Dietary preference, physical activity, and cancer risk in men: national health insurance corporation study

Many studies have investigated the protective effect of vegetable consumption against specific cancers, especially colon,[1, 2] prostate,[3] pancreas,[4] bladder,[5] lung,[1, 6‐8] and stomach cancer.[1, 9] Moderate-high levels of physical activity (PA) may also have a protective effect, specifically against breast,[10] colon,[2] prostate,[11] and lung cancer.[12]

Previous studies have suggested that PA may have beneficial impact on reducing the sex hormones or insulin resistance and improving immune system, maintaining a healthy body weight by balancing caloric intake with energy expenditure, which could lead to cancer prevention.[13] Vegetables and fruits are known to contain numerous potentially beneficial anti-oxidants, fibers, minerals, and phytochemicals that may help prevent cancer and contribute to maintenance of a healthy weight.[14]

Recently, American Cancer Society (ACS) has released the recommendation of PA and nutrition for cancer prevention. Their recommendation is "at least 30 minutes or more of moderate to vigorous activity, above usual activities, on 5 or more days of the week" and "eating five or more serving of vegetables and fruits each day".[15]

However, findings of the protective effects of vegetable diet and physical activity on cancer risk from numerous epidemiological studies have been inconsistent[3, 6] and only few studies distinguished the effect of vegetables and fruits.[1, 8] Moreover, most studies did not simultaneously evaluate the combination effect of diet and physical activity to cancer risk. As dietary intake and physical activity, like tobacco use, are cancer risk factors that can probably be modified through lifestyle changes.[15] The relationship of diet and PA to other cancer risk factors, such as smoking, alcohol, body mass index (BMI), and glucose intolerance, should be considered in attempts to identify the effect of diet and PA on cancer risk.

In this study we evaluated the effects of dietary preference and PA, as well as their combined effect on cancer incidence in a large, population-based Korean male cohort, adjusting for other known risk factors.

We analyzed the relationship between dietary preference and LPA and the incidence of major cancers among men older than 40 years from the National Health Insurance Corporation Study with a follow-up period of 3,124,596.7 person-years. Of these, 14,109 cases had been diagnosed with at least one primary cancer. This is one of the largest cohort studies to examine such associations and the first to examine the combined protective effects of diet preference and LPA on cancers, including lung, stomach, and liver cancer. We adjusted for age, smoking, and BMI and added job, fasting serum glucose level, and absolute alcohol consumption for multivariate analysis.

Our study showed that vegetables preference was inversely associated with lung cancer risk and that LPA was protective against stomach, lung, and liver cancer. To our knowledge, this is the first prospective cohort study suggesting that moderate to high LPA might be associated with a lower risk of stomach cancer. Our findings also suggested that the combination of both behaviors (vegetable preference and LPA) was more protective, especially against stomach cancer among never/past smokers and lung cancer among current smokers.

Lung cancer was the second common cancer for men in Korea. [18] Although the major risk factor for lung cancer is tobacco smoking, diet and physical activity [10, 12] can modify the risk. While some cohort studies found a clear inverse association between vegetable intake and the risk of lung cancer in men,[8] others showed little or no association.[6, 7] In some studies, a reduced lung cancer risk was associated not with intake of vegetables, but with intake of fruits.[7] Here we showed that vegetable preference was inversely associated with the risk of lung cancer, especially among current smokers, which is in agreement with other studies,[7, 8] but not among former or never smokers in multivariate analysis. The Netherlands Cohort Study also found no significant protective effect of vegetables in never smokers.[8]

Our finding that more regular and intense LPA helped prevent lung cancer, especially among current smokers, is consistent with other studies suggesting that physically active individuals have a lower risk of lung cancer.[3, 12] Interestingly, the combination group of both positive behaviors showed an additive effect in current smokers to reduce lung cancer risk 67%. As smoking history is the most important risk factor for lung cancer, we performed an additional analysis adjusting for pack-years of cigarette smoking (< 20, 20–29, 30–39, ≥ 40) in current smokers. That produced only minor changes in the RR of lung cancer associated with dietary preference (RR, 0.83; 95% CI, 0.67–0.97) or with LPA (RR, 0.84; 95% CI, 0.74–0.95). Thus, our study suggests that vegetable preference and regular and intense LPA could be employed together with smoking cessation to reduce the risk of lung cancer.

We found that physically active men, compared with inactive men, had a modest reduction in the risk of developing stomach cancer which is most common cancer in Korean men.[18] This protective effect was found in never/past smokers but not in current smoker. Little information exists on the relationship between physical activity and risk of stomach cancer, and our results are inconsistent with the findings of a case-control study in which stomach cancer was not associated with the Total Activity Index.[19]

Though chronic infection with Hepatitis C virus (HCV) or Hepatitis B virus (HBV) is a major risk factor for liver cancer,[20] several studies suggest that the consumption of vegetables or miso soup is significantly associated with an increased risk of liver cancer.[20] While vegetable preference was not associated with liver cancer risk in our study, moderate or high LPA reduced the risk slightly (RR = 0.88, 95% CI: 0.81–0.95). The protective effect of LPA against liver cancer did not seem to depend on smoking status, even if it was not significant in current smoker. We cannot exclude the possibility of a confounding effect for chronic liver disease, i.e., that subjects with chronic liver disease, a risk of liver cancer, reduced their PA. We had no data on infection status of hepatitis virus. However, this is the first prospective study to report that physical activity may reduce liver cancer risk, suggesting that further study is called for.

In our study, there was no significant association between low LPA or meat preference and colorectal cancer risk. ACS recommends increasing PA and vegetables intake for the colorectal cancer prevention,[15] however, this guideline depended on only two studies.[21, 22] Although several studies[1, 2, 22] also supported a significant reduction in the risk of colon cancer associated with vegetable consumption and PA but it was not consistent.[3, 17] Furthermore, recent large prospective cohort studies do not support the hypothesis that PA or vegetables intake is related to a lower incidence of colon cancer.[2] The heterogeneous results may come from the difference of study design or confounding factors.[1] Although our study has the advantage that other important cancer risk factors, such as smoking, alcohol, BMI, and glucose intolerance were considered in the analysis, the occupational physical activity (OPA) total caloric or specific nutritional intake were not collected.

Until now, especially for PA, there is no definited the cut-off level in guideline to prevent the cancer incidence.[15, 24] Our classifcation of moderate-high LPA based on frequency and duration of sweat- producing intensity is arbitrary and is different from frequency and duration of LPA recommended in the ACS or World Health Organization (WHO) guidelines on physical activity for cancer prevention.[15, 24] When We inferred the intensity of PA from this questionnaire based on the British Columbia physical activity guidelines, one of self-gauging rating factors of intense LPA was "sweat-producing" and "vigorous, sweat-producing LPA" could include the type of LPA such as jogging (7 Metabolic Equivalence Task (MET) per hour(hr)) and so on.[25] With the assumption that "vigorous, sweat-producing LPA" was 7 MET/hr, we could calculated the MET-hr/wk of our LPA category as follow: low LPA, < 10.5 MET-hr/wk; moderate LPA, 10.5–17.5 MET-hr/wk; and high LPA, ≥ 17.5 MET-hr/wk. Recent studies have shown that high physical activity at least 18 MET-hr/wk, which was similar to the category of high LPA in our study, was associated with decrease in colon cancer recurrence and mortality.[26] Another study also suggested that physical activity at least 9 MET-hour/week, which was similar to the category of moderate LPA, might lower the risk of death in breast cancer patients.[27] However, these studies focused on the survival benefit of PA in cancer patients, and further well-designed studies was needed to clarify the cut-off level of PA to prevent the cancer incidence.

Although our study showed that moderate LPA is significantly related with reducing the risk of lung, stomach and liver cancer, higher level of PA could be required to reduce the colorectal cancer risk, as the level of PA to show the survival benefit in colorectal cancer patients[26] was higher than that of breast cancer patients.[27] However, the proportion of subjects with high LPA (≥ 17.5 MET-hr/wk) was few, and the colon cancer incidence of Korea very lower than that of US,[18] which could lead to dilute the real associations. Continued monitoring and long-term follow-up study of our cohort is needed to clarify these associations.

Data on the role of vegetable intake and physical activity in preventing other cancers (e.g., head and neck, pancreas, gall bladder, kidney, and bladder cancer) are limited.[1] Vegetable consumption was not associated with a lowered risk of head and neck cancer in this study, regardless of smoking status. Many case-control studies suggest that fruit, but not vegetables, have a significant protective effect against head and neck cancer.[1, 28] In our study, LPA was not significantly associated with a lower risk of head and neck cancer in current smoker. The combination of both behaviors showed similar results. Comparison with other findings is limited by the absence of prospective studies of both behaviors on the risk of head and neck cancer, and the association warrants further investigation.[1]

While neither vegetable preference nor LPA was associated with gall bladder cancer risk, further studies need to be done for LPA because there have been inconsistent findings.[17] Evidence for an association between physical activity and vegetable intake and risk of pancreatic cancer is not consistent. Some case-control study suggest that physical activity and vegetable intake are associated with reduced risk[3] but that was not suggested in our cohort study or a recent prospective study.[29] Thus, the association between vegetable consumption and physical activity and pancreatic cancer risk warrants further investigation.[28]

While smoking avoidance is the most important behavior that can reduce cancer risk,[1] the daily consumption of vegetables or regular physical activity might also reduce risk, especially for lung, stomach and liver cancers. Our findings support the increase of vegetable preference and physical activity as powerful public health measures to reducing cancer[1] and its considerable burden on society.[12]

Our study has several limitations. First, there was a possibility of selection bias. In the study population, only government employees/teachers were included and all women were eliminated, which might have limitation of generalization. In addition, Korean diet is different from North American diet, these cultural differences should be considered to interpret our results. Second, calculating exact MET-hr/wk for LPA was impossible in this study for comparison with other studies,[26, 27] because the questionnaire included only about the frequency and duration of sweat-producing activity without specific activity type or intensity. Third, we could not collect the information about OPA or usual activity including household. In multivariate analysis, however, we adjusted for employment status, which might be a surrogate for OPA. In addition, when we stratified the subjects based on employment status, the results were similar to those for the whole population (data not shown). Only the association between LPA and stomach cancer (RR, 0.96; 95% CI, 0.82–1.12) was attenuated and not statistically significant in white collar workers. Forth, we did not collect the diet information in detail. Therefore, we could not analyze the differences in food consumptions between the three diet preferences groups based on study participant's opinion into which group he or she belongs to.

The consumption of common foods over the course of a year, however, is not encoded as a series of discrete episodic events, but preference is inferred from the "liking" of a specific diet effect on attitude, taste factors, and habit. [30] Further study is needed to clarify the association between reported dietary preference and actual food consumed. Fifth, there is a weak scientific evidence for the classification of dietary preferences as (1) meat, (2) vegetable or mixture of vegetables and meat. However, when we performed the analysis with various classifications, we couldn't find significant association between other classification of diet preference and cancer risk. Moreover, one Japanese study also showed that the combined intakes of meat and vegetables on colon cancer incidence.[17]

To our knowledge, this is first study for dietary preference and cancer risk. As few is known about the association between dietary preference and cancer risk, further studies are needed. Despite these limitations, our findings add to the evidence of the beneficial effects of vegetable preference on lung cancer risk and of physical activity on lung, stomach, and liver cancer risk. This study suggested that the additive effect of combined vegetable preference and LPA might reduce lung and stomach cancers.