Hot on the heels of a review from top nutrition scientists that cautioned against drinking cow's milk comes another study with another caution: drinking milk increases the risk of developing breast cancer, say the researchers. But this finding comes from an observational study, and there may be confounders that are not accounted for, says an expert not involved with the study.
The latest research was based on data from the long-running larger study called Adventist Health Study-2 (AHS-2), which is looking at diet and health among Seventh Day Adventists in North America. Past results from this study have suggested that Seventh Day Adventists have longer life spans and lower rates of some cancers, perhaps because of healthier lifestyles.
The latest analysis suggests that milk raises breast cancer risk, and the more you drink the higher your risk may be.
"Consuming as little as 1/4 to 1/3 cup of dairy milk per day was associated with an increased risk of breast cancer of 30%," first author Gary E. Fraser, MBChB, PhD, said in a press statement. Fraser is affiliated with the School of Public Health at Loma Linda University, California.
"By drinking up to 1 cup per day, the associated risk went up to 50%, and for those drinking 2 to 3 cups per day, the risk increased further to 70% to 80%," he added.
The findings were published February 25 in the International Journal of Epidemiology.
"The AHS study is provocative, but it's not enough to warrant a change in guidelines. The caution being espoused by the authors is not warranted given the observational nature of this study," commented Don Dizon, MD, director of Women's Cancers, Lifespan Cancer Institute at Brown University in Providence, Rhode Island. He was not involved with the study and was approached by Medscape Medical News for comment.
Because of its observational design, the study cannot prove that cow's milk causes breast cancer, Dizon emphasized.
"I'd want to see if the findings are replicated [by others]. Outside of a randomized trial of [cow's] milk vs no milk or even soy, and incident breast cancers, there will never be undisputable data," he said.
"Probably the biggest point [about this study] is not to overinflate the data," Dizon added.
He noted that the results were significant only for postmenopausal women, and not for premenopausal women. Moreover, analyses showed significant associations only for hormone receptor-positive cancers.
"We know that breast cancer increases in incidence with age, so this tracks with that particular trend. It suggests there may be confounders not accounted for in this study," he said.
Research so far has been inconclusive on a possible link between dairy and increased risk for breast cancer. Dairy has even been tied to decreased risk for breast cancer, according to the World Cancer Research Fund.
The current study included 52,795 Seventh Day Adventist women from North America who did not have cancer at the start of the study. Women had a mean age of 57.1 years, and 29.7% were black. At baseline, women reported their dietary patterns for the past year using food frequency questionnaires. For 1011 women, researchers double-checked food intake with 24-hour diet questionnaires, and verified soy intake by analyzing urine levels of soy isoflavones.
Data on invasive breast cancer diagnoses came from national registries in the US and Canada. Over the course of 7.9 years, 1057 women developed invasive breast cancer. Results were adjusted for a range of factors related to breast cancer risk, including diet, lifestyle, and family history of breast cancer.
Overall, women who consumed the most calories from dairy per day had 22% increased risk for breast cancer, compared with women with the fewest calories from dairy (hazard ratio [HR], 1.22; 95% confidence interval [CI], 1.05 - 1.40, P = .008). Women who drank the most cow's milk per day had 50% increased risk for breast cancer compared with women who drank the least (HR, 1.50; 95% CI, 1.22 - 1.84, P < .001).
Drinking full or reduced fat cow's milk did not change the findings (P for trend = .002 and P for trend < .0001, respectively).
No significant association was found between breast cancer risk and cheese or yogurt consumption (P = .35 and P = .80, respectively).
Need for Change?
US dietary guidelines are under review. A new version, which will cover pregnant women and children under age 2 for the first time, is expected later this year.
Current guidelines recommend that adults and children aged 9 and over drink three 8 oz glasses of milk per day, or equivalent portions of yogurt, cheese, and other dairy products.
"Evidence from this study suggests that people should view that recommendation with caution," Fraser said.
Milk Is Complex Topic
A top nutrition scientist agrees. Walter Willett, MD, DrPH, professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health in Boston, Massachusetts, told Medscape Medical News: "There is little scientific justification for the recommendation of 3 cups of milk per day. This new study adds a further reason for caution."
"This was a high-quality study conducted by experienced investigators," Willett said. Strengths of the study include the high soy intake and low consumption of foods from animal sources, factors that are hard to study in other populations.
Willett was a coauthor, along with David Ludwig, MD, PhD, also from Harvard, of the recent review published in the New England Journal of Medicine that questioned the science behind milk-drinking recommendations. An article about this review on Medscape Medical News has attracted a huge number of comments from our readers.
Milk is a complex topic, Willett explained. As a good source of essential nutrients, especially calcium and vitamin D, cow's milk has been touted to have several health benefits, especially decreased fracture risk. But Willett said calcium recommendations have probably been overstated, and current evidence does not support high milk intake for fracture prevention.
Other benefits include improved nutrition in low-income settings, taller stature, and decreased colorectal cancer risk. But cow's milk has also been linked to increased risk for some cancers, including prostate and endometrial cancer. Many of the benefits derived from nutrients found in milk may be obtained from other sources without these risks, according to Willett.
"Given the risks and benefits, we suggest a possible range from zero to two servings per day of dairy foods, including milk, cheese, and yogurt. If intake is zero or one serving, taking a calcium/vitamin D supplement would be good to consider," he said.
However, Fraser and Willett also suggested another option: replacing cow's milk with soy milk. Analyses from the current study showed no significant association between consumption of soy and breast cancer, independent of dairy (P for trend = .22).
In addition, substituting average amounts of soy milk for cow's milk was linked to a 32% drop in risk for breast cancer among postmenopausal women (HR, 0.68; 95% CI, 0.55 - 0.85, P = .002). However, these results were not significant among premenopausal women (HR, 0.70; 95% CI, 0.36 - 1.38, P = .31).
"The suggestion that replacing some or all of [cow's] milk with soy milk may reduce risk of breast cancer is consistent with other studies supporting a benefit of soy milk for risk of breast cancer," Willett said.
"If someone does choose soy milk, picking one with minimal amounts of added sugar is desirable," he added.
Drinking Milk, or Some Related Factor?
Fraser, the lead author of the current study, said in a statement that the results provide "fairly strong evidence that either dairy milk or some other factor closely related to drinking dairy milk is a cause of breast cancer in women."
That 'other' factor is probably complicated, but may be related to what humans have done to cows. To increase milk production, humans have bred cows to have higher levels of insulin-like growth factor, which in turn has been linked to some cancers, including breast cancer.
Sex hormones in cow's milk may also be involved. About 75% of a dairy herd is pregnant and the cows are by definition lactating. So the milk they produce may have higher levels of progestins and estrogens, which may play a role in hormone-responsive breast cancer.
Other factors that researchers did not measure in this study, such as poverty and the income of participants, may be at play.
But to know what's really going on, all agree that more research is needed.
"The overall evidence so far has not shown a clear increase or decrease in risk of breast cancer with higher [cow's] milk intake. Thus, this topic needs further examination," Willett said.
The study was funded by the National Cancer Institute at the National Institutes of Health, and the World Cancer Research Fund in the UK. Three of the authors report following largely vegetarian diets. All authors report regular and free use of dairy products without religious or other restrictions. No authors report associations with the soy product or dairy industries. Willett reports being a consultant during the design and early years of the Adventist study, but has not been involved with it for at least 8 years. Dizon has disclosed no relevant financial relationships.
Int J Epidemiol. Published online February 25, 2020. Abstract
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Cite this: New Study Suggests Milk Could Increase Breast Cancer Risk - Medscape - Feb 28, 2020.