A relationship between consumption of dairy products and risk of various cancers has been intensively investigated in the past but yielded inconclusive or conflicting results. Now a large new study comparing Chinese dairy consumption with that in the UK, shows that increased dairy consumption was linked to higher risks of liver cancer and female breast cancer.
The study, by researchers from Oxford Population Health, Peking University, and the Chinese Academy of Medical Sciences in Beijing, used data from the China Kadoorie Biobank Study, a long-term prospective study involving more than over 510,000 participants recruited from 10 geographically diverse areas across China, including both rural and urban regions. They compared this to data from the UK biobank.
Subjects were 59% female, 41% male, aged 30-79 years, and had no history of cancer at recruitment between 2004 and 2008. Food questionnaires were completed at the outset and participants followed for an average of 11 years, using national cancer and death registries and health insurance records to identify new cancer diagnoses, including both fatal and non-fatal events.
Participants were categorised into three groups according to how often they consumed dairy products (primarily milk):
- Regular consumers (at least once a week): 20.4% of the cohort
- Monthly consumers: 11.1%
- Non-consumers who never or rarely consumed dairy products: 68.5%
Average dairy consumption was 37.9 g/day overall and 80.8 g/day among regular consumers. This compares with an average consumption of around 300 g/day in participants in the UK Biobank cohort.
Over the course of the study, 29,277 new cancer cases were recorded, including 6282 lung, 2582 female breast, 3577 stomach, 3350 colorectal, and 3191 liver cancer cases.
Analyses correlating cases with consumption took into account a range of other factors potentially affecting cancer risk, including age, sex, region, family history of cancer, socio-economic status (education and income), lifestyle factors (alcohol intake, smoking, physical activity, soy consumption and fresh fruit intake), body mass index, chronic hepatitis B virus infection, and female reproductive factors.
Higher Dairy Intakes Linked With Risk of Liver and Breast Cancers
Results revealed that that higher regular dairy intake was associated with significantly higher risks of liver cancer and female breast cancer, both common types of cancer in China. Analyses indicated that for each 50-g/day intake, the risks increased by 12% and 17%, respectively.
There was also an increase in total cancer diagnoses, and an increased risk of lymphoma, though this was not statistically significant after correction for confounders. No association was found between dairy products and colorectal cancer, prostate cancer, or any other site-specific cancer.
The research, published in BMC Medicine, is the first major study to investigate dairy consumption and cancer risk in Chinese adults. The results conflict with previous studies on Western populations, which have suggested that dairy products may be associated with a lower risk of colorectal cancer and a higher risk of prostate cancer but have found no clear link for breast or other types of cancer.
Lead researchers Dr Maria Kakkoura and Associate Professor Huaidong Du told Medscape UK that, although they don’t know the reason for the difference, "there is clear evidence that colorectal cancer has a different incidence pattern in China compared with Western countries. Other risk factors, like adiposity, may have a stronger effect on the risk of colorectal cancer in Western countries than in China". Notably, the mean BMI in the study population was around 23 kg/m2, they said – by contrast in the UK it is 27.6 kg/m2.
Effects Not Necessarily Causal
Ian Givens, professor of food chain nutrition at the University of Reading, 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 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 insulin-like growth factor-I, known to promote cell proliferation and associated with higher risks of several types of cancer.
In addition, oestrogen 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 Dr Duane Mellor, registered dietitian and senior teaching fellow at Aston Medical School. "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 1000 people."
Similarly, Kevin McConway, emeritus professor of applied statistics at the Open University, 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, told Medscape UK: "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 UK, 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."
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