A recent study concludes that women who drink greater amounts of milk might have an increased risk of developing breast cancer than those who drink little or no milk.
According to the American Cancer Society, in 2019, there were around 268,600 new cases of breast cancer among women in the United States.
Over the years, scientists have uncovered a number of lifestyle-related risk factors for breast cancer; these include alcohol consumption, higher body mass index, and lower levels of physical activity.
Many scientists believe that there may also be nutritional risk factors, but as the authors of a recent study explain, “Results have been inconsistent for virtually all nutritional factors to date.”
Two food groups that have received a fair amount of attention are soy and dairy. Their impact on breast cancer has proven difficult to pinpoint.
However, because individuals who consume more soy are likely to consume less dairy, and vice versa, untangling the relationship has proven challenging.
Unperturbed, the authors of a recent study published in the International Journal of Epidemiology set out to look for links between milk and soy consumption and breast cancer.
To investigate, the authors delved into a unique dataset created as part of the Adventist Health Study-2; they used data from 52,795 women aged 30 or older.
Of these participants, 40% were either vegans, who consume no meat, dairy, or eggs, or vegetarians, who consume eggs and dairy, but at levels 60% lower than usual U.S. levels.
At the beginning of the study, the participants completed a food frequency questionnaire that gathered information about dairy and soy intake, among other factors. The participants also provided details about alcohol intake, levels of physical activity, family history of breast cancer, ethnicity, and gynecological and reproductive history.
Many of the participants consumed significantly more soy than the general population. And, importantly, 50% of the group consumed roughly the same amount of dairy as the U.S. average.
This stark contrast between soy-consumers and dairy-consumers allowed the authors to “evaluate independent associations between soy, dairy consumption, and breast cancer incidence with unusual clarity.”
The scientists followed the participants for an average of 7.9 years. During this time, there were 1,057 cases of breast cancer.
Overall, the authors concluded that soy did not have a protective effect against breast cancer; there was no relationship between soy and breast cancer risk.
When they analyzed the effect of dairy, however, they found a significant interaction. The authors concluded:
“Dairy foods, especially milk, were associated with increased risk, and there was a marked reduction in risk when substituting soy milk for an approximately equivalent quantity […] of dairy milk.”
The authors believe that “previous findings of a lower incidence of breast cancer risk among higher soy consumers may be at least partially due to a relative absence of dairy.”
In an interview, the study’s first author, Gary E. Fraser, Ph.D., explained, “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%.”
He went on to say that by drinking up to 1 cup per day, the associated risk increased by 50%. For those drinking 2 or 3 cups per day, the risk increased by 70% and 80%, respectively.
It is important to put these figures into perspective. The percentages above are describing relative increases rather than absolute increases.
In the current study, around 1 in 50 of the participants developed breast cancer within the follow-up period. If we were to increase this risk by 80%, that would give us an incidence rate of 1.8 in 50.
So, although these changes are statistically significant, looking at absolute differences makes the true effect size a little clearer.
Despite the large size of the study, there are some limitations. As with most large nutrition studies, this one was observational, making it impossible to confirm cause and effect. In other words, there may be other factors associated with milk consumption that are impacting breast cancer risk, such as coffee or tea intake.
Also, dietary information was only collected at one time; people may change their eating habits significantly over the years.
Overall, though, Fraser concludes that “Dairy milk does have some positive nutritional qualities, but these need to be balanced against other possible, less helpful effects. This work suggests the urgent need for further research.”
Because breast cancer is relatively prevalent and milk is widely consumed, understanding the true relationship between the two is an important topic to investigate.