Dietary Calcium Cuts Colorectal Cancer Risk by 17%

A major prospective study of more than half a million UK women conducted over almost 17 years has confirmed that dietary calcium lowers the risk of colorectal cancer. 

Cancer Research UK (CRUK), which funded the study, said that it demonstrated the benefits of a healthy, balanced diet for lowering cancer risk.

Colorectal cancer is the third most common cancer worldwide. Incidence rates vary markedly, with higher rates observed in high-income countries. The risk increases for individuals who migrate from low- to high-incidence areas, suggesting that lifestyle and environmental factors contribute to its development.

While alcohol and processed meats are established carcinogens, and red meat is classified as probably carcinogenic, there is a lack of consensus regarding the relationships between other dietary factors and colorectal cancer risk. This uncertainty may be due, at least in part, to relatively few studies giving comprehensive results on all food types, as well as dietary measurement errors, and/or small sample sizes.

Study Tracked 97 Dietary Factors

To address these gaps, the research team, led by the University of Oxford, tracked the intake of 97 dietary factors in 542,778 women from 2001 for an average of 16.6 years. During this period 12,251 participants developed colorectal cancer. The women completed detailed dietary questionnaires at baseline, with 7% participating in at least one subsequent 24-hour online dietary assessment.

Women diagnosed with colorectal cancer were generally older, taller, more likely to have a family history of bowel cancer, and have more adverse health behaviours compared with participants overall.

Calcium Intake Showed the Strongest Protective Association

Relative risks (RR) for colorectal cancer were calculated for intakes of all 97 dietary factors, with significant associations found for 17 of them. Calcium intake showed the strongest protective effect, with each additional 300 mg per day – equivalent to a large glass of milk – associated with a 17% reduced RR. 

Six dairy-related factors associated with calcium – dairy milk, yogurt, riboflavin, magnesium, phosphorus, and potassium intakes – also demonstrated inverse associations with colorectal cancer risk. Weaker protective effects were noted for breakfast cereal, fruit, wholegrains, carbohydrates, fibre, total sugars, folate, and vitamin C. However, the team commented that these inverse associations might reflect residual confounding from other lifestyle or other dietary factors.

Calcium’s protective role was independent of dairy milk intake. The study, published in Nature Communications, concluded that while “dairy products help protect against colorectal cancer”, that protection is “driven largely or wholly by calcium”.

Alcohol and Processed Meat Confirmed as Risk Factors

As expected, alcohol showed the reverse association, with each additional 20 g daily – equivalent to one large glass of wine – associated with a 15% RR increase. Weaker associations were seen for the combined category of red and processed meat, with each additional 30 g/ per day associated with an 8% increased RR for colorectal cancer. This association was minimally affected by diet and lifestyle factors.

Commenting to the Science Media Centre (SMC), Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London, said: “One theory is that the calcium may bind to free bile acids in the gut, preventing the harmful effects of free bile acids on gut mucosa.” However, the lactose content in milk also has effects on large bowel microflora, which may in turn affect risk.

Also commenting to the SMC, Dr David Nunan, senior research fellow at the University of Oxford’s Centre for Evidence Based Medicine, who was not involved in the study, cautioned that the findings were subject to the bias inherent in observational studies. “These biases often inflate the estimated associations compared to controlled experiments,” he said. Nunan advised caution in interpreting the findings, as more robust research, such as randomised controlled trials, would be needed to establish causation.

Dr Sheena Meredith is an established medical writer, editor, and consultant in healthcare communications, with extensive experience writing for medical professionals and the general public. She is qualified in medicine and in law and medical ethics.