Study: Dairy milk may slightly raise breast cancer risk

IN-DEPTH REVIEW OF RESEARCH

Study background:

The Seventh-day Adventist Church promotes a healthy lifestyle. Most members do not smoke or use alcohol, and they are encouraged to eat a vegetarian diet. Many also avoid caffeine. Adventists have varied diets. Two previous studies in California showed that a small percentage are total vegetarians; however, many are semi-vegetarian (eating meat less than once per week) or follow a lacto-ovo-vegetarian diet. About half of Adventists eat as much meat as the general population.

The Adventist Health Studies (AHS) are a series of long-term research projects at Loma Linda University that are designed to measure the link between lifestyle, diet, disease and mortality in Seventh-day Adventists. The Adventist Health Study-2 (AHS-2) began in 2002 with the goal of investigating the impact of selected foods that might change cancer risk.

Among women enrolled in ASH-2, almost 40 percent are strict about their diets (no meat, eggs or dairy) or lacto-ovo vegetarians (consume eggs and dairy). These women often consume soy as a source of protein, averaging up to 68 grams per day compared to about three grams per day for women in the general population. The ASH-2 group gets most of their dietary calcium (up to 80 percent) from non-dairy sources. Women in this group are novel due to their higher-than-average soy consumption.

Results from studies that looked at the link between soy and dairy intake and breast cancer risk have been inconsistent. Some suggest a link between high soy or high dairy consumption and increased breast cancer risk. Other studies suggest that increased intake of soy or dairy decreases breast cancer risk. Researchers of this study wanted to use the ASH-2 group to understand the link between soy and dairy intake and breast cancer risk.

Researchers of this study wanted to know:

Whether intake of soy milk, other soy products, dairy milk and other dairy products affect the risk of breast cancer.

Populations looked at in this study:

The Adventist Health Study-2 (AHS-2) is a group of 52,795 female, North American (Canada and the U.S.) Seventh-day Adventists.

Study design:

52,795 female participants (aged 30 years or older) were recruited from Adventist churches in the U.S. and Canada from 2002 to 2007.

• Participants completed a food frequency questionnaire based on their recall of food consumed a year prior to enrollment in the study. The questionnaire included:
• 51 questions related to how much and which types of soy milk and soy products they consumed.
  • These included questions about soy-based meat substitutes, tofu, soybeans and soy isoflavone supplements.
  • 17 questions related to how much and what types of dairy milk and dairy products they consumed.
  • Questions related to intake of meats, nuts/seeds and other dairy items.
  • The questionnaire also included questions about demographics (age, gender, etc.), family history of breast cancer, physical activity, alcohol consumption, past and current hormonal use (oral contraceptives and/or hormone replacement therapy), other medication use, breast cancer screening and reproductive and gynecological histories.  

Breast cancer cases from 53 cancer registries were used to identify study participants who developed breast cancer.

• Breast cancer cases were also identified by follow-up questionnaires and confirmed by medical record reviews.   

Study findings:

After almost 7.9 years of follow-up, 1,057 cases of breast cancer were identified among study participants. Of these:

• 906 cases were in postmenopausal women.
• 121 cases were in premenopausal women.
• 30 cases were in perimenopausal women.

Statistical analyses showed no link between intake of soy milk or soy products and increased breast cancer risk.

When participants were analyzed as a single group, higher intake of total dairy or dairy milk (either full-fat or reduced-fat dairy milk) was correlated with increased breast cancer risk.

• When researchers looked at women who were pre- or postmenopausal, higher intake of total dairy or dairy milk was associated with increased breast cancer risk only for postmenopausal women.
• Higher intake of dairy cheese, yogurt and total dairy fat was not linked to increased breast cancer risk regardless of menopausal status.

Strengths and limitations:

The major strength of this study was that it followed a large number of women (over 50,000) whose religion has rules about diet.

This study has major limitations including:

• It was observational. Women had to recall how much and what soy and dairy products they had consumed during the year prior to enrollment. The accurate recall of almost 53,000 participants likely varied.
• Participants may have under- or overreported how much soy and dairy they consumed, so that their diets appeared more aligned with Church teachings.
• After adjusting for a range of factors that this study measured, including diet and lifestyle factors, the link between dairy milk and increased breast cancer risk remained. Other factors may have been different among the women who drank more milk compared to those who did not. For example, pesticide levels in soy and or dairy products or participants’ level of income.
• Soy and dairy consumption were only measured at intake. It is possible the participants’ diets changed over the course of the study.
• A different study design is needed to show that drinking dairy milk increases breast cancer risk. Only a long-term, well-controlled prospective study that closely monitors the amounts and types of soy and dairy consumed would show whether dairy milk increases breast cancer risk.  
• The authors did not provide exact numbers on breast cancer cases by group; they only gave hazard ratios and relative risks. These estimates used the total proportion of breast cancer in study participants and reported relative risks.
• The population studied was predominantly White women from the U.S. and Canada. It is unknown whether these results can be applied to other populations; to Asian women, for example, who consume a lot of soy.

These limitations make it hard to draw direct causal conclusions that drinking dairy milk increases breast cancer risk. In other words, this study linked dairy milk with increased risk of breast cancer, but it did not show that drinking dairy milk caused breast cancer or if increased risk was due to some other factor. Whether dairy milk affects breast cancer risk has been researched extensively. In fact, a number of studies have shown that dairy (including milk) decreases the risk of breast cancer.

Context:

Media reports often claim certain foods can cause or cure cancer. These reports are often misleading and they increase confusion about how diet affects cancer risk. It is worth noting that most media articles covering this study only reported relative risk. The relative risk of drinking dairy milk and increased breast cancer risk in this study was scary— 50 to 80 percent! However, the absolute risk increase was much smaller: between 1 to 2 percent. About 1 to 2 in 100 women who drank no milk at all developed breast cancer, compared to 3 to 4 in 100 women who drank a glass or two of milk per day. 

Conclusions:

While U.S. dietary guidelines recommend consuming the equivalent of about three eight-ounce cups of milk per day, most studies in non-vegetarian cohorts have not shown that milk increases breast cancer risk. If anything, results have been inconsistent. Some studies have found no link between dairy and breast cancer risk. Others have shown that dairy products lower breast cancer risk, while a few have concluded that dairy increases breast cancer risk.

This study does not mean that women should assume their breast cancer risk will be increased if they drink milk. More research is needed to understand the effect of dairy milk on breast cancer risk,

Share your thoughts on this XRAYS article by taking our brief survey.

Posted 9/3/20