cancer Inst. 2000 92(16):1352-4.
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