Many studies have tied lifestyle factors such as body weight, alcohol consumption and physical activity to breast-cancer risk. Findings from studies examining dietary factors, though, have been inconsistent.
Dairy has been hypothesized to increase the risk, while soy foods are thought to help lower it. But, so far, there’s no clear-cut evidence that either theory is correct.
Now, findings from a large study suggest that cow’s milk, a beverage we’ve long been told to drink for our bones, increases breast-cancer risk. And swapping soy milk for dairy may offset that risk.
The latest research
The findings, published last week in the International Journal of Epidemiology, come from the Adventist Health Study-2 (AHS-2), an ongoing investigation of the link between diet and cancer among 96,000 Seventh-day Adventists living in the United States and Canada.
Nearly 40 per cent of the AHS-2 population are vegan (no meat, fish, eggs or dairy) or lacto-ovo vegetarian (eggs and dairy are allowed) and often eat soy as a protein source.
For this study, the researchers examined the diets of 52,795 female participants, average age of 57, who were initially cancer-free. After eight years of follow-up, 1,057 women had developed breast cancer.
Compared with women who drank very little or no cow’s milk, drinking one cup (250 ml) a day, either low-fat or full-fat, was tied to a 50-per-cent greater risk of breast cancer.
Even drinking less than one cup of milk, compared with none, was associated with an increased risk. Cheese and yogurt were not linked to breast-cancer risk.
The researchers accounted for other factors associated with the risk including age, family history, alcohol intake, physical activity and body weight. Intake of red and processed meat, fish, poultry, soy foods and nuts/seeds were also considered.
Soy foods and soy milk did not increase or decrease breast-cancer risk.
The findings also hinted that if you’re a regular milk drinker, replacing some of it with soy milk could lower the risk of breast cancer.
Drinking one cup of milk a day, compared with none, was found to raise the risk of breast cancer by 50 per cent. This doesn’t mean, though, that a woman’s personal risk for developing breast cancer was 50 per cent higher.
In North America, a woman with no known risk factors has a 1 in 8 chance of developing breast cancer in her lifetime, a 12-per-cent personal risk. A 50-per-cent increase, then, raises a woman’s personal risk to 18 per cent.
Strengths, limitations, plausibility
A credit to the study is its large and ethnically diverse sample size. It also included a wide range of dairy and milk consumption, from none to a lot, which is necessary to capture an effect.
Weaknesses included the study’s observational design, which can’t prove that drinking milk directly causes breast cancer; it can only uncover associations.
Participants’ diets were self-reported, therefore subject to error, and were measured only once at the onset of the study. However, it was revealed that most participants adhered to their dietary regimes for a decade or longer.
Despite the limitations, there are plausible ways in which cow’s milk could increase breast-cancer risk.
Dairy cows are lactating, and about 75 per cent of the herd is pregnant, which means that estrogen and progesterone end up in cow’s milk. Since breast cancer is a hormone-sensitive cancer, it’s possible that consuming bovine sex hormones in milk influences breast-cancer risk.
Consuming dairy and other animal foods has also been shown to increase blood levels of insulin-like growth factor-1, a hormone thought to promote certain cancers.
What to do?
These new findings do not warrant giving up cow’s milk. This is only one study, and while it does suggest that something is going on, further studies are needed to establish if, indeed, drinking cow’s milk increases breast-cancer risk.
Dairy is a key component of a blood-pressure-lowering diet, and it also appears to help guard against colorectal cancer.
That said, women who have a higher risk of breast cancer due to family history or other factors could consider switching to a calcium-fortified plant beverage. Among these milk alternatives, only soy milk and pea milk match the protein content of cow’s milk.
Leslie Beck, a Toronto-based private practice dietitian, is Director of Food and Nutrition at Medcan.
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