Milk Products and Colorectal Cancer

Basic Facts on Colorectal Cancer 

There are many known risk factors for colorectal cancer including the following, among others:¹ 

• Family history of colorectal cancer; 
• Being overweight or obese; 
• Not being physically active; 
• Alcohol; 
• Smoking; 
• Eating processed meat; 
• Eating a diet low in fibre; 
• Inflammatory bowel disease.

About 26,800 Canadians were diagnosed with colorectal cancer in 2017.¹ It is the second most commonly diagnosed cancer in Canada (excluding non-melanoma skin cancers). 

The evidence 

According to the Third Expert Report published in 2018 by the World Cancer Research Fund and the American Institute for Cancer Research, there is strong evidence that dairy products (total dairy, milk, cheese) are associated with decreased risk of colorectal cancer.² 

Potential mechanisms 

There are several potential mechanisms by which dairy products may reduce the risk of colorectal cancer involving several dairy components. 

Calcium 
Calcium may protect against colorectal cancer, with evidence for several plausible mechanisms.¹ 

In a dose-response meta-analysis of prospective observational studies, it was found that each 300 mg/d of total calcium decreased colorectal cancer risk by 8% and this reduction continued beyond 1000 mg/day.³ 

Several biological mechanisms may explain the protective effect of calcium against colorectal cancer.^{2, 3, 4} Calcium may: 

• Suppress proliferation, promote terminal cell differentiation and induce apoptosis of colorectal tumour cells; 
• Bind secondary bile acids (bile acids which are formed from the action of intestinal bacteria on bile acids synthesized by the liver) or ionized fatty acids so as to inhibit their ability to modify colonic cells; 
• Reduce colonic K-ras gene mutations. 

Vitamin D 
The evidence that vitamin D or foods containing it protect against colorectal cancer is limited but generally consistent.² There are plausible mechanisms for growth-restraining, anticancer effects of vitamin D on colon cells.⁵ As well, the effects of vitamin D and calcium are interrelated because both restrain cell proliferation and induce differentiation and apoptosis of intestinal cells.² Other potential mechanisms relate to improved immune function, reduced inflammation, inhibition of angiogenesis and regulation of microRNA expression with higher vitamin D status.² Furthermore, calcium-mediated effects are dependent on vitamin D levels. 

Conjugated linoleic acid (CLA) 
The anticancer effects of CLA have been demonstrated in numerous animal models. CLA has been shown to modulate eicosanoids production, reduce the synthesis of prostaglandins, interfere with cell signalling pathways, inhibit DNA synthesis, modify the fluidity of cell membranes, promote apoptosis, and modulate angiogenesis.^{6, 7} 

Evidence from human studies is limited; however, it does suggest that CLA may help protect against colorectal cancer. A study using data from the Swedish Mammography Cohort has demonstrated an inverse relationship between high-fat milk products and CLA and colorectal cancer.⁸ 

Other dairy components 
Butyric acid is another dairy fatty acid that has been suggested to play a beneficial role against colorectal cancer. It has been shown to inhibit proliferation and induce differentiation in tumour cell lines.⁷  

In animal models, lactoferrin, a milk protein, has been shown to suppress carcinogenesis in the colon and other organs. Additionally, lactoferrin may inhibit the growth of adenomatous polyps in humans.⁹  

Lactic acid bacteria found in fermented dairy products may also protect the epithelium surface of the colon. They may bind to the apical surface of colonic cells and suppress the excretion of toxins from putrefactive bacteria, thus protecting the epithelium surface.^{7, 10} 

Sphingolipids in milk fat are also a potent inhibitor of cell growth and may induce differentiation and apoptosis.¹⁰ 

Conclusion 

According to the World Cancer Research Fund International, the authority on diet and cancer, there is strong evidence that milk products (total dairy, milk, cheese) reduce the risk of developing colorectal cancer. 

Several components in milk products that could be responsible for this protective effect include: calcium, vitamin D, lactoferrin and butyric acid. 

Further research is needed to fully elucidate all the potential mechanisms by which milk products may help protect against colorectal cancer.