In this study, we estimated the number and proportion of cancer deaths attributed to excess body weight and observed their changes during 2006-2015 in China. Overall, about 45 385 cancer deaths (3.17% of all) were attributable to excess body weight in China in 2015, of which, 17 322 cancer deaths (1.21% of all) were attributable to obesity. PAFs were higher in women and subjects in the east and northeast. Renal cell carcinoma was the leading cause of EBW-attributable cancer deaths in men, whereas ovary cancer in women. From 2006 to 2015, the overall PAF attributed to overweight first decreased slightly and then increased, but the corresponding figures for obesity and EBW were both on the rise. Our results indicate that the burden of EBW-related cancer in China has increased over the past decade.
We estimated that 3.17% of all cancer deaths were attributed to EBW in 2015, which was higher than 0.32% of EBW-related cancer deaths in a previous study conducted by Wang and his colleagues11. This discrepancy can be explained by the following reasons: firstly, different prevalence rates of EBW (14.7-35.96% in our study VS. 5.8%-16.1% in the previous study) and RRs were used in these two studies. Secondly, twelve cancers were included in our study, compared with six cancers in the previous study. Another study conducted in China demonstrated that 3.5% of all cancers deaths in 2013 were attributable to high BMI 19, which was similar to our study. We also compared the combined PAFs for EBW and cancer mortality in our study with similar figures in previous studies from different countries as shown in Table S3. PAFs in our analysis were relatively lower than the corresponding estimates in the worldwide studies10, Australia study20, US study21, UK study22, Germany study23, but higher than that in the Japan study24. Because our PAF was estimated in 2015, and the relevant data in Japan study was in 2005, whereas the prevalence of obesity in East Asia increased from 2005 to 201525. The discrepancy of PAFs between our analysis and other studies could be partly explained by selected cancers, prevalence and RR, as well as different ethnicity. For example, previous studies showed that the prevalence of obesity in Europe, the United States and Australia was significantly higher than that in China7,25.
We also found more cancer deaths attributable to excess weight in the East and Northeast of China. This may be due to the higher prevalence of EBW in economically developed areas such as the East and Northeast9. In China, the prevalence of overweight and obesity in adults was positively correlated with income, but this correlation may change in the near future. In our study, overweight and obesity played a larger role in women than that in men, which was consistent with the studies shown in Table S3. The main explanation could be due to the higher prevalence of overweight and obesity in women than that in men in the past9.
In our study, obesity had greater impact on liver, breast, and endometrial cancers than overweight. It may be blame to that obese person were more likely to have adipose tissue and visceral fat than overweight person. A study conducted by Calle et al. found that adipose tissue contributed to breast and endometrial cancers in postmenopausal women by secreting endogenous estrogens, while visceral fat contributed to liver cancer by promoting non-alcoholic fatty liver disease and non-alcoholic steatohepatitis26.
In recent years, dietary patterns have changed significantly in China, with increased consumption of animal-based foods, refined grains and highly processed, high-sugar, high-fat foods, whereas levels of physical activity have declined in line with increased sedentary behaviors9. In this context, the prevalence of overweight and obesity in China has increased significantly. As a consequence, a corresponding increase in cancer deaths attributed to overweight and obesity has been seen over the last decade in this study. This finding was similar to another study in China11. In eastern China, however, we found that PAF did not always show an upward trend from 2006 to 2015. It may result from the different stages of nutrition transition and different allocation of health resources in different regions9.
Many efforts have been made in Europe and the United States to reverse the rising prevalence of overweight and obesity. For example, the United States has tried to prevent and control the obesity epidemic by increasing convenient sports facilities in communities, improving school diet environments, strengthening nutrition education, imposing taxes on sugary drinks, subsidizing healthy foods and using nutrition labels21,27. Denmark has introduced a saturated fat tax28, and the UK, in addition to its soft drinks tax, has banned TV and online adverts promoting foods high in sugar, fat and salt before 9pm and stopped "buy one, get one free" campaigns for unhealthy foods29. China has also carried out a series of activities aimed at promoting healthy diet and physical activity, such as The Chinese Student Nutrition Day, the Sunshine Campaign, "Three reduce three promote health action"30. In addition, in the "Healthy China 2030" plan outline, China has put forward overall health management policies, providing a good environment for the establishment of a comprehensive obesity prevention and treatment system31. However, COVID-19 home quarantine may increase sedentary behaviors and reduce opportunities for physical activity32, and some successful obesity control experiences from high-income countries may be not applicable to China. We still need to further develop comprehensive environmental and social measures as well as individualized measures to prevent and control overweight and obesity in China.
Our study could provide the latest evidence on the cancer burden attributable to overweight and obesity in China and examined longitudinal changes in PAF for cancer deaths and EBW during 10 years. A previous study also estimated the change of PAF from 2005 to 201511. However, they used the exposures in 2002 to predict PAF in 2015 and did not calculate the total PAF, whereas we calculated PAF in each year from previous exposures and current cancer deaths, so our results might be closer to reality. Moreover, in our study, data used to calculate BMI were obtained from anthropometric measurements, which were objective and made our results more reliable. However, our study has several limitations. Firstly, we did not include esophageal cancer, which was also considered by ICAR to be associated with EBW6, so our results may be underestimated. However, studies have shown that obesity is positively associated with the occurrence of esophageal adenocarcinoma, whereas inversely associated with the occurrence of squamous cell carcinoma33,34, accounting for 86.3% of esophageal cancers with definite tissue types in China16. So, the effect of this underestimation might be small. Secondly, the same RRs of cancer incidence were used as that of cancer mortality, which may lead to some bias, although a study have shown little difference35. Thirdly, we used the same RRs for both genders, but the strength of association may be actually different by genders. Fourthly, the RR of multiple myeloma was obtained from a pooled analysis conducted in European population, and it may not apply to the Chinese population. Finally, we assumed that the latency between excess body weight inducing cancer death was 10 years, whereas the assume in another study was 15 years5, and the choice of latency time may affect the PAF.