Background
Kidney cancer has been the common cancers in the world [
1,
2]. There are 48,210 new cases and 17,168 deaths in China in 2017, accounting for 12.27% and 12.39% of 393,042 new cases and 138,528 deaths worldwide, respectively [
2]. Kidney cancer mainly includes renal cell carcinoma with a proportion of 90–95% [
3]. The incidence of kidney cancer has increased year by year in recent years, rising by 2% to 3% compared with 10 years ago [
4]. The mortality of kidney cancer has been stable globally since the 1990s, while it decreases in most countries in recent years [
5]. In China, both incidence and mortality of kidney cancer seemed low, but previous study reported incidence of kidney cancer had greatly increased [
6]. The mortality trend of kidney cancer remains unknown.
Established risk factors for kidney cancer mainly include tobacco smoking, obesity/overweight, hypertension and chronic kidney disease [
5,
7‐
10]. It is currently accepted that there is a global epidemic of obesity [
11]. The prevalence of overweight or obesity in China adults is also rising greatly during 1993–2014 [
12,
13]. It is being concerned that obesity increased the risk of kidney cancer [
8,
14]. This relationship is attributed to abnormal secretion of adipokines, insulin resistance, higher estrogen level among overweight/obesity individuals [
9,
10]. However, with a rapid urbanization and transition to western dietary and lifestyle in China, people experienced an increasing exposure to obesity [
6,
15], and trends of kidney cancer death remains unknown. The prevalence of obesity/overweight is different between China and U.S. populations. Therefore, we aim to further estimate the time patterns of kidney cancer mortality attributable to high BMI to present the relative risk due to age, period and cohort of the mortality in China, compared with U.S., which could provide epidemiological evidence for kidney cancer prevention and control.
Discussion
The causes of kidney cancer are very complex, and the relevant literature shows that smoking, hypertension and obesity are the three most important risk factors [
27]. According to relevant studies, the summary relative risk estimate was 1.07 (95% CI 1.05–1.09) per unit of increase in BMI [
28]. With the socio-economic development, urbanization, intensified industrialization and changes in dietary and lifestyles, the exposure to kidney cancer risk factors in Chinese population has increased. From 1980 to 2000, global tobacco consumption showed an upward trend in most regions, and tobacco consumption in China increased the fastest; but decreased by 27% and 2% in the Americas and Europe [
29]. According to the Chinese national nutrition and health survey, the prevalence of hypertension in the population aged over 15 years old was 5.1% in 1958–1959, 7.7% in 1979–1980, 13.6% in 1991, and 17.6% in 2002, and the age-standardized rate also showed the upward trend [
15]. In 1982, overweight and obesity among the Chinese population were still very rare, at 6% and 0.6%, respectively. In 2014, the overweight rate reached 34.26% and the obesity rate was 10.98% [
15]. In overall, all these factors may be related to the increased mortality of kidney cancer in this present study.
Obesity/overweight is considered to be the increased risk for kidney cancer [
8,
14]. The prevalence of obesity/overweight is different between China and the U.S. Our study found that kidney cancer deaths attributable to high BMI showed an increasing tendency in China and men U.S., and the overall mortality rate of kidney cancer is still increasing in both genders in China but not in the U.S. Comparison to U.S., China has a serious attribution burden of kidney cancer death and BMI. The exposure to kidney cancer risk factors in Chinese population is increasing yearly. Potential kidney cancer risk factors include behavioural and environmental factors, comorbidities, and analgesics. Smoking, obesity, and hypertension represent established risk factors [
30]. In GBD 2017 study, spatiotemporal Gaussian process regression was used to estimate risk-attributable burden and risk exposure for many risks, typically those with rich age-sex-specific data. It synthesises noisy data by borrowing strength across space, time, and age to best estimate the underlying trends for a given risk. GBD study found considerable heterogeneity across super-regions in the leading risk factors. There are also marked spatial patterns for other risks such as high BMI in central America, north Africa and the Middle East, and Oceania [
22]. In our study, we obtained China and U.S. mortality data from GBD study to analyze the relationships between kidney cancer deaths and high BMI. The geographic heterogeneity in mortality rates of kidney cancer attributable to high BMI may be considerable between China and U.S.
In China, the increasing ASMR of kidney cancer attributable to high BMI was possibly associated with the prevalence of obesity [
12,
13]. The increasing attributable burden and death of kidney cancer may be related to the improved diagnosis level of kidney cancer [
6], and changes in exposure levels of kidney cancer risk factors in the population also have impacts on the mortality of kidney cancer [
31]. Differently, a stable trend or slight decline was observed for kidney cancer mortality in U.S., which may be explained by effective improving the rising obesity trend in the US adult population over the past decades [
32,
33]. In U.S., relevant researches reported that obesity prevalence remains need continue surveillance [
34], and the increasing mortality attributable to high BMI was observed in men U.S. Thus, this attributable burden about BMI and kidney cancer remains need be focused.
After adjusting the three effects using the APC model, we confirmed that the differences in age-specific mortality rate patterns exist. A similar trend of age effect on kidney cancer mortality attributable to high BMI was observed between China and the U.S. adults. The age effect increased in the two areas, which indicated that the risk of the mortality of kidney cancer attributable to high BMI tended to increase in the middle-aged and younger groups. This finding may suggest that aging has driven the trend of the mortality [
35,
36]. The interesting phenomenon is that age effect in the two areas was found to increase exponentially until age 70, when it continues to rise, albeit at a slower pace.
The cohort effects of the youngest and oldest age groups must be interpreted carefully because they are the small number of observations and they have larger standard errors than estimates for the middle cohorts [
37]. Overall, The general trends of cohort effect showed a continuously declined trend, which indicated a decreased risk of the mortality in younger generations. Younger generations may receive good education and have a strong awareness of health [
38], which may have a connection with the mortality of kidney cancer. For example, earlier birth cohorts had weak health awareness and they realized neither the occurrence of cancer nor the damage of obesity or overweight to human health. It is worth noting that cohort effect increased for 1902–1906 and 1907–1911 birth cohort in the U.S., which may indicate a higher risk in the earliest birth cohort. This finding should be treated carefully.
Here, period effects were found to be small or modest when birth cohort and age effects were both controlled. This study showed that the period effect might be the critical factor in the trend of kidney cancer mortality, because the period effect in China was significantly continuously increasing and its mortality rate was also continuously, while the period effect in U.S. slightly increased over the same period and its mortality rate tends to be stable. This relationship needs to be further verified. Increasing period effect was observed in both China and U.S. adults over the whole study period, which indicated the period risk factors might be contributed to the increase in the linear trend of the age-standardized mortality rate of kidney cancer attributable to high BMI through 1990 to 2017. However, the period trend increased rapidly in Chinese adults, compared with U.S. adults. The difference in period effect of kidney cancer was possibly related to the different risk exposure. In U.S., no significant changes in obesity prevalence in recent years [
34], and some measures were conducted for improving the increasing obesity prevalence in U.S. adults [
32,
33]. However, the prevalence of childhood obesity in the U.S. is rising during the past decades [
39], and an increasing trend in obesity was observed for U.S. women during 2005–2014 [
40]. Thus, this finding also needs to be further verified. Traditionally, obesity has been considered a problem in Western countries, while urbanisation in Asia has led to a sedentary lifestyle and overnutrition, setting the stage for the epidemic of obesity [
41]. According to the 2011 China Health and Nutrition Survey, the prevalence of obesity among both Chinese adults increased significantly over the past decades, especially in men [
42,
43]. Moreover, Chinese children may have a severe obesity problem. National study reported that 14.4% of children and adolescents were overweight, 11.9% were obese, and 36.8% did not meet screen-time viewing recommendations [
44,
45]. The possible reason of difference between the two areas was associated to the increasing obesity prevalence. As for the different increasing period effect, the underlying reason of different extent to the trend between the two areas was possibly attributable to the fact that the increase in adult obesity in U.S. has slowed down [
46]. Thus, Chinese population seems to face a more severe situation of attributable burden of kidney cancer. Our findings also may indicate inadequate measures or policies on obesity prevalence in Chinese adults, while U.S. has efforts in prevention and care of obesity, and in establishing collaborative weight management models [
47,
48], and the rates of awareness, treatment and control of disease are relatively low in China population [
5]. In addition, there have a higher relative risk of the mortality of kidney cancer attributable to high BMI in men than women. In China, it is more likely to be overweight/obesity in men compared with women [
42,
49]. This gender difference in the relationship of kidney cancer and obesity should be further studied.
In summary, there has rapid urbanization and increasing prevalence of obesity in China, and previous study also noted a decrease in all measures of physical fitness in normal-weight adults during 2000–2014 [
13], the risk of kidney cancer and high BMI need be focused. The increasing period effect indicated the period factors may be the key factor affecting the increasing mortality of kidney cancer. Thus effective measures such as promoting national fitness and low-fat dietary are needed for the prevention of obesity/overweight. It is necessary to reduce kidney cancer deaths in Chinese adults.
Limitations
There are also some limitations in the present study. First, the data from the GBD study were supplied by the governments of the various countries and districts, which may have substantially different systems for collecting vital statistics and methods used to confirm causes of death. These factors limit the comparability of the information in the two areas. Second, despite the mortality data estimated by GBD study which incorporates methods to adjust for incomplete or missing VR and VA data, general heterogeneity in data completeness and quality, and the redistribution of so-called garbage codes (insufficiently specific or implausible cause of death codes), there might be difficult to thoroughly avoid inaccuracy of data. Therefore, our results in the present study on epidemiology of kidney cancer mortality should be treated carefully.
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