Effects not necessarily causal
Ian Givens, PhD, professor of food chain nutrition at the University of Reading (England), said the study was “potentially very important for Chinese people, if it can be confirmed that dairy products affect the risk of breast and/or liver cancer differently in Chinese subjects to those in Western Societies, especially as dairy consumption in China is much lower than in most Western diets.”
He added: “As always it needs to be kept in mind that this type of study can only establish associations with disease risk, not cause.”
Dr. Kakkoura, nutritional epidemiologist at Oxford (England) University’s department of population health, said: “This was the first major study to investigate the link between dairy products and cancer risk in a Chinese population. Further studies are needed to validate these current findings, establish if these associations are causal, and investigate the potential underlying mechanisms involved.”
The researchers said that, while the results do not prove causation, “there are several plausible biological mechanisms that may explain these associations.” They pointed to higher dairy consumption potentially increasing levels of insulinlike growth factor-I, known to promote cell proliferation and associated with higher risks of several types of cancer.
In addition, estrogen and progesterone present in cows’ milk may play a role in increasing breast cancer risk, whilst saturated and trans-fatty acids from dairy products may increase the risk of liver cancer. As many Chinese people are lactase deficient, dairy products may also be broken down into products that affect cancer risk.
No justification for dietary change
Confounding factors may also have influenced the results, commented Duane Mellor, PhD, RD, RNutr, registered dietitian and senior teaching fellow at Aston University, Birmingham, England. “Those in the study who consumed dairy were more likely to live in cities and have other health conditions, including cardiovascular disease and diabetes – although some of these factors were considered in the analysis, not all of these covariates were, which could influence the findings.
“In my view this study alone does not provide strong evidence that reducing dairy intake would reduce cancer risk.”
He added: “Although the paper suggests a 12% increased relative risk for female breast cancer, this does not equate to 12 more cases per 100 individuals – in absolute terms this would be more like 1 or 2 cases per 1,000 people.”
Similarly, Kevin McConway, PhD, emeritus professor of applied statistics at the Open University, Milton Keynes, England, said: “An issue is that there were many differences between the people that consumed different amounts of dairy products, apart from their difference in dairy consumption. For instance, of those who never or rarely consumed dairy products, fewer than a third lived in urban areas, but of regular dairy consumers (at least once a week), 83% lived in urban areas. Regular consumers were considerably more likely to be well educated than those who never or rarely consumed dairy products, and there were other differences too.
“So if, as the researchers found, a greater proportion of the regular consumers than of the never or rare consumers had a cancer diagnosis, that could have been because of their different dairy consumption, or it could have been (in part or entirely) because of the different places they lived, or their different education levels, or any of the other factors on which the groups differed.
“One can never be sure that all the relevant factors have been adjusted for. That’s why the researchers rightly say that these results can’t establish whether the associations between dairy consumption and the risks of some cancers, that they found, are there because the dairy consumption differences change the cancer risks in a cause-and-effect way. They might, or they might not.”
He cautioned: “I don’t think anyone should decide to change their individual diet solely because of the results of this new study.”
Commenting on the study, Fiona Osgun, senior health information manager at Cancer Research UK, London, told this news organization: “This early-stage study found an association between dairy consumption and the risks of certain cancers, but that doesn’t mean that they’re causing them or that people need to avoid dairy. Dairy products can be part of a healthy balanced diet and, in the U.K., the Food Standards Agency regulates them to make sure they’re safe. There’s good evidence that dairy reduces the risk of bowel cancer, but no clear evidence for other cancer types, and this is no different for people who are lactose intolerant.”
A version of this article first appeared on Medscape UK.