Hormones and growth factors in dairy increase cancer risk.
by Ivan Oransky, The Scientist
February 2007
http://www.the-scientist.com/article/home/43585/
Article Summary: Cow's milk contains steroid hormones such as estradiol and testosterone, and peptide hormones such as IGF-1. Drinking milk has been shown to boost serum levels of certain hormones, particularly IGF-1, in humans. High levels of certain hormones, particularly IGF-1, have been shown to increase the risk of certain cancers. Some epidemiologists have connected those three dots and have suggested that cow's milk increases the risk of cancer. Large epidemiologic studies have appeared in major journals, reporting that prostate cancer -- particularly aggressive forms -- seem to be associated with dairy intake, and perhaps more strongly with total calcium intake. Such intake may double or triple the risk of aggressive prostate cancers, which kill about 2-3% of men. From this study and others presented at a McGill and Harvard-sponsored conference, Harvard epidemiologist Walter Willett to conclude that current US dietary guidelines, updated in 2005, promote too much milk. "I think it's not wise to recommend three [8-ounce] glasses per day for adults. Probably, a serving a day is OK; I don't see much reason that would be harmful. I'm concerned about two glasses a day, and three has a strong potential for harm."
While no one really disagrees that drinking milk is associated with higher serum levels of IGF-1, the mechanism is the subject of debate. There are also many other factors that are raising levels of IGF-1, including genetic differences. Other problems with associating hormone levels with cancer include difficulty in measuring hormones and a lack of a relationship in epidemiologic studies. And there are benefits to milk, including the calcium that can prevent osteoporosis; a weak protective effect against colorectal cancer; and decreased risk of fracture, insulin resistance, and stroke. Also unknown is whether the selective breeding of cows has selected for endocrine variants in modern cows. On average, cows are producing six times the milk they did in 1900.
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