Need an excuse to tuck into a cheeseboard? A handful of studies have recently suggested that dairy reduces the risk of colorectal cancer.
Also known as bowel cancer, this is the type of the disease worldwide and the second leading cause of cancer deaths. If dairy can be protective, it might seem a no-brainer that we up our consumption, but is it so simple?
What clouds the situation is how confident we should be about studies that find a correlation between two things, rather than proving that one directly influences the other.
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For instance, in a recent study, at the University of Oxford and her colleagues used on more than 540,000 women in the UK who had been regularly asked what they ate. During this time, more than 12,000 of them were diagnosed with colorectal cancer.
The researchers looked at 97 food or nutritional factors, such as vitamins and types of fat, extrapolated from those diets and linked the consumption of dairy, which is high in calcium, to protection against colorectal cancer. They also found that every additional 300 milligrams of calcium per day was associated with a 17 per cent reduction in risk.
鈥淭his is the most comprehensive single study ever conducted into the relationship between diet and bowel cancer, and it highlights the potential protective role of dairy, largely due to calcium, in the development of bowel cancer,鈥� says Papier.
Calcium 鈥渕ay protect against colorectal cancer by attaching to bile acids and free fatty acids in the colon, which helps reduce their cancer-causing potential鈥�, she says.
Her study chimes with another recent that followed聽more than 470,000 adults for over 20 years. This similarly found an association between reduced colorectal cancer risk and higher calcium intake, from any source, including cheese, milk and yogurt.
Both papers are based on people filling out questionnaires 鈥� a notoriously unreliable way of assessing diet. Nevertheless, they 鈥渁dd to and strengthen the existing evidence that consuming more dairy foods lowers bowel cancer risk and that it is the calcium in dairy products that appears to be beneficial鈥�, says at Newcastle University, UK.
To bolster the results they got from using questionnaires, Papier鈥檚 team did another analysis using a technique called Mendelian randomisation. This uses the genetic variation that occurs among people as a stand-in for the way researchers randomly assign participants to groups in experimental trials, so we can be more confident that trends represent causal links.
For the randomisation, the researchers were interested in a gene called LCT, which can act as a proxy for consuming milk because it codes for an enzyme needed to break down the sugar lactose. The team found that a variation in a single nucleotide near the gene was associated with a reduced risk of colorectal cancer. 鈥淭he Mendelian randomisation approach avoids some of the confounding that is problematical in conventional epidemiological studies of diet and cancer,鈥� says Mathers.
A third , led by at Brigham and Women鈥檚 Hospital in Boston, adds to the mounting evidence, but turns the spotlight away from calcium. His team followed some 87,000 women for 40 years and 45,000 men for 30 years, asking every four years about their yogurt consumption.
At least two servings a week of plain yogurt was linked to a 20 per cent lower rate of colorectal tumours that contained Bifidobacterium bacteria. This bacterium is found in yogurts and a strain has been linked to in the mammary glands of rats.
Ideally, trials would randomly assign people to consume dairy or not, thereby proving causation. But the follow-up period required for subsequent cancer diagnoses to take place makes this unrealistic. 鈥淚t is impractical or impossible to conduct a trial with yogurt randomised to people over 40 years,鈥� says Ogino.
Muddying the waters further, a recent stated that the link between dairy and various cancers is inconclusive.聽Instead, it emphasised what might seem very familiar advice: that a Mediterranean diet rich in fish, fruits and vegetables, but with limited dairy, may be the best approach.
Even so, Papier is convinced that calcium is key, and that other sources than yogurt can be beneficial. Her team looked at the impact of calcium regardless of whether it came from dairy or from other calcium-rich foods, such as tofu, canned fish and fortified plant-based milks. 鈥淐alcium was found to have a similar effect in both dairy and non-dairy sources,鈥� says Papier.
When it comes to calcium supplements, a 2018 from the World Cancer Research Fund and American Institute for Cancer Research suggests that they decrease the risk of colorectal cancer, but another found that they may increase the risk of polyps that are precursors to the condition.
Given the widespread use of these supplements, further research is needed, says Papier. The benefits of Bifidobacterium probiotics are also unknown, says Ogino.
To him, these studies illustrate the complex connection between our diet and the risk of cancer, with there being no shortcut to health. 鈥淚f people try to go to a supermarket and buy more plain yogurt, then that鈥檚 good, but if they just stop in a day or two, there is no effect.鈥�
People should try to follow all the cancer prevention 鈥� such as being active and not smoking 鈥� and 鈥渘ot focus on the potential benefits of one food or one nutrient鈥�, says Mathers.
