For most people, dairy products are foods with high nutritional value. Dairy products can not only provide high-quality protein, vitamins B2 and B12, calcium and other nutrients, but also lactose in milk can promote the absorption of calcium, iron, zinc and other minerals. However, the impact of dairy products on cancer has been controversial all over the world. The World Cancer Research Foundation / American Cancer Institute (WCRF / AICR) pointed out that dairy products may help prevent colorectal cancer, but higher dairy intake is associated with a higher risk of prostate cancer.
Because many Chinese people have difficulty digesting and absorbing lactose, and the intake of dairy products has been low, the relevant research carried out in the Chinese population is very limited.
Recently, a study published in BMC medicine has added important evidence for the impact of Chinese eating dairy products on cancer. The data of more than 10 years' follow-up of 500000 people in different regions of China show that among Chinese adults, more dairy products are associated with higher risk of liver cancer, female breast cancer and overall cancer. The paper points out that the study is "the first and largest prospective cohort study in China" on dairy products and cancer risk.
The study was jointly completed by experts from the University of Oxford, the school of public health of Peking University, Fuwai Hospital of the Chinese Academy of Medical Sciences, the Chinese Academy of Medical Sciences, Henan Center for disease prevention and control and China national food safety risk assessment center.
The research team also objectively stated in the paper that although a series of confounding factors have been adjusted, this study is not enough to confirm that this association is causality. These findings and the accumulation of evidence from more future studies may provide important information for evidence-based diet recommendations for cancer prevention for the Chinese population.
At present, the dietary guidelines for Chinese residents (2022) recommends that healthy people aged 2 and over drink 300 ml ~ 500 ml of milk or equivalent dairy products every day.
research method
This large-scale prospective study was conducted based on the Kadoorie Biological Sample Bank (CKB) in China, covering about 510000 people. The participants joined the study between 2004 and 2008, with an average age of 52, from five urban areas of Qingdao, Harbin, Haikou, Suzhou and Liuzhou, and five rural areas of Sichuan, Gansu, Henan, Zhejiang and Hunan. All participants did not have cancer at the time of joining the study.
The researchers recorded the participants' eating habits in the past year through a questionnaire. To facilitate the assessment of diet, the researchers divided all foods into 12 groups, namely rice, wheat products, coarse grain products, red meat, poultry, fish, eggs, total dairy products, fresh vegetables, pickled vegetables, fresh fruits, and bean products; The frequency of food intake was divided into five categories: daily, 4-6 days a week, 1-3 days a week, monthly and never / rarely. The life and health status of participants was determined according to the information of China disease surveillance system, cancer registry and national health insurance claim database.
Research results
Among all participants, the average daily intake of dairy products was 37.9 grams. 20.4% of the participants ate dairy products at least once a week (regular eating habits), and their average daily intake was 80.8 grams. 68.5% of people never or rarely eat dairy products. For both men and women, the intake of dairy products in urban areas is higher than that in rural areas.
After an average follow-up of 10.8 years, a total of 29277 people developed cancer. The researchers stratified by high-risk age, gender and region, and adjusted for family history of cancer, education, income, alcohol intake, smoking, physical activity, intake of soybeans and fresh fruits, and body mass index (BMI). The intake of dairy products was significantly positively correlated with the overall risk of cancer and the risk of cancer in some specific parts (P & lt; 0.05).
Specifically, for every 50g increase in dairy intake per day:
It was associated with a 7% increase in the overall risk of cancer (HR = 1.07, 95% CI 1.04 – 1.10);
It was associated with a 12% increased risk of liver cancer (n=3191) (hr=1.12, 95% CI 1.02 – 1.22) and was not associated with hepatitis B infection;
It was associated with a 19% increased risk of female breast cancer (n=2582) (hr=1.19, 95% CI 1.01 – 1.41);
It was associated with a 17% increased risk of lymphoma (n = 915) (HR = 1.17, 95% CI 1.07 – 1.29).
After adjusting the data several times, the researchers found that the association between dairy intake and the risk of lymphoma was not statistically significant. Moreover, no significant relationship between dairy intake and colorectal cancer or cancer in other parts was observed.
The researchers also found that compared with people who did not eat or ate less dairy products, people who ate dairy products regularly:
It is more likely to be female, with higher education, higher income, and generally better health, but the proportion of patients with cardiovascular disease or diabetes is also slightly higher.
The average height is higher (0.6cm high) and the legs are longer (0.3cm long).
Both body weight and BMI are low, with an average BMI of 0.4 kg / m2 in men and 0.5 kg / m2 in women. However, no clear association was observed between dairy intake and waist circumference or body fat.
summary
The paper concludes that among Chinese adults with relatively low dairy intake, higher dairy intake may be associated with higher risk of liver cancer, female breast cancer and lymphoma. The impact of dairy intake on cancer is independent of other lifestyle factors, including obesity. The possible mechanisms are:
Higher intake of dairy products may lead to an increase in plasma insulin-like growth factor-I (IGF-I), which plays a key role in cell proliferation and cancer development.
Milk contains high levels of branched chain amino acids, lactose and IGF-I, which can activate and enhance the mechanism of rapamycin complex 1 (mTORC1) signal transduction, so as to promote cell proliferation and may lead to cancer.
Saturated fatty acids (SFA) and trans fatty acids in dairy products may be associated with insulin resistance and elevated levels of proinflammatory cytokines, which may be risk factors for the development of liver cancer and lymphoma.
In recent decades, although the dairy intake of Chinese people has increased significantly (the average intake increased from 14.9 g / day in 1992 to 24.7 g / day in 2012), it is still far lower than that of other countries (for example, the per capita intake of the United States was about 400 g / day in 2015). The study's analysis of height also supports the role of dairy products in slowing bone loss.
The paper said that the study also has limitations. For example, the questionnaire only collected the intake of some major food groups, without the intake of total energy, specific nutrients (such as SFA and calcium) or specific dairy products. Moreover, although a large number of cancer cases have been recorded, the statistical power of some less common cancer sites (such as prostate cancer) is still very low. Even for common cancer types, the number of cases may not be enough to obtain reliable results in subgroup analysis (for example, it is difficult to analyze the risk of breast cancer related to estrogen receptor status).
In the future, more research is needed to further explore the potential mediating role of lactose intolerance between dairy intake and cancer risk. It is expected that future research can determine its causality and potential mechanism.
reference material
[1] Kakkoura， M.G。， Du， H。， Guo， Y。 et al。，（2022）。 Dairy consumption and risks of total and site-specific cancers in Chinese adults： an 11-year prospective study of 0.5 million people.BMC Med 20， 134 （2022）。 https://doi.org/10.1186/s12916-022-02330-3