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Unraveling the Dairy Enigma: Breast Cancer Risk and Calcium

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Although dairy products like cheese, yogurt, and milk are well-known calcium sources, their relationship with breast cancer remains a subject of ongoing discussion. While calcium is believed to have a protective role against breast cancer, the overall connection between dairy consumption and breast cancer has shown mixed results. An analysis in the American Journal of Clinical Nutrition sought to clarify this relationship by synthesizing data from 21 previous studies.

The Dairy Debate

Dairy consumption and its potential impact on breast cancer risk have been a contentious topic for years. Some research suggests that calcium, vitamin D, and conjugated linoleic acid—a beneficial dairy fat—could help regulate cancer cell growth and provide protection against breast cancer. On the other hand, dairy products may also raise insulin-like growth factor-1 (IGF-1) levels, promoting cancer cell growth. Additionally, concerns exist about the possible influence of hormones in cows, both naturally occurring and those administered in farming, on breast cancer risk.

Shauna Lindzon, a consulting dietitian in Toronto, Ontario, who teaches nutrition at a cancer support center, says that dairy is often discussed in the context of cancer and nutrition. She notes that while most people recognize dairy as a source of essential nutrients like calcium, they often worry about potential negative effects, such as inflammation or harmful hormones.

What the Research Says

The aforementioned study analyzed data from over 1 million women who participated in 21 different cohort studies, with follow-up periods ranging from eight to 20 years. The researchers assessed dietary habits using food frequency questionnaires, examining the intake of milk, hard cheese, ricotta/cottage cheese, yogurt, and ice cream.

They investigated the relationship between calcium, dairy products, and the risk of developing invasive breast cancer overall, as well as the risk of developing breast cancer subtypes based on estrogen receptor status. Breast cancer cells use hormone receptors to fuel their growth, making it essential to understand how hormones from dairy products may play a role.

According to Marji McCullough, Senior Scientific Director of Epidemiology Research at the American Cancer Society and one of the study's researchers, the analysis showed that overall dairy consumption is unlikely to increase breast cancer risk. In fact, fermented dairy products may actually lower the risk, particularly for more challenging estrogen receptor-negative tumors.

What You Need to Know

The study largely alleviated concerns about dairy and breast cancer risk. Researchers found weak or nonexistent associations between the dairy products examined, calcium (from food or supplements), and the risk of developing overall or estrogen receptor (ER)-positive breast cancer. Moreover, certain dairy foods like yogurt, ricotta cheese, and cottage cheese were found to be weakly associated with a reduction in breast cancer risk.

McCullough states that small amounts of yogurt consumption were linked to a lower breast cancer risk, particularly for estrogen receptor-negative tumors. However, the beneficial effects of ricotta and cottage cheese were only observed in studies outside North America. This discrepancy could be attributed to differences in food regulations, processing, farming practices, and nutrient content, as North American versions of these cheeses may contain fewer helpful probiotics.

Lindzon, unsurprised by the study's findings, emphasizes that dairy is safe for consumption, whether one has breast cancer or not. The study aligns with recommendations from the American Institute for Cancer Research. McCullough also highlights that, based on these findings, it is unlikely that dairy products or high-calcium diets increase breast cancer risk. However, as dairy foods may lower the risk of some cancers while increasing the risk of others, the American Cancer Society does not specifically recommend dairy consumption for cancer prevention.

For those who enjoy dairy products, rest assured that they are likely safe to consume and not associated with an increased risk of breast cancer. If you choose to avoid dairy for any reason, ensure you obtain sufficient calcium from alternative sources.

William H. McDaniel, MD

Dr. Robert H. Shmerling is the former clinical chief of the division of rheumatology at Beth Israel Deaconess Medical Center (BIDMC), and is a current member of the corresponding faculty in medicine at Harvard Medical School.

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