In this cross-sectional analysis of middle-aged adults we found beneficial associations of dietary fiber from all sources and from cereal and fruit with lung function and COPD. Of note, these beneficial associations of higher fiber intake were not explained by greater intake of other dietary nutrients including carotenoids, vitamins C, D, E, omega-3 fatty acids, nor lower intake of cured meat, nor by a large array of risk factors for reduced pulmonary function and COPD.
Higher intake of dietary antioxidants (e.g. vitamins C and E, β-carotene) (28
), foods rich in antioxidants (e.g. fruits and vegetables) (32
), omega-3 fatty acids (35
) and fish consumption (35
) have been associated with better lung function and reduction of COPD symptoms and mortality; conversely, frequent cured meat consumption was associated with an obstructive pattern of lung function and increased odds of COPD (38
). Some studies found stronger associations for fruit intake than for individual fruit-related nutrients such as vitamin C and carotenoids (9
), suggesting other associated nutrients may be more relevant in protecting the lung from oxidative stressors. Dietary fiber is thought to contribute to the beneficial health effects on cardiovascular endpoints of intake of fruits, whole grain and vegetables (3
), but has seldom been studied in relation to lung disease. In the Singapore Chinese Health Study, nonstarch polysaccharides, a major component of dietary fiber, reduced the incidence of chronic bronchitis symptoms (13
). Relevant to interpretation of previous findings on diet and lung disease that did not consider the role of fiber, in the Singapore study, adjustment for fiber intake eliminated univariate associations with intake of antioxidant micronutrients or fruit (13
The hypothesis that higher dietary intake of fiber might protect against reduced lung function and COPD is biologically plausible. Both airway inflammation and oxidative stress play roles in the pathogenesis of chronic bronchitis, emphysema (5
) and deterioration of lung function (40
). Our observed beneficial association of dietary fiber with lung function and COPD may be due to the antiinflammatory and/or antioxidant properties of fiber (2
). Fiber intake has been associated with reduced level of C-reactive protein (43
), a marker of systemic inflammation. Fiber may modulate inflammation by several mechanisms including slowing the absorption of glucose (47
), decreasing lipid oxidation (2
), or influencing the production of antiinflammatory cytokines by the gut flora (48
). Another possibility is that some constituents of fiber, such as trace elements or associated nutrients (e.g. flavonoids), that we cannot assess from our questionnaire, may have beneficial effects on the lung (13
). Another mechanism that theoretically could come into play is a substitution effect, replacing the intake of foods with detrimental effects. However, it is notable that in another study, fiber intake was protective for chronic bronchitis symptoms independently of a deleterious association with a dietary pattern characterized by high intake of preserved and fresh meat, sodium and refined carbohydrates (50
). Further, the effect of fiber in our study persisted after adjustment for preserved meat which was related to increase risk of adverse respiratory events in two studies (38
). Thus we believe that our study adds to growing evidence that fiber intake per se may have beneficial effects on the lung, independent of intake or other foods or nutrients or dietary patterns.
The concern might be raised that fiber intake simply reflects related behavior or clinical characteristics, such as smoking (), that influence respiratory risk. However, the association with dietary fiber remained significant after careful adjustment for smoking as well as a wide variety of demographic, lifestyle, and dietary characteristics. Moreover, the protective effect of dietary fiber on lung function was similar in both smokers and nonsmokers () indicating that the protective effects of dietary fiber on lung function was independent of smoking behavior (51
Neither sex, ethnicity, nor smoking status significantly modified the associations of total fiber intake with lung function measurements (). Finding a protective effect of fiber intake on lung function in both smokers and nonsmokers is not unexpected. In nonsmokers, fiber intake may protect against deleterious effects of indoor and ambient air pollutants (25
). In stratified analyses for COPD, however, the effect of dietary fiber was limited to whites and a test for interaction by race was significant (). Similarly, Stevens et al reported a protective role of cereal fiber on the development of diabetes limited to white ARIC participants (52
). The lack of association among African-Americans in ARIC may be due to the smaller sample size (2,798 African-Americans vs. 9,099 white participants), lower intake of dietary fiber (16.5% of African-Americans vs. 21.1% of white participants were in the highest quintile of total fiber intake), and lower prevalence of COPD (11.0% in African American vs. 15.9% in white participants). Although upon stratification by smoking the inverse association appeared to be limited to current and past smokers, the test for interaction by smoking was not statistically significant and as expected, there were far fewer cases of COPD among never smokers, limiting power.
The association between carotenoids intake and lung function was attenuated after adjustment for fiber (Table S1
). This is consistent with a previous report that the inverse association between antioxidant vitamins and chronic bronchitis symptoms diminished after adjustment for fiber (13
). The earlier studies on anti-oxidant vitamins and lung function did not consider the role of fiber (28
), and therefore it is unclear whether the previously reported associations with those vitamins were independent of fiber intake.
We found significant associations of lung function and COPD with cereal and fruit fiber, but not with vegetable fiber. These findings are consistent with a pooled analysis of studies of diet and coronary heart disease in which fiber from cereal and fruit, but not from vegetables, was inversely associated with disease risk (53
). The lack of association with vegetable fiber also mirrors results from the Singapore Chinese Health Study, a population with low intake of whole grain cereals, in which fiber from fruit and soy, but not from vegetables, was associated with reduced risk of chronic bronchitis symptoms (13
). It is possible that fiber from cereal or fruit may have physiologic effects that are more beneficial to respiratory system than fiber from vegetables.
Limitations of our analysis should be noted. The GOLD criteria for COPD are based on post-bronchodilator FEV1
); however, as in other large epidemiologic studies (55
), we did not administer a bronchodilator. Thus, we cannot distinguish reversible from nonreversible airway obstruction and as a result, our spirometry-defined COPD group may contain asthmatics. Further, a temporal relationship between fiber intake and lung function and COPD can not be established in this cross-sectional analysis. It is possible that participants with symptoms or a diagnosis of COPD could have responded by changing to a healthier diet with increased intake of fruits and whole grains. However, few studies linked diet with COPD during the study period (1987-1989), making it quite unlikely that participants with COPD would have been advised to make these dietary changes. Although we had two measures of pulmonary function at both visits 1 and 2, they were only three years apart which is probably insufficient to detect longitudinal effects of fiber intake (59
). Nevertheless, we performed supplementary longitudinal analyses (Table S2
); we found significant inverse associations of declines in FEV1
, FVC and percent predicted FEV1
with cereal fiber, but not with total or fruit fiber.
We conclude that higher dietary intake of fiber was related to better lung function and reduced prevalence of COPD. These beneficial associations with fiber intake were independent of other nutrients or dietary patterns that have been suggested to have either beneficial or deleterious effects on respiratory health. To our knowledge, this is the first study to examine fiber intake in relation to pulmonary function and COPD. Our findings suggest that greater intake of dietary fiber may protect against deterioration of lung function, and possibly against COPD.