Study: Dairy milk may slightly raise breast cancer risk

Summary:

Debate continues about whether consuming soy or dairy products increases breast cancer risk. This study looked at a large number of women and found no link between soy and breast cancer risk. The study did find that postmenopausal women who drank dairy milk have a small increase in breast cancer risk. (09/04/20)

Relevance:

This article is most relevant for Women who consume dairy or soy

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This article is relevant for:

Women who consume dairy or soy

This article is also relevant for:

• Healthy people with average cancer risk
• Women under 45
• Women over 45

Contents

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

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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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:

• The Academy of Nutrition and Dietetics
• The United States Office of Disease Prevention and Health Promotion
• The American Institute for Cancer Research

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

• NCT04374747: Fruit and Vegetable Intervention in Lactating Women to Reduce Breast Cancer Risk. This trial is only for nursing mothers. This study will look to see if eating at least 8 to 10 daily servings of fruits and vegetables reduces breast cancer biomarkers.
• NCT03805516: Central Obesity and Cancer Prevention for Chinese American Women. This trial is evaluating the effectiveness of a smartphone-based intervention to reduce obesity and breast cancer risk among Chinese American women in San Francisco.
• NCT03448003: Comprehensive Lifestyle Change To Prevent Breast Cancer. This trial at MD Anderson in Houston looks at how well lifestyle changes work to prevent breast cancer. Premenopausal women 18 years and older with intact breast and ovaries are eligible.
• NCT04125914: HEALTH4Families: Optimizing a Weight Management and Health Behavior Intervention for BRCA+ and Lynch Syndrome Families. This trial studies how well weight management and health behavior intervention lower the risk of cancer in patients with hereditary breast and ovarian cancer mutations or Lynch syndrome mutations.
• NCT03779867: Acute Exercise Intervention in Reducing Breast Cancer Risk in Healthy Participants (ACE). This trial at the Fred Hutchinson Cancer Research Center in Seattle studies how well acute exercise in healthy participants reduces the risk of breast cancer in healthy participants.
• Energetics and Lifestyle in Inherited Syndromes (ELLIE’s Study). ELLIE’s Project is designed to look at factors, such as weight, Body Mass Index, metabolism, dietary habits and activity levels that may affect cancer risk in people with inherited mutations linked to cancer.
• NCT03314688 Lifestyle, Exercise, and Nutrition Study Early After Diagnosis. This study is looking at a dietary and physical activity guidelines intervention soon after diagnosis may effect treatment outcomes.  This study is open to women newly diagnosed with breast cancer who will receive chemotherapy.

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.

Rating Details:

Relevance: Medium-Low

• This study had significant limitations, which reduced relevance.
• There is no clinical utility. Many factors affect breast cancer risk. Why only dairy milk increased breast cancer risk (and only in postmenopausal women) and not dairy products such as yogurt or cheese which are made of milk did not is not clear. The results of this study suggest that if you are a premenopausal women, drinking dairy milk does not increase breast cancer risk.
• It is not generalizable. One of the issues in general with nutrition research is that when something is taken out of a diet, something else is added.  In this study, some of the women who did not drink milk were also vegan and likely ate more vegetables and fruit, and less processed meat than some women who drank milk.
• The results of this study would only be clinically relevant if the results were confirmed by a controlled prospective trial.  For example, how much and how often a woman drank milk over many years would need to be closely monitored and their diets would need to be identical to women who did not drink milk.  This type of study would be very difficult to conduct.

Scientific Strength: Medium-Low

• This is a prospective observational study. It does not have controls. Participants recalled their diet the year prior to enrollment. Dairy intake was not controlled after enrollment and indeed, may have changed over the study period. .
• Effect of diet is inherently useful clinical information. It is directly applicable for most people.
• The sample size was large (52,795 participants with 1,057 cases of breast cancer). Study participants were unique in that their religion encourages a healthy lifestyle and diet.
• The statistical tests was complex but appropriate for this type of data. There were significant limitations, which limited how generalizable this data is. For example, it is not known whether some other factor was not measured in this study (e.g. wealth or change in diet over time) affected breast cancer risk.
• Whether drinking dairy milk increase breast cancer risk is unclear and requires additional study.

Research Timeline: More Research Needed

• While this study looked at a large number of women with unique diets much more research is needed to figure out if drinking dairy much causes an increases a woman’s breast cancer risk.