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Study: Dairy milk may slightly raise breast cancer risk

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Contents

At a glance Clinical trials
Strengths and limitations Guidelines              
What does this mean for me? Questions for your doctor 
In-depth Resources


STUDY AT A GLANCE

This study is about:

Whether consuming soy or dairy products increases breast cancer risk.


Why is this study important?

Many studies have looked at soy or dairy intake and breast cancer risk. Some studies show there may be a link while others do not. These conflicting results lead to confusion about whether a diet that includes soy or dairy products increases breast cancer risk. 


Study findings: 

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. Almost 53,000 North American members enrolled in this study. They are unique in that they eat and drink a lot of soy products. While many also consume dairy products, they drink much less dairy milk than women in the general population.

None of the participants had breast cancer at the time of enrollment. They filled out a dietary questionnaire that asked them to recall their diet in the year prior to enrollment. Among the participants:

  • Almost 40% were strict vegetarians (no meat, eggs or dairy) or lacto-ovo vegetarians (consumed eggs and dairy).   
    • These women often ate soy as a source of protein, consuming up to 68 grams of soy per day (equivalent to about one-quarter cup of soy milk or about 2.4 ounces of tofu). This is much more than the average woman, who consumes about 3 grams of soy per day.
  • Most got dietary calcium (up to 80%) from non-dairy sources. 

After almost eight years of follow up, 1,057 cases of breast cancer were diagnosed among participants. The researchers used complex statistical tests to see if there was a link between the types and quantity of soy and dairy products consumed and breast cancer risk. The results showed:

  • no link between soy products and breast cancer.
  • no link between dairy milk and breast cancer for premenopausal women.
  • postmenopausal women who reported drinking more dairy milk (either full fat or reduced fat) at the start of the study had slightly higher rates of breast cancer than those who drank less milk.
  • the link between breast cancer and dairy was only for dairy milk. No link was found between breast cancer and cheese, yogurt or other dairy products.


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.
  • The major limitation of this study is that it depended on participants’ recall of how much dairy and soy they had consumed the year prior to enrollment. 
  • Finding links between individual foods and cancer risk is very difficult. The study design did not allow researchers to say that drinking milk increased breast cancer risk. It may have been something else entirely.


What does this mean for me?

Avoiding dairy milk is unlikely to protect you from breast cancer. In this study, the link between drinking dairy milk and increased breast cancer risk was quite small. About 1 to 2 women in 100 who drank no milk at all got breast cancer compared to 3 to 4 in 100 women who drank a daily glass or two of milk. 

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Reference

Fraser GE, Jaceldo-Siegl K, Orlich M, Mashchak A, Sirirat R and Knutsen S. Dairy, soy, and risk of breast cancer: those confounded milks. International Journal of Epidemiology. 2020, 1-12.   

 

Disclosure

FORCE receives funding from industry sponsors, including companies that manufacture cancer drugs, tests and devices. All XRAYS articles are written independently of any sponsor and are reviewed by members of our Scientific Advisory Board prior to publication to assure scientific integrity.

This article is relevant for:

Women who consume dairy or soy

This article is also relevant for:

Healthy people with average cancer risk

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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 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 . The of drinking dairy milk and increased breast cancer risk in this study was scary— 50 to 80 percent! However, the 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

Expert Guidelines Expert Guidelines

The American Cancer Society recommendations on nutrition and physical activity for cancer prevention include: 

  • Achieve and maintain a healthy weight throughout life. 
  • Adopt a physically active lifestyle. 
  • Consume a healthy diet with an emphasis on plant-based foods 
  • Limit processed and red meats: 
    • Eat at least 2.5 cups of vegetables and fruits every day. 
    • Chose whole grains instead of refined grain products. 
    • Drink no more than 1 alcoholic drink per day (women). 

Other experts, including the following, also provide guidelines for nutrition and health: 

Expert Guidelines Expert Guidelines

The American Cancer Society (ACS) guidelines on exercise, nutrition and weight for cancer prevention recommend the following: 

Diet and nutrition

  • Follow a healthy eating pattern, including:
    • foods that are high in nutrients in amounts that help you get to and stay at a healthy body weight.
    • a variety of vegetables, fiber-rich legumes (beans and peas), and whole fruits in a variety of colors. Consume at least 2½ to 3 cups of vegetables and 1½ to 2 cups of fruit each day, depending on your calorie requirements. 
    • whole grains rather than refined grains. At least half of the grains you eat should be whole grains. 
  • A healthy eating pattern that limits or does not include:
    • red and processed meats.
    • sugar-sweetened beverages. 
    • highly processed foods and refined grain products.
  • It is best not to drink alcohol. People who choose to drink alcohol should:
    • have no more than 1 drink per day (women) or 2 drinks per day (men).

Exercise

  • Exercise regularly.
    • Adults should get at least 150 minutes of moderate-intensity activity (equal to a brisk walk) or 75 minutes of vigorous activity (heart rate is increased, breathing is faster and you are sweating) each week, preferably spread throughout the week.
    • Physical activity has been shown to lower the risk of several types of cancer, including breast, endometrial, and colon. It also reduces the risk of other serious diseases such as diabetes and heart disease.

Weight

  • Achieve and keep a healthy weight.
    • Being overweight or obese is a risk factor for many cancers, including breast, colon, endometrial and pancreatic. You can control your weight through regular exercise and healthy eating.

Other experts, including the following, also provide guidelines for exercise, nutrition and health: 

Updated: 07/19/2022

Questions to Ask Questions to Ask Your Doctor

  • What factors are likely to affect my risk of breast cancer?
  • What type of diet should I eat to improve my health and well-being?
  • Should I avoid any foods or beverages?
  • Can you refer me to a nutritionist?

Open Clinical Trials Open Clinical Trials

The following are studies focused on nutrition and cancer prevention. 

Multiple cancers

Breast cancer

Colorectal cancer

Visit our Featured Research Page and Research Search and Enroll Tool to find additional studies enrolling people with, or at high risk for cancer.

Updated: 12/05/2021

Find Experts Find Experts

You can find a registered dietician in your area using the search tool from the Academy of Nutrition and Dietetics.  

Updated: 11/13/2021

Who covered this study?

The Washington Post

Scary headlines aside, drinking milk probably does not cause breast cancer This article rates 5.0 out of 5 stars

VegNews

Dairy milk consumption linked to 80 percent spike in breast cancer risk This article rates 1.0 out of 5 stars

Slash Gear

Study warns drinking milk may drastically increase breast cancer risk This article rates 1.0 out of 5 stars

Yahoo! Life

Drinking dairy ups your risk of this cancer by 80 percent, study finds This article rates 0.5 out of 5 stars

How we rated the media

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