The Japanese Lung Cancer Smoking Paradox

Asian man smoking but less likely to get lung cancer than American men
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Japanese men smoke more on average than men in the United States and Europe but are less likely to develop lung cancer. The seeming incongruity that those in Japan smoke more but have a lower lung cancer rate than the United States has been coined the “Japanese smoking lung cancer paradox,” and researchers have set out to evaluate the reasons why this occurs. Though it's likely that a combination of factors is at play, the role of diet, genetics, the efficiency of filters, carcinogens present in cigarettes, age of smoking onset, and more, may help to explain this confusing phenomenon. We will look at the rates of lung cancer relative to smoking in Japan and other Asian countries, as well as probable and possible reasons for the paradox.

The Japanese Lung Cancer Smoking Paradox: Statistics

The rates of lung cancer attributable to smoking differ not only in Japan and the U.S. but among other Asian countries as well.

Rates of Lung Cancer vs. Smoking in Japan and the U.S.

The proportion of lung cancer in smokers and never smokers differs between Japan and the United States. While in the United States the "odds ratio" of lung cancer in male smokers vs non-smokers was 40.1 (in other words, male smokers were 40 times more likely to develop lung cancer than male non-smokers in the U.S.), the odds ratio in Japan was 6.3. In other words, male smokers in Japan were only 6.3 times as likely to develop lung cancer as male non-smokers. 

It's well known that lung cancer in never smokers is more common in many Asian countries, the paradox does not appear to be due to proportions alone. In other words, the lower relative risk of lung cancer in smokers does not appear to be due to a higher incidence of lung cancer in never smokers.

In addition, the difference between Japan and U.S. does not appear to be related to the amount of smoking, or pack years of smoking. The men from the United States and those from Japan smoked for essentially the same number of years, and also averaged the same number of cigarettes daily.

Relative Rates of Lung Cancer in Other Asian Countries

The lung cancer paradox is clearly true and is not isolated to Japan. The relative risk of lung cancer in a 2016 study found that compared with the 40:1 relative risk in the United States, smokers in Korea were only 4.0 to 4.6 times more likely to develop lung cancer than non-smokers. The relative risk in Japan in this study was 3.7 to 5.1, and that in China was 2.4 to 6.5.

A Word of Caution

Researchers caution that the paradox should not be misinterpreted to mean that smoking is safer for Asians.

Rates of Other Smoking-Related Conditions

Smoking is associated with not only lung cancer, but heart disease, stroke, and much more. In a 2018 study it was found that though the relative risk related to smoking of both ischemic heart disease and stroke were similar in Japan and the United States, even though the relative risk of lung cancer was significantly lower in the Japanese population.

Probable Reasons for the Paradox

Reasons that likely contribute to the higher risk of lung cancer in the United States vs some Asian countries include:

Lower Levels of Carcinogens in Japanese Cigarettes

One reason for the paradox could be that Japanese cigarettes contain lower levels of carcinogens (cancer-causing ingredients). In the United States, roughly 70 of the chemicals found in cigarettes are thought to cause cancer. While tobacco is present in most cigarettes, the type and proportion of these other chemicals can vary widely.

Genetic Factors

Genetic factors could, in some way, lead to Japanese men being more resistant to the development of lung cancer. Though lung cancer is often thought of as being a smoking-related disease, there are many causes, and some people carry a predisposition to the disease. This may be easier to understand if you have followed the stories about Angelina Jolie, and the decisions she faced upon learning she had a hereditary predisposition to breast cancer. In fact, one of these "breast cancer genes" known as BRCA2 is linked with a 2-fold risk of developing lung cancer in women who smoke.

Genetic factors (proteins that are coded for in genes) can affect how carcinogens affect the body, but can also affect the formation of carcinogens in the body, or their breakdown and elimination. There appear to be genetic differences that lead to reduced uptake of some carcinogens from tobacco smoke in Japanese people, such as tobacco-specific nitroasamines, polycyclic aromatic hydrocarbons (PAHs), 1,3-butadiene, and other substances. People of Japanese ethnicity also experience less nicotine uptake (more unchanged nicotine remains in the body) due to lower activity of the primary enzyme that metabolized nicotine in the body (CYP2A6).

While nicotine does not appear to play a large role in initiation of cancer, it does appear to play a role in promoting a cancer already present.

Epigenetic Factors

In addition to genetic factors (structural changes in DNA), epigenetic factors may play a role as well. Epigenetic factors refer to ways that DNA is "read" differently rather than structural changes. Stated in a different way, epigenetics refers to the modification of gene "expression" (genes being turned "on" or "off") rather than alterations in DNA itself. Studies have found that DNA methylation patterns in smokers differ between south Asians and Europeans.

Possible Reasons for the Paradox

In addition to genetic tendencies and carcinogen content of cigarettes, there are other factors that may account for, or at least contribute to, the differences between smoking and lung cancer risk in the U.S. and Japan. These may include:

Alcohol Consumption

High alcohol consumption is linked with a higher risk of lung cancer, though some types of alcoholic beverages may result in more risk than others. Genetic factors may also play a role in this factor, as one study found no link between alcohol and lung cancer in Chinese men.

Fat Intake

Lower fat intake in the diet is associated with a lower risk of developing lung cancer, and Japanese males tend to have a lower fat intake than men in the U.S.

Filter Efficiency

There is more use of activated charcoal in the filters of Japanese cigarettes than cigarettes available in the United States. You may be familiar with activated charcoal as it is used in emergency rooms to treat some poisonings and overdoses. Activated charcoal binds some chemicals, but certainly not all. In addition, activated charcoal is used in some health food supplements.

Age of Smoking Onset

American men begin smoking, on average, at least 2.5 years earlier than Japanese men. In general, lung cancer risk increases with the number of pack years smoked or the total number of cigarettes. But those who begin smoking at younger ages appear to be more at risk than those who begin smoking later, even if both groups were to smoke the same number of cigarettes.

Lifestyle Factors Other Than Smoking

Certain dietary factors are associated with a lower risk of lung cancer. In addition, exercise—even small amounts—appears to reduce the risk of lung cancer.

While obesity is increasing in the U.S., as are the rates of obesity-related cancers, this is not an explanation for the paradox. In fact, people who have a high body mass index (are overweight or obese) tend to have a lower rate of lung cancer than those who are slender.

What You Can Do With This Information

Certainly, genetic factors are beyond our control, but American men who smoke may wish to consider limiting their alcohol intake and consumption of high-fat foods. Make sure to check out these 10 tips for preventing lung cancer, whether or not you smoke. Keep in mind that lung cancer can, and does, occur in lifelong never smokers. Anyone with lungs can get lung cancer.

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