The answer to that question is unknown, although it may have its origins in the underbelly of the American food system. Let us unravel how red meat consumption may be associated with breast cancer risk. The U.S. Nurses’ Health Study II provides some striking evidence: compared with women who ate one serving of red meat per week, women who ate 1.5 servings of red meat per day had a 22% increased risk of breast cancer (2).
What was done: the U.S. Nurses’ Health Study II
This was a longitudinal cohort study of 116,430 female registered nurses who were 24 to 43 years of age when the study began. In 1991, the nurses filled out a validated food frequency questionnaire, which asked about their usual dietary intake and alcohol consumption in the past year. “Red meat” items were defined as:
- Hot dogs
- Salami, bologna, and similar deli meats
The nurses were followed-up until 1 June 11, or date of breast cancer diagnosis or death, if either of those came first.
What was found: the association between red meat consumption and breast cancer risk
Among all women, those who ate red meat 1.5 times per day had a 22% increased risk of breast cancer, compared with those who ate red meat once per week.
The risk of breast cancer increased by 13% per additional daily serving of red meat.
This association was independent of other important breast cancer risk factors, such as age, smoking status, oral contraceptive use, childbirth factors, body mass index, alcohol intake, and caloric intake.
The really concerning finding comes here:
The risk of breast cancer increased by 54% per additional daily red meat serving among current oral contraceptive users.
This figure is a substantial increase to the above-cited 13% risk increase for users and non-users combined.
On a positive note, the authors observed that poultry consumption before menopause reduced post-menopausal breast cancer risk. Each additional daily serving of poultry was associated with a 25% lower risk of post-menopausal breast cancer. They also observed that substituting servings of red meat with servings of poultry, legumes, nuts, and fish was associated with reduced breast cancer risk.
What else do we know?
The authors conducted follow-up research after this study was published. Using the same dataset, they hypothesized that eating red meat earlier in life would be particularly bad in terms of breast cancer risk (2, 3). In an interview with the Harvard School of Public Health, Maryam Farvid, the lead investigator, said:
We developed this hypothesis based on the results from atomic bombings of Hiroshima and Nagasaki. Girls and young women who were exposed to this radiation had a higher risk of breast cancer later. But women who were exposed at age 40 or older did not have an increased risk.
True to form, consumption of red meat during adolescence was associated with a higher risk of pre-menopausal breast cancer (developing earlier in life), but not post-menopausal cancer (developing later in life) (3). Mammary glands appear to be more susceptible to carcinogenic factors during development and growth; red meat consumption appears to be no exception.
Why is red meat a potential cause of breast cancer?
The authors provide two mechanistic hypotheses. The first is through the carcinogenic by-products of cooking meat at high temperatures (i.e. grilling or barbequing). The second mechanism that the authors propose is through ‘hormone residues of the exogenous hormones for growth stimulation in beef cattle’ (1).
This latter hypothesis – hormone residues in beef cattle – is intriguing and alarming. Unfortunately, the authors do not expand on the policy implications. The topic is certainly political and has huge implications for all women (and men) consuming red meat in the United States. Studies from international contexts with different cattle industry regulations would be useful for comparison with this study in the American context.
The risk associated with eating red meat appeared to be the highest among women who took oral contraceptive pills. Adolescence appeared to be a critical time period in life where the effects of eating red meat were the strongest. How do we explain these results, given that several breast cancer risk factors are hormonal in nature, or thought to act through hormonal pathways? Whether hormone residues or some other factor related to the meat is the culprit, some kind of biological interaction appears to be occurring between red meat and sex hormones.
Given the ubiquitous consumption of red meat and oral contraceptive pills, these issues demand attention.
1) Farvid MS, Cho E, Chen WY, Eliassen AH, Willett WC. Dietary protein sources in early adulthood and breast cancer incidence: prospective cohort study. BMJ 2014;348:g3437
2) Harvard School of Public Health. News: Red meat consumption and breast cancer risk. http://www.hsph.harvard.edu/news/features/red-meat-consumption-and-breast-cancer-risk/ (accessed 5 November 2014).
3) Farvid MS, Cho E, Chen WY, Eliassen AH, Willett WC. Adolescent meat intake and breast cancer risk. Int J Cancer 2014; Published Online First 15 September 2014: doi: 10.1002/ijc.29218
Image source: Lou Ferrigno