Milk Products and Breast Cancer

Basic facts on breast cancer 

Breast cancer is the most common cancer among Canadian women (excluding non-melanoma skin cancers).¹ There is no single cause of breast cancer; however, there are many risk factors including the following, among others:^1,2

• Personal history of breast cancer
• Family history of breast and other cancers
• Breast cancer (BRCA) gene mutations
• Hormone replacement therapy
• Oral contraceptives
• Breast density
• Obesity

The evidence

With regards to diet and lifestyle factors, the 2020 IARC World Cancer Report published by the WHO International Agency for Research on Cancer notes that alcohol, low physical activity and postmenopausal obesity are associated with an increased overall breast cancer risk.² This report makes no mention of dairy products in the context of discussing risk factors associated with breast cancer. 

According to the Third Expert Report published in 2018 by the World Cancer Research Fund International and the American Institute for Cancer Research, limited evidence suggests that consuming dairy products may decrease the risk of premenopausal breast cancer.³ For postmenopausal breast cancer, the report found no associations with regards to milk or dairy products. The report also judged there is limited evidence that consuming diets high in calcium may reduce the risk of breast cancer in both premenopausal and postmenopausal women. 

A 2021 systematic review of the evidence on yogurt, cultured fermented milk, and health found that fermented milk product consumption (including yogurt) is associated with a reduced risk of breast cancer.⁴ In terms of fat content, researchers also found the consumption of milk fat in fermented milk products was associated with a reduced risk of breast cancer. 

A 2020 dose-response meta-analysis of prospective studies explored the associations between various food groups and breast cancer risk.⁵ Kazemi et al. reported on linear and nonlinear dose-response relationships for total dairy food as well as milk, yogurt and cheese intakes:

• For total dairy, there was no association with breast cancer for each additional 200 g/day;
• For milk, no association was observed for each additional 200 g/day, but intakes greater than 450 g/day were associated with increased risk;
• For cheese, each additional 30 g/day was associated with a 5% reduced risk of breast cancer;
• For yogurt, the risk of breast cancer decreased by approximately 7.5% with an increasing intake up to 100 g/day. No significant association was observed for each additional 200 g/day increase in yogurt intake. 

In 2019, a meta-analysis of case-control studies by Chen et al. found no associations between intakes of low-fat/skim milk, whole milk or yogurt and breast cancer.⁶  

A 2015 meta-analysis of 22 prospective cohort studies involving a total of 1,566,940 participants evaluated the association between dairy intake and breast cancer risk.⁷

• Higher dairy consumption (>600 g/day) was associated with a 10% lower risk of breast cancer, as compared to lower dairy consumption (<200 g/day). Moreover, for each additional:
  • 250 g/day dairy intake, risk was reduced by 3%
  • 500 g/day dairy intake, risk was reduced by 6%
  • 750 g/day dairy intake, risk was reduced by 9%
• Higher yogurt and low-fat dairy intakes were associated with a risk reduction of 9% and 15%, respectively.
• No association was observed between whole milk, low-fat/skim milk, cheese, butter or high-fat dairy and breast cancer risk.

Potential mechanisms

The mechanisms by which the consumption of milk and milk products may influence the risk of breast cancer are not clear. Nevertheless, there are key components in milk products, namely calcium, vitamin D, lactoferrin, and milk fat which appear to play important anticarcinogenic roles.

Calcium

A 2016 meta-analysis has shown that there is an inverse dose-response relationship between calcium intake and breast cancer risk among both premenopausal and postmenopausal women.⁸ Calcium participates in the regulation of apoptosis (cell death), cell proliferation and differentiation.

Animal studies have shown that a high calcium intake inhibits hyperproliferation of mammary glands and can inhibit mammary carcinogenesis. The anti-proliferation and pro-differentiation properties of calcium may also decrease benign proliferative epithelial disorders.⁹

Vitamin D

Vitamin D plays potential mechanistic roles through its anti-cancer and anti-inflammatory properties. Multiple meta-analyses indicate vitamin D plays a protective role against breast cancer.^10-13 For example, a 2021 meta-analysis found a higher prevalence of vitamin D insufficiency in patients newly diagnosed with breast cancer.¹⁰ 

Lactoferrin

Bovine milk lactoferrin has also been found to be protective against breast cancer. It appears that lactoferrin has the ability to interact with certain receptors and modulate the genetic expression of molecules involved in the cell cycle and apoptosis.¹⁴

Milk fat

Milk fat naturally contains a number of unique components such as conjugated linoleic acid (CLA), vaccenic acid, phospholipids, accompanied by the fat-soluble vitamins A, D3 and K. These milk components have antioxidant, anti-inflammatory and immunostimulating properties that may in turn exert anticarcinogenic effects.¹⁵ Some experimental and in vitro studies have shown the CLA found in milk products may protect against mammary carcinogenesis.^16,17

Conclusion

The totality of the evidence suggests that the consumption of dairy products is not adversely related to beast cancer risk and may in fact reduce the risk of breast cancer, particularly in premenopausal women. Milk products are a major source of calcium, and the available evidence suggests diets high in calcium may reduce the risk of breast cancer in both premenopausal and postmenopausal women. Vitamin D, lactoferrin and milk fat may also be important anti-cancer milk components. More research is needed to elucidate the precise mechanisms by which dairy foods may reduce breast cancer risk.