Ask the Expert: Dairy Products
Q: Do dairy products cause cancer? And if you don’t drink milk, how do you get all the calcium you need?
A: Recent scientific studies have suggested that dairy products may be linked to increased risk for prostate cancer, testicular cancer, and possibly for ovarian and breast cancers.
Prostate cancer has been linked to dairy products in several studies. In Harvard’s Physicians Health Study, including more than 20,000 male physicians, those who consumed more than two dairy servings daily had a 34% higher risk of developing prostate cancer than men who consumed little or no dairy products. Several other studies have shown much the same thing.
Several mechanisms have been proposed to explain this association. Dairy product consumption increases levels of insulin-like growth factor I (IGF-1) in the bloodstream. IGF-1 is a potent stimulus for cancer cell growth. High IGF-1 levels are linked to increased risk of prostate cancer and breast cancer.
Calcium may also play a role in prostate cancer. Although the body needs some calcium, a high calcium load results in a lowering of blood levels of activated vitamin D. In the case of prostate cancer, where vitamin D is thought to be protective, high dairy intake could therefore have a harmful effect.
For ovarian cancer, galactose, a component of the milk sugar lactose, has been under study as a possible culprit. A recent analysis of studies examining a relationship between dairy product consumption and ovarian cancer risk found that for every 10 grams of lactose consumed (the amount in one glass of milk), ovarian cancer risk increased by 13 percent.
In contrast to these findings, a 2001 Norwegian study concluded that milk consumption reduces breast cancer risk in premenopausal women. However, breast cancer is uncommon in premenopausal women (only 0.6 percent in this study). Also, the study relied solely on self-reported milk intake during adulthood and the recollection of milk intake during childhood, not on key blood markers related to breast cancer risk or overall dairy product intake.
The nutrient that was hypothesized to be protective was conjugated linoleic acid (CLA), a component of dairy fat that has been shown to increase risk for heart disease. A number of other studies have shown either no association between breast cancer incidence and dairy product consumption or an increased risk with dairy intake.
In Asia, where whole grains, vegetables, fruits, tofu, soymilk, and other soy products are commonly consumed and milk is not a normal part of the diet, people are generally healthier and breast cancer is much rarer than in the United States and Europe.
So, what calcium sources are most healthful? A moderate amount from a variety of plant sources seems to be best. There’s plenty of easily absorbed calcium in dark leafy greens, such as bok choy, kale, mustard greens, collard greens, and turnip greens, as well as broccoli, dried beans, figs, almonds, calcium-fortified juices, and soymilk and other non-dairy milks. Plus, these foods contain other cancer-fighting nutrients that aren’t present in dairy products.
Chan JM, Stampfer MJ, Ma J, Gann PH, Gaziano JM, Giovannucci E. Dairy products, calcium, and prostate cancer risk in the Physicians’ Health Study. Am J Clin Nutr. 2001;74:549-554.
Larsson SC, Orsini N, Wolk A. Milk, milk products and lactose intake and ovarian cancer risk: a meta-analysis of epidemiological studies. Int J Cancer. 2006;118(2):431-441.
Hjartaker A, Laake P, Lund E. Childhood and adult milk consumption and risk of premenopausal breast cancer in a cohort of 48,844 women - the Norwegian women and cancer study. Int J Cancer. 2001;93(6):888-893.