In 2 large cohorts of US women and men, we found that participants with a higher total fiber intake had a lower risk of newly diagnosed COPD, even after adjustment for many potential confounders. The potentially beneficial effect was independent of other dietary factors, such as omega-3 and cured meat. Only cereal fiber was significantly associated with newly diagnosed COPD.
Many studies have examined antioxidants (or foods rich in antioxidants such as fruits and vegetables) in relation to lung function or COPD (2
). The main epidemiologic evidence in support of an antioxidant–COPD association suggests that vitamin C and fruits and vegetables have beneficial associations with lung function, in both cross-sectional and longitudinal analyses. Relatively little attention has been paid to other foods or nutrients except for cured meats, which appear to have a deleterious effect on COPD risk (5
Two studies have investigated the relation of a novel antioxidant, dietary fiber, to COPD or COPD symptoms (12
). Butler et al. (12
) reported that nonstarch polysaccharides, a major component of dietary fiber, had an independent, inverse association with the incidence of cough with phlegm in 63,257 Chinese Singaporean women and men. In the Atherosclerosis Risk in Communities Study of 11,897 US women and men, Kan et al. (13
) investigated the association between dietary fiber and respiratory phenotypes, using both a cross-sectional design (i.e., level of lung function and COPD prevalence) and a longitudinal design (i.e., mean changes of lung function). In the cross-sectional analysis, they reported a negative association of dietary fiber intake from all sources and from cereal and fruits with the level of lung function and the prevalence of COPD; in the longitudinal analysis (only 3 years apart), they reported a significant inverse association of decline in lung function with cereal fiber but not with total or fruit fiber.
We now report a significant, independent association between total fiber intake and the risk of COPD, particularly in women. Gender influences the epidemiology, diagnosis, and presentation of COPD, in addition to physiologic and psychologic impairments (23
). In the study of Butler et al. (12
), results were adjusted for sex by statistical modeling; stratified results were not reported. Kan et al. (13
) formally tested the interaction between sex and total fiber intake on the level of lung function, and they did not find a statistically significant interaction. As our formal test for interaction between fiber intake and sex was not statistically significant, it supports the likelihood that the difference we observed between men and women might be due to chance. We faced a statistical power issue in men because the number of cases was much lower in men compared with women; the confidence interval for men actually included a strong inverse association. In men, we initially found a borderline significant negative association between the risk of newly diagnosed COPD with total fiber and cereal fiber intakes, associations that became nonsignificant after adjustment for cured meat. In women, we found independent effects of cured meat intake and fiber intake on COPD risk. In earlier work by our group, a very strong association was reported between cured meat intake and the risk of newly diagnosed COPD in men, with a relative risk for daily consumers of similar magnitude to ever smoking (6
); in women, the magnitude of the association was smaller but also highly significant (5
COPD is an oxidant- and nitrosant-related disease (24
) with characteristic airway inflammation. Along those lines, a recent study found that the mortality risk of individuals with inflammatory respiratory diseases was significantly lower for those who reported the highest intake of whole-grain foods (11
). The biologic explanation for a potential benefit of fiber intake is related to both its antioxidant and antiinflammatory properties. Even if the exact mechanism between dietary fiber and inflammation is unclear (7
), it has been reported in epidemiologic data that fiber intake is associated with both a lower level of C-reactive protein and various proinflammatory cytokines, such as interleukins 6 and18 and tumor necrosis factor α (25
), and a higher level of the antiinflammatory cytokine adiponectin (27
). Moreover, we cannot exclude that our findings are not an effect of fiber per se, but that they are due to other constituents of whole grains, including lignans.
Our primary findings for cereal fiber are consistent with the longitudinal finding of Kan et al. (13
). Together, these data support the hypothesis that fiber from cereal, or another constituent of whole grains, may have physiologic effects that are more beneficial to the respiratory system than fiber from fruits or vegetables. Identification of the predominant mechanism for the beneficial effects of dietary fiber on COPD risk will require further study (7
The study has few potential limitations. First, newly diagnosed COPD was defined by a self-reported physician's diagnosis of COPD, and no lung function results were available. Nevertheless the questionnaire-based definition of newly diagnosed COPD was validated in a subset of the Nurses’ Health Study (17
). Second, we acknowledge that the association between fiber intake and newly diagnosed COPD may be due, in part, to residual confounding by cigarette smoking. To minimize this possibility, we adjusted multivariate models to multiple time-varying measures of tobacco exposure (smoking habits, pack-years, and pack-years2
) that were assessed biennially since 1976, and analyses were stratified according to smoking status. Finally, we note the difficulty of studying the health effects of any specific nutrient given the complex interrelations among intakes of different components of diet. Our fiber–COPD findings merit replication in other populations, preferably from cohorts with higher intake of fiber and different patterns of dietary intake.
In summary, the intake of fiber, and particularly cereal fiber, was negatively associated with the risk of newly diagnosed COPD in women. Similar, but weaker, associations were seen among men. Our results support the current dietary guidelines that recommend that Americans increase their daily consumption of whole grains (29
). Besides potential prevention benefits for cardiovascular diseases, diabetes, and cancer, fiber or another constituent of whole grains might also be involved through antiinflammatory effects, in the pathogenesis of COPD. For COPD prevention, the most important public health message remains smoking cessation, but our data suggest that diet, another modifiable risk factor, might also influence COPD risk.