Table 3 shows the frequencies of the eight possible combinations of the three types of tobacco used. Each lifelong pattern of tobacco use was represented as a binary variable, indicating “ever” or “never” smoked. Cigarettes were by far the most frequently used type. The cigar and pipe smokers tended to have smoked cigarettes as well. All types of tobacco use were associated with ORs >1.0. The combinations that included cigar smoking were associated with the largest excess risks.
Table 4 shows ORs for prostate cancer by cumulative exposure to the smoking of cigarettes, cigars, and pipes. The results for each type of tobacco smoking are presented either without or with adjustment for the other two types. All three types of tobacco smoking showed evidence of weak associations with prostate cancer. The ORs were higher for cigar smoking than they were for cigarette or pipe smoking.
To examine the joint effects of tobacco smoking and BMI on prostate cancer risk, we chose to restrict our analysis to subjects who smoked only cigarettes to prevent the misclassification of exposure that could arise from either ignoring the use of the other types of tobacco or from attempting to construct composite tobacco exposure variables.
Table 5 shows ORs for prostate cancer associated with exclusive cigarette smoking, using as the reference group nonsmokers of any tobacco; that is, smokers of cigars and pipes were excluded from this and the following analyses. There was a relatively small increase in risk associated with the highest level of exposure, expressed in terms of pack-years. The 95% confidence intervals for all estimates were broad, however.
To examine whether the effects of cigarette smoking were modified by BMI, we repeated the analyses shown in Table 5 within levels of BMI. We found that the effects of smoking were similar in the two strata pertaining to the lower two tertiles of BMI, so we combined these strata.
Table 6 shows the results of this analysis. The ORs refer to a common reference group consisting of the men in the lower category of BMI who never smoked. Although in the lower stratum of BMI smoking was associated with ORs <1.0, the data were also reasonably compatible with no effect. Furthermore, there was no tendency for the ORs to increase with increasing pack-years of cigarette smoking. Nonsmokers in the higher stratum of BMI had a much lower risk than the reference group, whereas “ever smoking” was associated with an OR equal to that of the reference group. The ORs increased steadily, with increasing exposure expressed as pack-years.
Because the ORs in the higher stratum of BMI tended to increase monotonically with increasing duration and amount smoked per day (results not shown), we have presented the results in Table 6 using another reference group consisting of the men who never smoked with BMI >26.660. This approach allowed us to estimate the risk of cigarette smoking relative to not smoking among men of similar BMI. Table 7 shows that among the men in the highest tertile of BMI, cigarette smoking was associated with prostate cancer; the patterns by duration, amount, and quitting were those that would be expected of a causal relationship.
Consistent with the results of other studies, we found weak positive first-order associations between tobacco smoking and prostate cancer (Tables 3 and 4). 3,13 The associations appeared stronger for pipe and, especially, cigar smoking than for cigarette smoking. It was not possible to study the effects of cigar and pipe smoking further. It was possible, however, to examine further the risk associated with cigarette smoking, and we found a striking interaction between BMI and cigarette smoking (Table 6). Ostensibly, the risk of prostate cancer was lower among nonsmokers with high BMI than among nonsmokers with low BMI, the reference group. Among smokers with high BMI, the risk was equivalent to that of the reference group, whereas among smokers with low BMI the risk appeared to be less than that of the reference group.
One could attempt to interpret these results using a common reference group, or one could examine them by rows to study the modification of the effect of smoking by BMI, or by columns to study the effects of smoking among men with similar BMI. Adopting the latter perspective, among men with high BMI, smokers experienced an increased risk of prostate cancer (Table 7). This conclusion rests on the observation that risk was lower among nonsmokers with high BMI than among nonsmokers with low BMI, the reference group (Table 6). Although these results could have occurred by chance or could have been manifestations of bias, they might also reflect the underlying biological relations between BMI, smoking, and blood levels of sex hormones, which will be discussed below. Among men with low BMI, smoking appeared to be protective (OR = 0.70; 95% confidence interval = 0.41–1.20) (Table 6). Because the confidence interval was wide, this result is also compatible with smoking having no effect among men with low BMI. It could also have occurred by chance or could have been due to bias. Alternatively, it could reflect the underlying biological relations. Before considering possible biological explanations of our results, we will consider the limitations of the study and possible biases.
Although exposure to tobacco and to the covariates may have been misclassified to some extent, it is unlikely that misclassification of any of these variables could explain the effect modification by BMI that we observed. The quality of the data derived from proxies may have differed from that derived from the subjects themselves. We had few proxy respondents in this analysis, however, and we adjusted for respondent status. The methods of subject accrual ensured that the case and control series were population-based. Although selection bias could have occurred because of nonresponse, it did not appear to be appreciable. 9 We doubt that recall bias was appreciable, because the interviews elicited information about regular, repeated exposures to tobacco, which are usually not difficult to recall.
Control of confounding is problematic in studies of prostate cancer, because only a small fraction of cases can be attributed to known determinants. 14 Recognized determinants include age, race, family history, 14 and country of residence. 1,2 Confounding by race was unlikely, because almost all subjects were of European origin. We had no information about family history, but it is unlikely that there would have been a strong relation between the confounder (family history) and the exposure variable (smoking) in only one specific stratum of BMI.
Because a high-fat diet may be a determinant of prostate cancer 15–17 and because cigarette smoking tends to be associated with a high-fat diet, 18,19 it is possible that our results could be confounded by dietary fat. Given that smoking tends to increase smokers’ energy expenditure, 20 it is possible that smokers among the highest tertile of BMI may have increased their caloric intake by increasing their fat consumption to maintain their body weight to a greater extent than smokers among the lower tertiles.
Because BMI is inversely associated with energy expenditure from physical activity, it is also possible that the effect modification we observed could be related to levels of physical activity if it is a determinant of prostate cancer risk. The evidence for this relation is conflicting, 21–24 however, and thus, it is unclear how physical activity might be involved.
Interpretation of Results
Because tobacco smoke is known to have tumor-initiating, -promoting, and -progressing effects on the development of other cancers, 5 it is plausible that it could have similar effects on prostate cancer. The increases in risk observed with increasing duration and amount of cigarette smoking and the decrease observed after quitting ≥10 years before diagnosis among the men in the highest tertile of BMI (Table 7) are suggestive of effects involving tumor promotion.
Some aspects of the interaction between smoking and BMI might be explained by the effects of those factors on testosterone levels. Testosterone has been shown to act as a promoter of prostate cancer in an animal model, 25 and higher total testosterone levels in men are associated with increased risks of prostate cancer. 26 Small changes in hormone levels might cause large changes in risk. 27
Increasing BMI is associated with decreasing levels of total serum testosterone. 28,29 This relation might explain why the risk of prostate cancer was lower among nonsmokers with high BMI than among nonsmokers with low BMI (Table 6).
The association of cigarette smoking with higher levels of total serum testosterone 28,30,31 might explain the increased risk of prostate cancer associated with cigarette smoking among men with high BMI relative to not smoking among men with similar BMI (Table 7). The effects of the increase in total serum testosterone associated with smoking on prostate cancer risk might be more apparent among obese men who tend to have lower testosterone levels (Table 7) than among men with lower BMI who would tend to have higher testosterone levels (Table 6). 28,29
Nevertheless, attempts to explain prostate cancer incidence on the basis of serum hormone levels should be considered cautiously, because circulating hormone levels do not reflect intraprostatic hormonal activity. 27 Furthermore, the effects of smoking and obesity on serum testosterone may be counteracted by their effects on sex hormone binding globulin (SHBG): smoking is associated with increased levels of SHBG, and obesity is associated with decreased levels of SHBG. 28,29 SHBG binds a portion of circulating testosterone; unbound testosterone is considered to be the biologically active form. Thus, increasing levels of SHBG are associated with decreasing risks of prostate cancer, when levels of other hormones are controlled for. 26 Consequently, the increase in total serum testosterone associated with smoking that may increase risk is accompanied by an increase in SHBG that may decrease risk. The net effect of these opposing processes on prostate cancer risk may depend on BMI (Table 6). It is possible that the point estimates of the ORs in Table 6 that suggest a protective effect of smoking among men with low BMI could reflect such a net effect.
It has been suggested that smoking might also increase prostate cancer risk by reducing endogenous estrogen levels. The evidence cited for the effects of smoking on estrogen levels, however, came from studies involving women, 32–34 and Dai et al31 showed that smoking in men was not correlated with total serum estradiol or serum estrone.
We found two lines of evidence for an effect of tobacco smoking on the risk of prostate cancer. Cigar and pipe smoking seemed to be associated with the development of prostate cancer. In addition, among men with high BMI, cigarette smoking was associated with an increased risk of prostate cancer.
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Keywords:© 2001 Lippincott Williams & Wilkins, Inc.
prostatic neoplasms; smoking; body mass index; case-control studies