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Diet plays a key role in human health, but most of the research has been in Western countries. There is limited data in low- and middle-income countries, including China, where dietary patterns differ from those in Western countries. Over the past 10 years, CKB has generated important new evidence about the roles of several plant- and animal-based dietary factors in risks of major diseases that have worldwide relevance.

Plant-based dietary factors

In many regions of China, particularly inland Sichuan and Hunan provinces, spicy foods are widely consumed, often from a very young age. Overall 30% of the CKB participants reported consuming spicy foods daily (or on most days), with large variations across the 10 study areas, ranging from around 3-5% in Zhejiang province and neighbouring Suzhou city (coastal areas) to about 70% in Sichuan and 100% in Hunan province.

We demonstrated that people who consumed spicy foods regularly (i.e. 6-7 days a week) had a 14% lower risk of all-cause mortality compared with those who ate spicy foods less than once a week (BMJ, 2015).

Eating spicy foods was inversely associated with mortality from several major diseases, including cancer, ischaemic heart disease (IHD), and respiratory diseases. For site-specific cancer, regular spicy food consumption was associated with 19%, 11% and 10% lower risks of oesophageal, stomach and colorectal cancer, respectively. These results are important and can help to dispel concerns about the potential harmful effects of spicy foods (particularly when eaten in large quantities) on gastrointestinal cancers (IJE, 2021).

Fresh fruit and vegetables are rich sources of potassium, dietary fibre, antioxidants, and many other potentially active compounds. Daily consumption of fresh vegetables was universal across CKB participants in all 10 study areas (so difficult to study effectively), while only approximately 40% of participants reported consuming fresh fruit, mainly apples and oranges, 6 to 7 days a week. We demonstrated that daily consumption of 100 g fresh fruit was associated with about one-third lower risk of cardiovascular disease (CVD) mortality (NEJM, 2016). The associations observed in CKB were stronger than those previously reported in Western studies. The reasons for greater beneficial effects in CKB are not fully understood but may partly reflect the fact that Chinese people typically consume raw fruit, while in Western populations many consume cooked or processed (such as canned or frozen fruit or fruit juice). Higher fruit consumption was also associated with lower risks of deaths from cancer (particularly digestive tract cancer) and chronic obstructive pulmonary disease (COPD) (IJE, 2017). Higher fruit consumption was also associated with a lower incidence of new-onset diabetes, and, among those with diabetes, lower rates of microvascular and macro-vascular complications (PLOS Med, 2017). These findings have important health implications, especially given the long-standing concern, particularly in Asian countries, about the potential adverse effects of fruit on glycaemic control due to the high sugar (i.e. glucose and fructose) content of many fruits.

Soybean is a major source of plant-based protein and fibre in the Chinese diet. In a pooled analysis of data from CKB and 26 other prospective cohort studies involving more than 1 million participants, soybean intake was not associated with incident type 2 diabetes (J Nutr 2021). However, in a pooled analysis of CKB and eight other prospective studies, there was a weak, albeit significant, inverse association of soybean intake with breast cancer, with each 10 mg/day increment in soy isoflavone intake associated with 3% lower risk of breast cancer (Euro J Epidemiol, 2020).

In China, coarse grains are a major type of staple foods distinct from white rice and refined wheat products, and are comparable with whole grains consumed by Western populations. We found modest beneficial associations of higher intakes of coarse grains with risk of cardio-metabolic diseases (Nutrients, 2018), with each 100 g/day higher intake associated with 14% and 13% lower risks of diabetes and ischaemic stroke, respectively.

Animal-based dietary factors

Except for fish, animal-based foods tend to be less healthy than plant-based foods. Consistent with previous studies, we found that higher intake of red meat was associated with higher risk of diabetes, with an 11% higher risk for each 50 g/day higher intake of red meat. The association was independent of adiposity, but appeared more pronounced in people with higher bodily iron stores. We also demonstrated a positive association of fish intake with risk of diabetes, which was chiefly mediated through higher levels of adiposity (Diabetologia, 2020).

Egg is a major source of dietary cholesterol, but the available evidence does not support a significant association between egg consumption and CVD risk (BMJ, 2020). In CKB participants, however, we found a modest but highly significant inverse association of egg consumption with CVD and main CVD types. Overall, daily egg consumption was associated with 12%, 26%, 10% and 28% lower risks for IHD, haemorrhagic stroke, ischaemic stroke and CVD death, respectively, compared to non-consumers (Heart, 2018).

The consumption of dairy products has increased significantly in China over the last few decades, even though many Chinese are genetically lactose intolerant. Previous epidemiological studies have reported modest beneficial associations between dairy products and many health outcomes including obesity, diabetes, CVD, osteoporosis and fracture, and certain cancers (such as colon cancer). However, there have been some concerns about the potential adverse effects of dairy consumption on certain cancers (e.g. breast cancer and prostate cancer) due to the high content of growth factors and hormones (e.g. insulin-like growth factor 1: IGF-1), oestrogen and progesterone in dairy products. Indeed, analysis of CKB data found that a 50 g per day higher consumption of dairy products was associated with 7%, 12%, 17% and 19% higher risks of total cancer, liver cancer, female breast cancer and lymphoma, respectively (BMC Medicine, 2022).

Impact of research

Research based on CKB data has contributed important local evidence to enhance nutrition in China. In response to the growing burden of chronic diseases, the Chinese government has developed a Healthy China 2030 Plan, with diet prioritised as one of the leading actionable areas. Several key CKB findings were incorporated into prevention guidelines for chronic diseases in China (Chinese Circulation Journal, 2020) and elsewhere.

The key challenges in nutritional epidemiology are related to reliable exposure assessment, establishment of cause-effect associations and likely biological mechanisms involved in linking dietary factors with specific diseases. As well as continuing to explore the relationships of dietary factors with a wider range of other health outcomes (e.g. autoimmune diseases), our future research will assess their associations with novel biomarkers (e.g. metabolites, proteins, and gut microbiomes) that should become available in the near future.