Answer to a Reader's Question:
"In ecologic data, correlations exist between per capita meat...consumption and prostate cancer mortality rate [1 study cited]. In case control and prospective studies, the major contributors of animal protein, meat, dairy products and eggs have frequently been associated with a higher rate of prostate cancer...[23 studies cited]." (quoted from Giovannucci, E. Dietary influences of 1,25 (OH)2 vitamin D in relation to prostate cancer: a hypothesis. Cancer Causes and Control, 9: 567-582, 1998).
Here is a quote from my book (Campbell, TC and Campbell TM II, The China Study, Startling Implications of Diet, Weight Loss and Long-Term Health, 2005, 417 pp.):
"Under unhealthy conditions...IGF-1 becomes more active, increasing the birth and growth of new cells while simultaneously inhibiting the removal of old cells, both of which favor the development of cancer [7 studies cited]. ...consuming animal-based foods increases the blood levels of this growth hormone, IGF-1 [3 studies cited].
With regard to prostate cancer, people with higher than normal blood levels of IGF-1 have been shown to have 5.1 times the risk of advanced-stage prostate cancer...when men also have low blood levels of a protein that binds and inactivates IGF-1 [1 study cited], they will have 9.5 times the risk of advanced stage prostate cancer....fundamental to this finding is the fact that we make more IGF-1 when we consume animal-based food like meat...[3 studies cited]."
In effect, these data, summarized from multiple studies and including BOTH observational data AND mechanistic explanations is far more reliable than citing an occasional study. This kind of analysis also becomes much more impressive because it refers to a general property of a group of foods, in this case animal-based foods of which meat is a major component. And finally, an increased risk of 9.5 times is huge, really quite extraordinary.
A very similar situation exists with breast cancer. For example, there is a very strong relationship between breast cancer rates and the consumption of animal-based foods when comparing various countries [Carroll, K. K. Cancer Res., 35: 3374-3383, 1975; Carroll, K. K. and Khor, H. T. In: Progress in Biochemical Pharmacology: Lipids and Tumors, Vol. 10, pp. 308-345. New York: S. Karger, 1975]. Although this does not prove a cause-and-effect relationship, it nonetheless becomes much more convincing when we consider the mechanism by which meat and other animal based foods work. For example, animal based foods increase blood (circulating) levels of estrogen and other female hormones. According to one of the leading breast cancer research groups in the world, they say this about estrogen and breast cancer:
"...there is overwhelming evidence that estrogen levels are a critical determinant of breast cancer risk" [Pike, M. C., et al. Epidemiol. Revs., 15: 17-35, 1993; Bernstein, L. and Ross, R. K. Epidemiol. Revs., 15: 48-65, 1993] and furthermore, estrogen directly participates in the cancer process [Bocchinfuso, W. P., et al. Endocrinology, 141: 2982-2994, 2000; Atwood, C. S. et al J. Endocrinol., 167: 39-52, 2000]".