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Does Dairy Consumption Lead to an Increased Risk of Breast Cancer?
A Research Review by Emily Hertzberg
October 24, 2008
What’s all the hype about?!?
In the past few decades’ breast cancer has become the most invasive form of cancer in the developed world. Research has shown that most breast cancers are “sporadic,” while only five to ten percent of cases are due to inherited genes. The “sporadic” cases are caused by “certain cells that undergo mutations that leads to cancer” (Bissonauth, 2008, p. 52). Radiation, sun exposure, increased breast density, obesity, and certain carcinogenic chemicals often cause these mutations in the DNA. Each year, more and more risk factors are investigated for their role in tumor development. The more risk factors that are identified, the more hope exists among women that breast cancer can be avoided by certain life style changes. Environment may play an even larger role than genetics in its impact on the rate of breast cancer occurrences.
A surge of research has been published on risk factors of breast cancer in the last ten years. One main risk factor that has been questioned is the correlation between dairy consumption and breast cancer. There seems to be inconsistencies and contradictions among the research that has led to a greater controversy in the medical field. Originally, several prospective studies from the late 1990’s found a positive correlation between high-fat dairy products and breast cancer. But in later, more current case-studies and epidemiological studies have found no significant association between dairy consumption and breast cancer, and most have actually found an inverse relationship between the two, stating that the calcium and vitamin D function as protection factors against tumor growth. The research is split and it is difficult to disregard the older research just because it is contradicted by updated research. Therefore, it is necessary to examine both sets of research in order to understand the discrepancies in each study.
Why the hypothesis makes sense- IGF-1:
The original hypothesis states that dairy intake is positively related to the risk of breast cancer was not completely ill founded. The basis for this claim is that dairy has a high concentration of fat, saturated fatty acids, insulin-like growth factors (IGF-1), estrogenic hormones, and (bovine) growth hormones (bGH) (Parodi, 2005). Outwater, Nicholson, & Barnard (1997) developed the “IGF-I, estrogen, and bGH hypothesis,” which states that the three factors play a role in the positive correlation between dairy consumption and breast cancer.
To further the reason to look back at older research, a study was conducted on the relationship between diet and IGF-I levels. IGF-1 or insulin-like growth factor -1 is a growth factor central to the regulation of growth processes in the human body. The level of IGF-1 circulating in the body is usually a result of genetics and age, but can also be affected by diet. The mechanism of IGF-1 (at the time the article was written) had not been clearly explained, but researchers believed it may play a role in tumor growth by “enhancing cell proliferation and inhibiting apoptosis [cell death]” (Norat et al., 2006, p. 92). Results indicated that high levels of circulating IGF-1 were positively related to the intake of milk and cheese, but not with yogurt (usually of a lower-fat content). Women who drank more than 400 g/day of milk had a mean difference of IGF-1 levels of 8% greater than the group of women who drank less than 25.1 g/day (Norat et al., 2006).
Another prospective study hypothesized that high circulating IGF-1 levels would be positively associated with a high risk of break cancer. Hankinson, Willett, Colditz, et al. (1998) found that in the whole group study, there was no association between IGF-1 levels and breast cancer risk, but when they looked at pre-menopausal women, they found a relative risk of 2.33 overall, and 4.58 for women who were less than fifty years of age at the time of plasma collection. This observation led researchers to look at the differences in IGF-1 levels of pre- and post-menopausal women and found that naturally, IGF-1 levels decrease with age. When one looks at these two studies together, it seems likely that a positive correlation exists between dairy consumption and an increased risk of breast cancer. If milk or cheese consumption causes an increase in circulation IGF-1 concentrations (Norat et al., 2006), and another study found a positive correlation between high levels of IGF-1 and an increased risk of breast cancer (Hankinson et al., 1998), then through the associative property, milk/cheese consumption leads to higher risk of developing breast cancer.
Why the hypothesis is refuted:
The overall findings from several current epidemiologic studies do not support a strong enough of association between dairy consumption and an increased risk of breast cancer. Some associations were found but lost significance when other factors were accounted. In Moorman and Terry (2004), when referring to the earlier studies that had found a positive relationship, the later is addressed as “less than compelling” (p. 12).
In a meta-analysis, Parodi found that the amount of IGF-1 consumed daily from milk products is minute when compared to the amount produced by the human body (2005). He claimed studies show convincing evidence that the IGF-1 found in dairy product is not part in the etiology of breast cancer in humans. The IGF-1 mechanism is very complex but researchers believe that cancer is more likely influenced by secretion of IGF-1 naturally in the body by “autocrine/paracrine secretions” (Bissonauth, 2008, p. 54), rather than circulating levels from ingestion.
Almost all the epidemiological studies found an inverse correlation between dairy consumption and breast cancer. Dairy contains calcium and vitamin D that have been studied for their anti-carcinogenic effects. Calcium plays a role in cell proliferation and differentiation and vitamin D is a regulator in the absorption and metabolism of calcium in the body (Moorman & Terry, 2004). Butyric (found only in dairy) and Linoleic Acids have also been found to be anti-cancer agents. Both induce cell differentiation and regulate cell death. (Bissonauth, 2008).
Statistically, almost all recent studies have found insignificant relative risk due to dairy consumption.
Vitamin D Butyric Acid
This is not the end… Still more questions to be answered:
The debate over the proposed question: does dairy consumption lead to an increased risk of breast cancer is not over. Although a majority of printed research agrees that dairy products have more of an inverse relationship, or act as a protection factor against tumor development, there are still some researchers who believe otherwise. There is evidence from a rat study that show that milk consumption promotes development of DMBA-induced mammary tumors. The study divided rats into four groups, whole milk, artificial whole milk, non-fat milk, and artificial non-fat milk. The results found a positive association with whole milk and non-fat milk (Li-Qiang et al., 2007). More research must be done in order to replicate the findings with humans.
There have also been several common problems identified throughout all the research. The assessment of dairy intake is formulated from self-questionnaires, food diaries, and diet records, which can often lack reliability and can lead to misclassification. Small misclassifications in dairy intake can impact the significance of the findings. A small missing piece of a persons diet can skew the results.
Also, one must not overlook the dietary habits of someone who ingests a high dairy diet. People who eat a diet high in fat usually are little preoccupied by other health woes. Someone who drinks a lot of whole milk may also consume large amounts of butter, other high-fat foods, and may tend to eat a lot of “unhealthy” food. On the other side of the spectrum, people who consume less dairy or low-fat diets may also be more conscious of their diet and may tend to consume more fruits, vegetables, and whole grains. It is difficult to separate the role of dairy from the other dietary habits and be able to identify which part of ones diet is increasing the risk of breast cancer.
More studies must be done to look at the effects of small diet changes. I also think it would be useful to study more about IGF-1 and settle it’s effects on the human body. Recently, many people have been switching to organic milk, which claims to only use dairy cows without added hormones or antibiotics. It would be useful to perform a prospective study to identify differences in drinking organic versus regular milk and their individual health implications. For now, according to the given research, one does not have to stay away from dairy, in fact it may be beneficial to drink the FDA suggested amount of milk for it’s other health benefits, but like my mother always said, “Most things can be good, in moderation.”
Bissonauth, V., Shatenstein, B., & Ghadirian, P. (2008). Nutrition and breast cancer among sporadic cases and gene mutation carries: An overview. Cancer Detection and Prevention, 32, 52-64.
Hankinson, S.E., Willett, W.C., Colditz, G.A., Hunter, D.J., Michaud, D.S., Deroo, B. & et al. (1998). Circulating concentrations of insulin-like growth factor 1 and risk of breast cancer. The Lancet, 351, 1393-1396.
Li-Qiang, Q., Jia-Ying, X., Hideo, T., Jue, L., Jun, A., Kazuhiko, H., et al. (2007). Consumption of commercial whole and non-fat milk increases the incidence of 7,12-dimethylbenz (a) anthracene-induced mammary tumors in rats. Cancer Detection and Prevention, 31, 339-343.
Moorman, P. G., & Terry, P.D., (2004). Consumption of dairy products and the risk of breast cancer: a review of literature. The American Journal of Clinical Nutrition, 80, 5-14.
Norat, T., Dossus, L., Rinaldi, S., Overvad, K., Gronbaek, H., Tjonneland, A., et al. (2007). Diet, serum insulin-like growth factor-1 and IGF-binding protein-3 in European women. European Journal of Clinical Nutrition, 61, 91-98.
Outwater, J.L., Nicholson, A., & Barnard, N. (1997). Dairy products and breast cancer: the IGF-1, estrogen, and bGH hypothesis. Medical Hypotheses, 48, 453-461.
Parodi, P.W., (2005). Dairy product consumption and the risk of breast cancer. Journal of the American College of Nutrition, 24(6), 556S-568S.
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