Got milk? You’ve likely seen the dairy industry’s ads over the years, promoting milk as part of a healthy diet. But there’s a growing body of research that indicates that consuming cow’s milk is linked to a heightened risk of cancer.
A new study that followed more than 50,000 women for about 8 years found a 50 percent increase in breast cancer risk in women who consumed the largest amounts of cow’s milk, compared to those who consumed the least.1
The new research was part of the Adventist Health Study-2 (AHS-2) and followed over 50,000 women for about 8 years. Women in the 90th percentile of calories from total dairy products were 22 percent more likely to be diagnosed with breast cancer over the 8-year follow-up, compared to those in the 10th percentile. For cow’s milk specifically, women in the 90th percentile had a 50 percent increase in risk. The first author said that as little as 1/4 to 1/3 cup of cow’s milk daily was associated with a 30 percent increase in risk. The risk increased along with the quantity consumed: one cup daily was associated with a 50 percent increase, and two to three cups daily was associated with a 70-85 percent increase.
These results were driven primarily by postmenopausal breast cancer, and the results were similar for full-fat and reduced fat milks. Interestingly, they also found in their analysis that replacing cow’s milk with soy milk was associated with a 32 percent decrease in breast cancer risk.1
There are strong links between dairy products and prostate cancer,2-4 but previous data on dairy’s influence on breast, endometrial, and ovarian cancers has been inconsistent.5-12 One issue with studying this association in an North American or European population is the high average intake of dairy products – not enough zero-dairy consumers to compare to high-dairy consumers.
This research makes a valuable contribution to the field because of the population they studied. Whereas the American population as a whole generally has a high dairy intake, the Adventist Health Study-2 population has a true spectrum of dairy intake, allowing meaningful comparisons between zero intake, small intake, and larger intake of dairy. About 8 percent of the participants in this study are vegan and 40 percent are lacto-ovo-vegetarian, and their average soy intake is higher than typical for the U.S. Some vegetarian and nonvegetarian Adventists have low milk intake, while others have typical American intakes.1
The authors noted the results of this study were consistent with those of a 2016 study, also on the AHS-2 population, which found that vegans had a 22 percent lower risk of breast cancer compared to non-vegetarians, however, there was no difference in risk between lacto-vegetarians and non-vegetarians.13
Vegetarian dietary patterns and the risk of breast cancer in a low-risk population.
The possible role of female sex hormones in milk from pregnant cows in the development of breast, ovarian and corpus uteri cancers.
Dairy products, calcium, and prostate cancer risk: a systematic review and meta-analysis of cohort studies.
Effects of dairy products, calcium and vitamin D on ovarian cancer risk: a meta-analysis of twenty-nine epidemiological studies.
Dairy Products Intake and Endometrial Cancer Risk: A Meta-Analysis of Observational Studies.
Milk and yogurt intake and breast cancer risk: A meta-analysis.
Animal protein is a nutritional signal that increases the production of insulin like growth factor 1 (IGF-1). Animal protein – especially cow’s milk protein – is the primary dietary factor that increases IGF-1.14,15 The effect of dairy protein on IGF-1 is a plausible route through which cow’s milk could increase cancer risk, and there is abundant research linking elevated IGF-1 levels to greater risk of breast, prostate, and colorectal cancers.2,16-19
Galactose, a component of lactose, is known to promote oxidative stress, inflammation, and premature aging in animals. Elevated markers of oxidative stress and inflammation have been observed in humans who consume large amounts of cow’s milk.20 Lactose intake specifically has been linked to ovarian cancer risk.5-7
Greater exposure to estrogen is known to increase the risk for breast cancer.21,22 It is worth noting that modern dairy cows are often pregnant while lactating – about 75 percent of the herd is pregnant at any given time, which means estrogen and progesterone are secreted into their milk. Although these hormone levels are low, there is evidence they can be absorbed and, due to long-term exposure to low doses, may affect our biology or, like endocrine-disrupting chemicals in plastics, disrupt the actions of our own hormones.1,12,23,24
Does milk intake promote prostate cancer initiation or progression via effects on insulin-like growth factors (IGFs)? A systematic review and meta-analysis.
Milk consumption and circulating insulin-like growth factor-I level: a systematic literature review.
Circulation insulin-like growth factor peptides and colorectal cancer risk: an updated systematic review and meta-analysis.
Insulin-like growth factor 1 (IGF1), IGF binding protein 3 (IGFBP3), and breast cancer risk: pooled individual data analysis of 17 prospective studies.
Milk, Fruit and Vegetable, and Total Antioxidant Intakes in Relation to Mortality Rates: Cohort Studies in Women and Men.
Exposure to exogenous estrogen through intake of commercial milk produced from pregnant cows.
The Fate of Synthetic and Endogenous Hormones Used in the US Beef and Dairy Industries and the Potential for Human Exposure.
The researchers expect that the reduction in risk associated with drinking soy milk was partly due to reduced cow’s milk consumption and partly due to protective effects of soybeans. Soy phytochemicals called isoflavones have anti-estrogen effects in breast tissue. The research suggests that soy intake helps to protect against initial breast cancer development (especially postmenopausal breast cancer), breast cancer recurrence, and breast cancer mortality.25-27 However, the most powerful protective effects of soy foods against breast cancer are during adolescence.28
Soy isoflavones also have anti-cancer effects that are unrelated to estrogen. Soy intake is linked to decreases in prostate, lung, stomach, and colorectal cancers.29-32
I recommend including soybeans with their beneficial isoflavones, fiber, and plant protein along with other beans, lentils, and split peas in a Nutritarian diet. Edamame, cooked dry soybeans, and tempeh are the most healthful forms. Unsweetened soy milk and tofu are acceptable, but they are missing some of the fiber from the whole soybean.
Related: Don’t Fall for the Myths About Soy
Meta-analysis of soy intake and breast cancer risk.
Epidemiology of soy exposures and breast cancer risk.
Early intake appears to be the key to the proposed protective effects of soy intake against breast cancer.
Soy food consumption and risk of prostate cancer: a meta-analysis of observational studies.