Harvard’s Physicians’ Health Study followed 20,885 men for 11 years and found that having 2.5 dairy servings each day boosted prostate cancer risk by 34%, compared to having less than half a serving daily. Studies have found that dairy boosts the amount of insulin-like growth factor (IGF-I) in the blood which promotes cancer cell growth.
It’s not surprising that milk might affect the growth of cancer cells when you think about the fact that dairy milk’s sole purpose is to support the rapid growth of a newborn calf. After the age of weaning, calves (like all mammals) have no need for milk at all. Humans are the only mammals who drink the milk of another species after being weaned.
Other studies have suggested a link between dairy and breast cancer as well. One study found that for women who have had breast cancer, just one serving of dairy per day can increase their chance of dying from the disease by 49%.