Beyond the Headlines: Is it Milk or Calcium That Cuts Bowel Cancer Risk?
Depending upon your perspective, a recent study purporting the protective effect of daily milk consumption against developing bowel cancer may be a glass half full, half empty, or just empty.
“Having a large glass of milk every day may cut the risk of bowel cancer by nearly a fifth, according to the largest study conducted into diet and the disease,” reported The Guardian on 8 January (with similar sentiments echoed by many other publications).
The article, by science editor Ian Sample, goes on to underline that the results of the analysis showed specifically that a daily intake of 300 mg of calcium, “about the amount found in half a pint of milk,” was linked to a 17% reduction in bowel (colorectal) cancer risk, with non-dairy sources of calcium acknowledged to have “a similar protective effect.”
But that nuance disappeared when he quoted the first author of the study, Keren Papier, PhD, from the Cancer Epidemiology Unit at the University of Oxford, Oxford, UK. She said: “This comprehensive study provides robust evidence that dairy products [my italics] may help prevent colorectal cancer, largely due to the calcium they contain.”
Cancer Research UK, which funded the study, was even less subtle when it proclaimed on its website, in an article by Sophie Wedekind: “Bowel cancer risk could be reduced with an extra glass of milk.”
Where did I go wrong?
While their piece, like that in The Guardian, went on to deliver the necessary qualifications to that statement (including that the results pertain only to women), alongside more moderate quotes from Papier, the message that lingered on in the minds of many readers was clear.
Writing in the Comments section at the bottom of the Cancer Research UK piece, Mrs A Gibson, said: “I was very upset about this article as I have drunk milk every day since I was born, some 69 years ago and yet I got bowel cancer in May 2019, no warning signs and saw two doctors who said there were no concerns, I’d had tummy issues and one borderline bowel screening test!,
Where did I go wrong.”
Stephen Mac added: “Many thanks for this information. I have porridge oats every day, made with water, but will switch now to low fat milk. I usually only consume milk in coffee or tea so hopefully this is a small step in the right direction.”
Another commenter, Katie, picked up on the detail of the piece, however, and questioned its tenor.
“So it’s calcium that has the impact, not milk itself? Seems misleading to go with this as the headline when there are millions of people around the world who are intolerant to dairy and/or don’t want to buy into the dairy industry.”
Some good, some bad
But what did the study actually show?
Papier and colleagues looked at data from the Million Women Study, which included 1.3 million women with an average age of 56 years between 1996 and 2002 who were invited to the National Health Service (NHS) Breast Screening Programme in England and Scotland.
From that, the researchers selected almost 550,000 women who had completed a dietary questionnaire and for whom there was information on cancer occurrence and death over almost 17 years of follow-up.
Specifically, they looked at 97 foods, drinks and nutrients to determine their association with the likelihood of developing colorectal cancer, finding that 17 were linked to the disease. For example, for every 20 g of alcohol (the equivalent of a large glass of wine) drunk per day there was a 15% higher risk of bowel cancer, while each 30 g of red and processed meat eaten per day increased the risk of the disease by 8%.
On the other hand, each 300 mg of calcium consumed per day reduced the risk of colorectal cancer by 17%. Other items linked to a lower risk of the disease included dairy milk, yogurt, breakfast cereals, fruit and whole grains, alongside riboflavin (vitamin B2), folate, vitamin C, and minerals such as magnesium, phosphorus and potassium.
A questionable impact?
Experts not related to the study were quick to point out its limitations:
“While this study highlights a potential benefit of increased dietary calcium, journalists and the public should approach the findings with caution,” warned David Nunan, PhD, a Senior Research Fellow at the Centre for Evidence Based Medicine, University of Oxford, Oxford, UK.
He pointed out that the headline-grabbing 17% reduction in colorectal cancer risk tells only part of the story, as that is the relative, not absolute, risk reduction. “In the UK, approximately 5% of women (one in 20) are diagnosed with colorectal cancer annually,” he explained.
“Using the 17% relative risk reduction figure from the study, if 100 women increased [their] calcium intake to 345 mg per day, the number of diagnoses could drop from five to four—a difference of one fewer case per 100 women.”
Nunan added: “However, this means 95 out of 100 women would remain unaffected by cancer regardless of their calcium intake.”
“Cows’ milk is a rich source of dietary calcium and riboflavin, and the relationships between milk, calcium and riboflavin and risk of cancer incidence were almost identical,” pointed out Tom Sanders, DSc, PhD, Professor emeritus of Nutrition and Dietetics at King’s College London (KCL), London, UK.
“A weaker protective relationship was found with breakfast cereals, especially wholegrain cereals but this could be confounded by the fact that breakfasts are consumed with milk.”
He added: “The limitations of the study are that it is observational and not a randomized controlled trial. It is also difficult to separate the effects of individual nutrients from an effect of milk per se.”
The authors of the study also acknowledged that the results are limited by not being able to include some dietary items, such as butter, due to the format of the survey, and they could not determine whether calcium supplements were beneficial.
“Additionally, although the women in the cohort are representative of middle-aged and older women living in the UK, they are predominantly of European ancestry,” they write. “Therefore, the results are not necessarily generalisable to other populations, or to populations where a large majority cannot digest lactose (including e.g. many Asian populations).”
Consequently, they conclude: “Additional research is needed to investigate overall health benefits or risks associated with higher calcium intakes.”
The study was funded by Cancer Research UK and the UK Medical Research Council.
Prior to his retirement, Sanders was a consultant to the Breakfast Cereals Advisory Board of the Food and Drink Federation.
One author of the study declares a relationship with AbbVie, while her husband declares relationships with BioNTech SE – ADR, Amazon, CureVac BV, NanoString Technologies, Google/Alphabet Inc Class C, NVIDIA Corp, Microsoft Corp.
No other relevant financial relationships declared.
Liam Andrew Davenport is a medical reporter with more than 20 years’ experience covering a wide range of specialties and topics in the field.