Applied nutritional investigationDietary fiber intake modifies the association between secondhand smoke exposure and coronary heart disease mortality among Chinese non-smokers in Singapore
Introduction
Cardiovascular disease (CVD) remains the number 1 cause of death worldwide [1]. Although most cases are due to risk factors related to individual behavior and lifestyle (e.g., active tobacco use, unhealthy diet, and lack of physical activity), a strong argument has been made to evaluate factors that are not personally modifiable [2]. With only 7.4% of the world's population living in areas that are covered by smoke-free legislation [2], exposure to secondhand smoke (SHS) is one of the most common sources of indoor air pollution worldwide, with as many as 35% of adults and 40% of children regularly exposed [3]. SHS exposure increases the relative risk for coronary heart disease (CHD) and overall CVD by an estimated 25% to 30% [4], [5]. Approximately 600,000 deaths were attributable to SHS in 2004 [3]; however, much uncertainty remains surrounding this estimate due to the potential influence that other risk factors (e.g., individual lifestyle, community, and societal factors) have on the magnitudes of effect across various populations or population subgroups [2], [5]. Therefore, although a major priority is the continued support of programs aimed to reduce smoking initiation and increase smoking cessation; there is also a need to identify factors that modify the effects of SHS exposure on CHD outcomes [5].
SHS contains a complex mixture of PM2.5 (particulate matter less than 2.5 microns in diameter) constituents that have been implicated in CVD development [6], [7] by inducing proinflammatory responses through the generation of oxidative stress [8], [9], [10]. Diets are a source of compounds with anti-inflammatory and antioxidant properties, such as ω-3 polyunsaturated fatty acids (PUFAs) [11], [12], carotenoids [13], [14], and fiber [15], [16], [17], [18], [19], that may counteract the adverse effects of SHS exposures on disease risk. Epidemiologic evidence implicates diet as a modifier of the relationships between ambient air pollutant exposures and adverse respiratory end points [20] and total mortality [21]. Additionally, we have demonstrated that fiber consumption modifies the relationship between childhood SHS exposure and risk for chronic respiratory symptoms among adult non-smokers [22].
The potential modifying effect of dietary factors on the relationship between air pollutant exposures and cardiovascular outcomes has not yet been extensively investigated [23], [24], [25], [26]. In a repeated-measures study, Baccarelli et al [23] reported that the adverse effects of ambient PM2.5 exposures on heart rate variability, an independent predictor of cardiovascular mortality, were abrogated among those with higher intake of methyl nutrients including B6, B12, and methionine. Similarly, supportive evidence has been provided from trials where those who were randomized to receive daily supplementation of fish oil as a source of ω-3 PUFAs had reduced effects of particulate matter on heart rate variability or biomarkers of oxidative stress [24], [25], [26]. Although compelling, these previous studies were limited by their small size and primary focus on subclinical indicators of cardiovascular health. To date, there are no studies that have investigated the possible modifying effect of diet on the relationship between non-ambient sources of air pollution and CVD mortality.
Air pollutant exposures are universal and often unintentional and, in the case for SHS exposure reductions, require substantial behavioral changes that are exceedingly difficult to implement in individuals or populations. Recommending avoidance is not always feasible; thus, identifying dietary factors that ameliorate the cardiovascular-related adverse effects of air pollution will have beneficial public health consequences [27]. We hypothesized that intake of specific dietary components, such as carotenoids (i.e., β-cryptoxanthin and lutein), ω-3 PUFAs, fiber, isothiocyanates, and soy isoflavones, would influence mortality due to SHS-associated CVD. By informing the mechanisms that underlie the diet–SHS interactions, we can provide insights into specific diet-based prevention strategies. We present the first evaluation of the potential modifying role of dietary factors with known antioxidant and/or anti-inflammatory properties on the relationship between SHS exposure and risk for CHD mortality using data from a large prospective population-based cohort of lifetime non-smoking Chinese adults in Singapore.
Section snippets
Materials and methods
The design of the Singapore Chinese Health Study has been described previously [28]. Briefly, the cohort was drawn from a population of men and women of Chinese ethnicity, ages 45 to 74 y, who were permanent residents or citizens of Singapore and resided in government-built housing estates that housed 86% of the population during the period of participant enrollment. The study was restricted to individuals belonging to the two major Chinese dialect groups in Singapore: Cantonese and Hokkien.
Results
The average age at the follow-up 1 interview was 59.9 y (SD, 7.5) and the average length of follow-up was 5.5 y (SD, 1.3) with a total of 161 422 person-years of follow-up. The 311 identified CHD deaths were due almost exclusively to acute myocardial infarction (47.3%; ICD 9 code 410.0) and chronic ischemic heart disease (51.1%; ICD 9 code 414.9). Chronic ischemic heart disease accounted for 58.1% of CHD deaths in men compared with 48.4% in women.
Women represented 77% of lifetime never-smokers.
Discussion
In a large, population-based cohort of Chinese adults from Singapore, we provide the first epidemiologic evidence for the modifying effect of dietary fiber on the relationship between SHS exposure and risk for CHD mortality. A 62% increased risk for CHD mortality for those living with at least one smoker as compared with living with no smokers was observed among those who were within the lowest quartile of fiber intake. There was no association between SHS exposure and risk for CHD mortality
Conclusion
Diets high in nutrients with antioxidant and anti-inflammatory properties have long been promoted as defenses against many chronic conditions, including CHD resulting from the harmful effects of SHS exposure. We now provide evidence that a diet high in fiber may also ameliorate these harmful effects.
Acknowledgments
This work was supported by the National Institutes of Health, USA [RO1 CA55069, R35 CA53890, R01 CA80205, and R01 CA144034]. We thank Siew-Hong Low of the National University of Singapore for supervising the fieldwork of the Singapore Chinese Health Study. We also thank the Ministry of Health in Singapore for assistance with the identification of mortality outcomes via database linkages. Finally, we acknowledge the founding, long-standing principal investigator of the Singapore Chinese Health
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MLC designed the analytic strategy, conducted the data analyses, interpreted the results, and wrote the manuscript. LMB directed the implementation, helped in designing the analytic strategy, results interpretation, and writing of the manuscript. WPK directed the Singapore Chinese Health Study implementation, including quality assurance and control, and edited the manuscript. RW maintains the Singapore Chinese Health Study's dataset, including constructing the analytic data set for this analysis, assisted in designing the analytic strategy, and edited the manuscript. JMY directed the study implementation, including quality assurance and control, helped with results interpretation and editing of the manuscript.