In China, almost one-quarter of all cancers in men can be attributed to smoking. And this figure is expected to rise over the coming decades because of the widespread use of tobacco, warn researchers from China and the United Kingdom.

Their study, led by Zheng-Ming Chen, DPhil, from the University of Oxford, United Kingdom, indicates that approximately three-quarters of men in China are ever-regular smokers, compared with less than one-thirtieth of women.

As a consequence, there is an increased risk for lung, liver, stomach, and esophageal cancers, among others, in men, a substantial proportion of which is directly attributable to smoking.

"The tobacco-related cancer risks among men are expected to increase substantially during the next few decades as a delayed effect of the recent rise in cigarette use, unless there is widespread cessation among adult smokers," the team reports.

"Encouragingly, the female smoking rate remains low," the researchers note, adding that this phenomenon is unexpected and without explanation.

In fact, "if smoking rates remain low in women, tobacco may soon be responsible for most of the difference in life expectancy between men and women in China," Dr Chen said in a press statement.

He pointed out that "widespread smoking cessation" will offer China one of the most effective, and cost-effective, strategies for avoiding cancer and premature death over the next few decades.

The study was published in a September 1 supplement to Cancer that focuses on lung cancer in China.

To determine the emerging tobacco-related cancer risks in China, the researchers examined data on 210,259 men and 302,632 women 30 to 79 years of age from 10 areas in China. All were enrolled in the nationwide prospective China Kadoorie Biobank study from 2004 to 2008.

Of the men, 74.0% were ever-regular smokers, which included 6.7% who stopped smoking by choice. Ever-regular smoking was significantly more prevalent in rural than in urban areas (78.2% vs 66.4%; P < .00001).

In contrast, just 3.0% of women were ever-regular smokers. And the prevalence of smoking was much higher in women born in the 1930s than in those born in the 1970s (10.1% vs 0.7%).

The fact that the mean age for starting smoking remained similar in women, at 27 years, suggests that there has been a progressive decline in smoking among women. However, for men, the mean age for starting smoking decreased over time. It was 26 years for those born in the 1930s and approximately 20 years for men born in the 1970s.

After more than 1.4 million man-years of follow-up, it was determined that 8566 men 40 to 79 years of age developed cancer. And after more than 2.15 million woman-years of follow-up, 8525 women 40 to 79 years of age developed cancer.

The overall cancer risk was significantly higher in men than in women (relative risk [RR], 1.44), and was also greater in urban (RR, 1.55) than in rural areas (RR, 1.39).

The risk was significantly higher for lung (RR, 2.51), liver (RR, 1.32), stomach (RR, 1.34), and esophageal (RR, 1.47) cancers, as well as for an aggregate of five other minor sites (RR, 1.52). For lung cancer, the relative risk was greater for nonadenocarcinoma than for adenocarcinoma (5.83 vs 1.78).

The team calculated that more cancers could be attributed to ever-regular smoking in men than in women (23% vs 2%).

For men who quit smoking, the risk for all cancers declined as the number of years without smoking increased. The relative risk was 1.35 for men who had not smoked for less than 5 years, 1.22 for men who had not smoked for 5 to 14 years, and 0.98 for men who had not smoked for at least 15 years (P < .001 for trend).

"The first generation of men in China to experience the full extent of tobacco risks will probably be those who were born during the 1970s or 1980s and reached adulthood during the 1990s or 2000s when cigarette consumption in China was high," the researchers explain.

Cities experienced widespread smoking earlier than rural areas because of the limited availability of cigarettes in rural areas before the 1980s. As a result, the smoking "epidemic" is more advanced in urban than in rural men, they point out.

"However, this urban/rural difference is likely to diminish or even be reversed over the next few decades because rural men born during more recent decades not only have tended to start at the same age as their urban counterparts but also have a higher smoking prevalence and smoke cigarettes almost exclusively," the team writes.

As expected, for cancers primarily associated with smoking, the observed relative risk in China is lower than that seen in Western populations. However, because of the high rates of cancer in Chinese never-smokers, the absolute risk is comparable in the two populations.

"Indeed, the lung cancer rates among male and female never-smokers in the current study were more than 3 times those reported for 1990 US never-smokers, perhaps partly because of exposure to indoor air population from cooking and heating," Dr Chen and colleagues write.

With many smokers having quit smoking as result only of ill health, the findings highlight "the need for effective educational programs and campaigns to promote voluntary cessation among adult smokers for more health gains," they conclude.

This study was funded by the Kadoorie Foundation, Wellcome Trust, the UK Medical Research Council, the British Heart Foundation, Cancer Research UK, and the National Natural Science Foundation of China. The authors have disclosed no relevant financial relationships.

Cancer. 2015;121(S17):3097-3106. Abstract