Dear Monica, I enjoyed your post on diet and breast cancer. However, I cannot believe that you did not mention the direct relationship between the intake of casein (milk protein) and the growth of mammary tumors. T. Colin Campbell in his The China Study outlines how this protein turns on tumor growth and the lack of it in the diet turns off cancer growth. “
Before saying anything else, let me first say that I completely support anyone who does not care to consume dairy products, for whatever reason. Dairy is certainly not essential to a healthy diet. There are plenty of other ways to get calcium and vitamin D. (And those who do not consume dairy need to take care to be sure they do.) I recently did a podcast episode on the pros and cons of dairy. You can listen to it here.
Now, to Campbell’s book: Lots of people commenting on this blog over the years have referenced this book as a definitive scientific rationale for various dietary practices. Obviously, Campbell’s prose is compelling. But I have to be honest with you: The science behind his conclusions is less so.
You’re right: There are recent (2007) studies showing that milk increases the incidence of chemically-induced breast tumors in rats. Interestingly, I also found a 2007 study showing that soy milk does the same thing. And another (2006) showing that fermented milk (yogurt) prevented tumors; and another (2001) showing that soy protein was preventive.
It appears that studies on things that cause or prevent tumors in rats injected with carcinogens might not provide a definitive answer to the question: Do dairy products cause breast cancer in humans? For that, it makes sense to turn to studies that compare what people eat to their risk of breast cancer.
Do dairy products increase breast cancer risk in humans?
The so-called “China Study” was a nutritional analysis conducted in rural China in the 1980s. This study purportedly found a link between the intake of animal protein and an increased risk of cancer and other disease. In the intervening 20 years, many researchers have tested this conclusion, specifically with regards to dairy and breast cancer. Here’s a brief sampling:
2007 American Journal of Clinical Nutrition (5000 subjects in England and Scottland): Childhood dairy intake was not associated with breast cancer risk.
2007 Cancer Epidemiology (2000 subjects in U.S.): Reduced breast cancer risks were associated with increasing milk consumption from ages 10-29, probably because of the cancer-preventive effects of vitamin D.
2006 Cancer Causes and Controls (5000 subjects in Italy): Consumption of milk and diary products did not increase breast cancer risk (and, in fact, consumption of skim milk slightly reduced risk).
2005 Journal of the American College of Nutrition (meta-analysis of 52 different studies): Evidence does not support an association between diary product consumption and the risk of breast cancer.
2005 Nutrition and Cancer (study looking back 30 years and across 38 countries): No substantial effect of milk consumption on risk of breast (and other) cancers.
2004 American Journal of Clinical Nutrition (meta-analysis of 46 studies): No strong association between the consumption of milk or other dairy products and breast cancer risk.
2002 Journal of the National Cancer Institute (90,000 women followed for 16 years): “We found no association between intake of dairy products and breast cancer in postmenopausal women. Among premenopausal women, high intake of low-fat diary foods was associated with a reduced risk of breast cancer.”
2002 International Journal of Epidemiology (meta-analysis of 8 prospective studies involving 350,000 subjects in N. America and W. Europe): “We found no significant associations between intake of meat or dairy products and risk of breast cancer.”
Dairy is not essential to a healthy diet but it does not appear to cause breast cancer
Obviously, I would have had to have continued for quite a bit longer to work my way back to the China study. But there didn’t seem to be much reason to continue. (And I wasn’t cherry-picking, either…these are the studies that came up in response to my query!)
So, let me end this post the way I began: Dairy products are not essential to a healthy diet. Feel free to eliminate them. But I don’t actually see evidence to suggest that doing so will reduce your risk of breast cancer. And, in fact, because dairy products are one of the only sources of vitamin D in the American diet, eliminating them might well increase your risk if you’re not careful to get that nutrient from other food sources.