To date, the evidence on whether eating dairy products reduces the risk of cancer has been inconsistent. Dairy products may be associated with a lower risk of colorectal cancer and a higher risk of prostate cancer in Western populations, but no clear link has been found for breast or other types of cancer. These findings, however, may not be applicable to non-Western populations, where dairy consumption and the ability to metabolize dairy products differ greatly.
For example, cheese and butter consumption in China is much lower than in Western populations, as is milk and yogurt consumption. Furthermore, due to a lack of lactase, a key enzyme for breaking down the milk sugar lactose, most Chinese adults are unable to properly metabolize dairy products.
Researchers from Oxford Population Health, Peking University, and the Chinese Academy of Medical Sciences, Beijing, published the findings of a new large-scale study in BMC Medicine today to determine whether dairy products affect cancer risk differently in Chinese people. This study gathered information from over 510,000 participants in the China Kadoorie Biobank Study.
The participants (59 percent female, 41 percent male) had no prior history of cancer and were recruited from ten geographically diverse regions across China between 2004 and 2008. Each participant (aged 30-79 years) completed a questionnaire about how frequently they consumed various food products, including dairy products, when they were recruited. The participants were divided into three groups by the researchers: regular dairy consumers (at least once a week), monthly dairy consumers, and people who never or rarely consumed dairy products (non-consumers).
Participants were followed for an average of 11 years, and the researchers identified new cancer diagnoses using data from national cancer and death registries as well as health insurance records. There were both fatal and non-fatal events included. Age, gender, region, family history of cancer, socioeconomic status (i.e. education and income), lifestyle factors (i.e. alcohol intake, smoking, physical activity, soy consumption, and fresh fruit intake), body mass index, chronic hepatitis B virus infection (for liver cancer), and female reproductive factors were all considered in the data analyses (for breast cancer).
The study discovered:
Overall, approximately one-fifth (20%) of participants consumed dairy products on a regular basis (primarily milk), 11% consumed dairy products monthly, and 69 percent were non-consumers. The overall average consumption in the study population was 38g per day, with regular dairy consumers consuming 81g per day (compared with an average consumption of around 300g per day in participants from the UK Biobank).
During the study period, 29,277 new cancer cases were recorded, with lung cancer (6,282 cases) having the highest rate, followed by female breast (2,582 cases), stomach (3,577 cases), colorectal (3,350 cases), and liver cancer (3,191 cases).
In China, liver and breast cancer are among the most common types of cancer, accounting for approximately 393,000 and 368,000 new cancer cases each year, respectively.
**** While these findings do not prove causation, the researchers believe there are several plausible biological mechanisms that could explain these associations. Increased dairy consumption, for example, may raise insulin-like growth factor-I (IGF-I) levels, which promote cell proliferation and has been linked to an increased risk of several types of cancer. Female sex hormones (such as oestrogen and progesterone) found in cow’s milk may play a role in an increased risk of breast cancer, while saturated and trans-fatty acids found in dairy products may increase the risk of liver cancer. Dairy products may also be broken down into products that affect cancer risk for the majority of Chinese people who do not produce enough lactase.
‘This was the first major study to investigate the link between dairy products and cancer risk in a Chinese population,’ said Dr Maria Kakkoura, Nutritional Epidemiologist at Oxford Population Health and the study’s first author. More research is required to validate these current findings, determine whether these associations are causal, and investigate the potential underlying mechanisms involved.’
Although China’s average dairy consumption remains much lower than that of European countries, it has increased rapidly in recent decades.
Associate Professor Huaidong Du, Senior Research Fellow at Oxford Population Health and one of the study’s senior co-authors, added, ‘While our findings suggest a direct link between regular dairy consumption and certain cancers, it is important to remember that dairy products are a source of protein, vitamins, and minerals.’ It would be unwise to reduce dairy consumption solely based on the findings of the current study, or without ensuring adequate protein, vitamins, and minerals from other sources.’