Research is done to test a hypothesis (McGuire & Beerman, 2018). You may or may not have heard about a research study done in 2014 that looked at dietary protein intake and breast cancer rates. The research article written by Farvid, Cho, Chen, Eliassen, & Willett (2014) stated that protein sources in pre-menopausal women are important when it comes to breast cancer incidence later in life. There is certainly merit to this statement, and this hypothesis has been studied since this article was published. Personally, however, I would argue that while protein sources are important, consuming red meat is more of a risk factor for breast cancer versus a definitive statement that red meat consumption does cause breast cancer. These types of studies come down to causative and correlative relationships, simple and complex relationships or interactions, and lifestyle, environmental, and genetic factors. Although this study was well-designed and thorough, admittedly there are many other factors involved, so one cannot state conclusively that red meat indeed causes breast cancer. In fact, researchers stated that red meat intake during early adulthood was associated with an increased risk of breast cancer in premenopausal women” (Farvid et al., 2014). In my opinion, the key word here is “risk”. 

In 1989, researchers Farvid, Cho, Chen, Eliassen, & Willett (2014) started with a sample size of 116,460 female registered nurses ages 24 to 43. In 1991, 97,813 participants answered the food frequency questionnaire regarding dietary intake of common foods over the previous year. To try to reduce error, they excluded those that had either extremely low or extremely high caloric intake ranges, diagnosis of cancer, heart disease, stroke, or diabetes prior to returning the questionnaire in 1991, or inconclusive data on age or consumption of red meat. The remaining 88,803 female nurses were included in the results of the study. This food frequency questionnaire was given on five separate years and adjustments were made based on changes in food supply and serving sizes.       

The results of the Nurses’ Health Study II cohort proved a strong/positive correlative link between red meat consumption early in life and breast cancer incidence later in life. However, from this study, we cannot state conclusively that there is a definite cause-and-effect/causative relationship. These results were complicated by the complex relationships that the researchers found throughout the duration of the study with the food frequency questionnaire and the assessment of other variables. These assessments did not find a simple relationship, which would have been only studying red meat consumption and rates of breast cancer. Instead they found a variety of other factors that may have interacted to increase the risk of breast cancer in women who were eventually diagnosed. For example, “For each serving/day of total red meat, the risk of breast cancer was 54% higher among women who currently used oral contraceptives (relative risk 1.54, 1.13 to 2.08) and 11% higher in women who were former users (1.11, 1.02 to 1.22), with no association in non-users (1.04, 0.84 to 1.28)” (Farvid et al., 2014, p. 4). Based on the correlative nature of the study, one could argue that oral contraceptive use is a risk factor for breast cancer as well - with or without consumption of red meat.     

Overall, this seems to be a strong and reliable study with important findings and implications. It was carried out well in a manner that was easy to collect data on their large sample size with the food frequency questionnaires. The researchers also automatically excluded those that were diagnosed with cancer and other diseases prior to returning the study (Farvid et al., 2014). This shows the researchers weren’t just looking for ways to influence the hypothesis of their study - especially if it was determined that those diagnosed did in fact consume higher amounts of red meat. It was also important for researchers to adjust the list of foods in the food frequency questionnaire over the years (Farvid et al., 2014). This could certainly skew the results if an abundance of unhealthy foods became more common over the years but were not added to the list for participants to report their consumption habits. Further adding to the validity of the study was that many other risk factors for breast cancer were collected and noted from the participants. These risk factors included “age, height, weight, family history of breast cancer, history of benign breast disease, smoking, race, age at menarche, parity, age at first birth, menopausal status, postmenopausal hormone use, age at menopause, and oral contraceptive use” (Farvid et al., 2014). Since these factors are already known to increase the chance of breast cancer, it was certainly important to note any inherent risk factors for it such as the varying lifestyle, environmental, and genetic factors. Again, this is important to note with or without consumption of red meat. Finally, it was important to include several types of protein sources to determine any positive or negative correlations of any type of protein. Researchers differentiated between red meat and other protein rich foods such as, poultry, fish, eggs, legumes, and nuts” to determine any possible link or association in the incidence of breast cancer in these women (Farvid et al., 2014). From these results, it was determined that when replacing even one serving of red meat for one of the “other protein rich sources” participants had a lower incidence of breast cancer (Farvid et al., 2014). Therefore, although these findings prove to be important, how they all may work together to potentially cause breast cancer is yet to be determined. In addition, while there are many factors that make this a strong study, it is not strong enough to definitively state that red meat consumption is more than just a risk factor for breast cancer.       

One of the biggest issues I have with this study is with the food frequency questionnaire. Even though potential errors tried to be minimized, relying on participant recall of foods and serving sizes from their adolescence for an entire year is not a very accurate way of gathering data. Current research states, “Under-reporting of EI [energy intakes] is a consistent finding when using dietary assessment methods which rely on self-reports of food and drink intake” (Foster et al., 2019). Even when given lists of common foods, under-estimates of serving sizes are common and made even more difficult when trying to remember specific foods and specific serving sizes from many years prior. For a study that relies on accurate data, perhaps a longitudinal study having participants keep track of daily food intake would have been a better option. This would likely have eliminated risk of excluding foods unintentionally (just because they may have forgotten what and how much they ate throughout one year of their life) and allowed researchers to get a better idea of what the participants were actually eating and how much throughout the year in question. 

Another issue that makes this a weaker study is that researchers ended up making multiple comparisons between many factors related to incidence of breast cancer. Because of these multiple comparisons, there is a potential for type I errors. Type I errors tend to show a “false positive” correlation for the hypothesis. Therefore, while researchers found a strong, positive correlation between red meat and breast cancer incidence, there are many other factors that may have played a part in these rates. Furthermore, one single study is difficult to fully answer 100% of questions that arise. Oftentimes, follow-up studies need to be done to adequately answer (and perhaps prove or disprove the hypothesis). Perhaps, to know that there were no other interactions, a study would have to feed individuals a diet rich in red meat with little else and test inflammatory markers. There are certainly ethical issues with this, and the goal would never be to cause breast cancer for the sake of research. However, testing for inflammatory markers such as C-reactive protein (CRP) and interleukin 6 (IL-6), which is commonly associated with breast cancer risk (Mills, 2017) may show more of a causal relationship compared to the correlative relationship this research found. 

Lastly, this study focused on a relatively select group of participants. The participants were female nurses and predominantly white. Although the sample size was very large, perhaps a more diverse group of ethnicities and backgrounds would have led to more conclusive results that may be applied across a broader range. Or may have even produced results that may be specific to certain demographics. In addition, because there was such a large group of participants, we do not know specifics about anyone filling out the questionnaire. Since we do not know more about each participant and because their diet was not controlled to eliminate other factors involved, we cannot determine if red meat consumption alone is what led to breast cancer for some. Finally, exercise is “considered by many authors to be a factor reducing the risk of cancer development and premature cancer-related death” (Siewierska, 2018). Therefore, it would be helpful to know what the participants' exercise habits looked like and how this may have altered the rate of breast cancer in these women.  

In order to state conclusively that red meat causes breast cancer, researchers would need to remove all other possibilities and potential interactions. The Nurses’ Health Study II cohort found that those who ate less red meat and substituted at least one serving of red meat for other protein sources had lower instances of cancer. Although, due to the number of comparisons that were made, there is a possibility of type I errors throughout the study, which cannot make this statement conclusive. Therefore, while I do agree that consumption of red meat should be limited, I do not think that consumption of red meat early in life automatically means that one will get breast cancer later in life. There seem to be many more factors involved. While this seems to be a highly correlative relationship, I do not believe it is 100% causative. What are your thoughts?    

References

Farvid, M. S., Cho, E., Chen, W. Y., Eliassen, A. H., & Willett, W. C. (2014). Dietary protein sources in early adulthood and breast cancer incidence: prospective cohort study. BMJ (Clinical research ed.), 348, g3437. https://doi.org/10.1136/bmj.g3437

Foster, E., Lee, C., Imamura, F., Hollidge, S. E., Westgate, K. L., Venables, M. C., Poliakov, I., Rowland, M. K., Osadchiy, T., Bradley, J. C., Simpson, E. L., Adamson, A. J., Olivier, P., Wareham, N., Forouhi, N. G., & Brage, S. (2019). Validity and reliability of an online self-report 24-h dietary recall method (Intake24): a doubly labelled water study and repeated-measures analysis. Journal of nutritional science, 8, e29. https://doi.org/10.1017/jns.2019.20

McGuire, M., & Beerman, K., (2018). Nutritional Sciences: From Fundamentals to Food (3rd ed.). Boston, MA: Cengage Learning. 

Mills R. C., 3rd (2017). Breast Cancer Survivors, Common Markers of Inflammation, and Exercise: A Narrative Review. Breast cancer : basic and clinical research, 11, 1178223417743976. https://doi.org/10.1177/1178223417743976

Siewierska, K., Malicka, I., Kobierzycki, C., Paslawska, U., Cegielski, M., Grzegrzolka, J., Piotrowska, A., Podhorska-Okolow, M., Dziegiel, P., & Wozniewski, M. (2018). The Impact of Exercise Training on Breast Cancer. In vivo (Athens, Greece), 32(2), 249–254. https://doi.org/10.21873/invivo.11231