Little is know about the relation between dietary fiber intake and the incidence of respiratory diseases, especially chronic obstructive pulmonary disease (COPD). The authors investigated this issue among 111,580 US women and men (Nurses’ Health Study and Health Professionals Follow-up Study). 832 cases of newly diagnosed COPD were reported between 1984 and 2000. The cumulative average intake of total fiber and of fiber from specific sources (cereal, fruit and vegetables) were calculated from food frequency questionnaires and food composition database, and divided into quintiles. After adjustment for 11 factors (age, sex, smoking, energy intake, body mass index, US region, physician visits, physical activity, diabetes, intakes of omega-3 and cured meat), total dietary fiber intake was negatively associated with risk of newly diagnosed COPD: RR for highest vs. lowest intake=0.67 (95% confidence interval, 0.50–0.90), P for trend=0.03. For specific fiber sources (cereal, fruit and vegetables), only cereal fiber was significantly associated with newly diagnosed COPD independently of other fiber sources: RR for highest vs. lowest intake=0.77 (95% confidence interval, 0.59–0.99), P for trend=0.04. These data suggest that a diet high in fiber, and possibly specifically cereal fiber, may reduce risk of developing COPD.

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of mortality in the United States and in Europe (1). With the increase in cigarette smoking in developing countries, COPD is expected to become the third leading cause of death worldwide by 2020. Over the last decade, there has been growing interest in the identification of foods or nutrients with antioxidant or anti-inflammatory properties that affect level of lung function or COPD symptoms (2). Most of these epidemiological studies were cross-sectional, but the few longitudinal studies have reported a negative association between fruits, vegetables and vitamin C intakes with lung function decline. Using an overall approach to assess diet, we recently reported in large cohorts of US women (3) and men (4) that a “prudent” dietary pattern (i.e, with ample intake of fruits, vegetables, fish, whole grains) was associated with a decrease risk of newly diagnosed COPD, whereas a “Western” diet (i.e, with ample intake of cured and red meats, desserts, refined grains, French fries) was associated with an increase risk of newly diagnosed COPD. We also demonstrated that frequent consumption of cured meat (one item of the Western diet) was associated with an increased risk of COPD (5, 6). However, no others specific foods or nutrients were investigated in relation with respiratory diseases.

Despite the anti-inflammatory and antioxidant properties of fiber (7), little is known about its relation with lung function and COPD. The paucity of data is striking in comparison to the extensive scientific literature on the relation of dietary fiber with cardiovascular diseases (8), diabetes (9) and cancer (10). A recent study found that a high whole-grain food intake was associated with a lower risk for respiratory disease mortality (11). Dietary fiber intake also was reported to be associated with a reduced incidence of cough with phlegm (12) and with a highest level of lung function and a lower prevalence of COPD (13).

We therefore investigated prospectively the association between dietary fiber intake and risk of newly diagnosed COPD in two large prospective US cohorts.

Participants with the highest intake of fiber were less often smokers, more physically active and reported a higher total energy intake, as compared to those with the lowest intake of fiber (Table 1). The intake of specific fiber types (cereal, fruit and vegetable) was correlated both in women and in men: cereal and fruit fibers (r=0.30 in women, 0.25 in men); cereal and vegetable fibers (r=0.25 in women, 0.24 in men); and fruit and vegetable fibers (r=0.42 in women, 0.38 in men).

In two 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 with lung function or COPD (2). The main epidemiological evidence in support of an antioxidant-COPD association suggests that vitamin C and fruits and vegetables have beneficial associations with lung function, both in cross-sectional and in 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 (5,6,22).

Two studies have investigated the relation of a novel antioxidant, dietary fiber, to COPD or COPD symptoms (12,13). Butler et al. reported that non-starch 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 (12). In the ARIC study (Atherosclerosis Risk in Communities, 11,897 US women and men), Kan et al. investigated the association between dietary fiber and respiratory phenotypes, both using a cross-sectional design (i.e. level of lung function and COPD prevalence) and a longitudinal design (i.e. mean changes of lung function) (13). In the cross-sectional analysis, they reported a negative association between 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., results were adjusted for sex by statistical modeling; stratified results were not reported. Kan et al. 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 since the number of cases was much lower in men as compared to 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 non-significant 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 RR 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 biological explanation for a potential benefit of fiber intake is related to both its antioxidant and anti-inflammatory properties. Even if the exact mechanism between dietary fiber and inflammation is unclear (7), it has been reported in epidemiological data that fiber intake is associated both with a lower level of CRP and of various proinflammatory cytokines such as IL-6, IL-18 and TNF-α (25, 26, 27) and with a highest level of the anti-inflammatory cytokine adiponectin (27, 28). 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-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). Secondly, 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, multivariate models were adjusted with multiple time-varying measures of tobacco exposure (smoking habits, pack-years and pack-years²) which 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 inter-relations 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 which 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 anti-inflammatory 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 influence COPD risk.