Corrine Hanson, Elizabeth Lyden, Lisa Weissenburger-Moser, Jeremy Furtado, Debra J. Romberger and Tricia D. LeVan
Chronic Obstructive Pulmonary Disease (COPD) is the third leading cause of death in the U.S. and worldwide,1-4 resulting in an economic and social burden that is both substantial and increasing. Veterans are a group of people who are disproportionately affected by COPD. COPD affects over 40% of the adult veteran population in the urban Midwest, which is five times greater than the general U.S. population.5 Cigarette smoking is the main etiologic agent for COPD risk and is common among VA healthcare users, hence the prevalence of COPD is expected to continue and increase in upcoming decades.5,6 Veterans with COPD suffer high morbidity, including poor quality of life, activity limitation and exacerbations leading to emergency department visits and hospitalisations.7 Therefore, a better understanding of contributing factors to the development, progression and burden of this pervasive disease among veterans is critical in order to design effective interventions.
Epidemiological studies have shown a positive association between intake or serum levels of certain micronutrients, especially those with anti-oxidant or anti-inflammatory functions, and lung function. For example, the National Health and Nutrition Examination Surveys (NHANES) of the U.S. population have repeatedly demonstrated that increased intake or serum levels of some micronutrients, including vitamins E, D, C, A, fibre , and omega-3 fatty acids are positively associated with measures of lung function.8-19 These results suggest that diet and nutrition may impact lung function and that nutrition interventions could be useful in those with lung disease. However, there is little data available on the intake of these nutrients, or the serum levels of nutrients with anti-oxidant properties in veterans. Thus, the purpose of this analysis was to examine the intake and serum levels of nutrients which have previously been associated with lung health in a population of veterans with COPD.
Materials and Methods
This was a secondary analysis of data obtained from a cross-sectional study of agricultural exposures and COPD in veterans seeking health care at the General Medicine clinics of the Omaha Veterans Affairs (VA) Medical Center.20 Briefly, all subjects had greater than 2 years’ experience working on a farm. Subject demographics and smoking habits were obtained by in-person and telephone interviews. A participant was considered to be a smoker if they had smoked more than 100 cigarettes in their lifetime. All veterans underwent spirometry and if they had a FEV1/FVC < 0.70, then post-bronchodilator spirometry with 0.083% albuterol was performed. COPD status was ascertained for each participant using the Global Initiative for Chronic Obstructive Lung Disease (GOLD) definition of FEV1/FVC < 0.7021 or by clinical assessment. FEV1 and FVC were adjusted for height, weight, age, gender and ethnicity based on NHANESIII reference equations.22 All participants signed a written informed consent document at study enrolment. This study was approved by the VA Nebraska Western Iowa Healthcare Systems Institutional Review Board.
Nutrient Intake. A Food Frequency Questionnaire (FFQ) was mailed to the home address of the participant, with a follow up phone call in 2 weeks if the FFQ had not been returned. FFQs were analysed by trained personnel in the Harvard University Department of Nutrition. The FFQ administered was the Willet FFQ, which has been validated in adults of all ages and sexes and among a variety of socio-economic groups.23-28 The FFQ allows for analysis of absolute nutrient intake values from foods and supplements. In addition, studies show that after adjustment for absolute energy intake, the Willett FFQ is robust in validity and reliability in comparison to other validated FFQs.29-31
Nutrient intake values calculated from the completed FFQs were compared to the appropriate Dietary Recommended Intake (DRI) values for the appropriate gender and life stage. The Recommended Dietary Allowance (RDA) was used when a nutrient level had been established; if no RDA had been established, the Adequate Intake level (AI) was used. The RDA and the AI may both be used as goals for individual intake.32-35 Recommended Dietary Allowances are set to meet the needs of almost all (97 to 98 percent) individuals in a group, while the AI for life stage and gender groups is believed to cover the needs of all individuals in the group, but lack of data prevents the ability y to specify with confidence the percentage of individuals covered by this intake. Specific nutrients of interest were chosen based on prior associations with COPD as reported in the literature and included vitamin A, vitamin D, fibre , vitamin E, vitamin C, carotenes and other vitamin A-related compounds, omega-3 and omega-6 fatty acids. For intake of omega 3 and omega-6 fatty acid, the Adequate Macronutrient Distribution Range (AMDR) was used. While no defined intake level at which potential adverse effects of n-6 polyunsaturated fatty acids was identified, the upper end of the AMDR is based on the lack of evidence that demonstrates long-term safety and human in vitro studies which show increased free radical formation and lipid peroxidation with higher amounts of omega-6 fatty acids.32 The upper end of AMDR for omega-3 fatty acids is based on maintaining the appropriate balance with omega-6 fatty acids, as this has been implicated in the risk for chronic disease.36
Serum Nutrient Level. Serum nutrient level was measured at the Biomarker Research Laboratory at the Harvard School of Public Health. Concentrations of lutein+zeaxanthin, β-cryptoxanthin, lycopene, α-carotene, β-carotene, retinol, γ-tocopherol, and α-tocopherol in serum samples were measured using the method described by Hess et al. with some modifications.37 Serum samples were mixed with ethanol containing rac-Tocopherol (Tocol) as an internal standard, extracted with hexane, evaporated to dryness under nitrogen, and reconstituted in ethanol, dioxane and acetonitrile. Samples were quantitated by high-performance liquid chromatography (HPLC) on a Restek Ultra C18 150mm X 4.6mm column, 3μm particle size encased in a Hitachi L-2350 column oven to prevent temperature fluctuations, and equipped with a trident guard cartridge system (Restek, Corp. Bellefonte, PA). A mixture of acetonitrile, tetrahydrofuran, methanol, and a 1% ammonium acetate solution (68:22:7:3) was used as mobile phase, flow rate 1.1 ml/min, using a Hitachi Elite LaChrom HPLC system comprised of an L-2130 pump in isocratic mode, an L-2455 Diode Array Detector (monitoring at 300nm and 445nm), and a programmable AS-2200 auto-sampler with chilled sample tray. The system manager software (D-7000, Version 3.0) was used for peak integration and data acquisition (Hitachi, San Jose, CA). Every run included two replicates each of a two-level serum pool sample set. For external quality control, the laboratory participates in the standardisation program for carotenoid analysis from the National Institute of Standards and Technology U.S.A.
[Table 1 available in download]
Statistical Analysis. Descriptive statistics (counts, percentages, means, and standard deviations) were calculated for all variables. The Spearman correlation coefficient was used to look at associations between lung function measures (Forced Expiratory Volume in One Second, FEV1; Forced Vital Capacity, FVC; FEV1/FVC ratio) and intake and serum level of nutrients. Multivariable regression models adjusted for the possible confounding of age, smoking, body mass index (BMI), energy intake (for models containing intake of nutrients) and serum cholesterol (for models containing the serum tocopherols). Intake and serum level of nutrients were log-transformed for normality and used in the regression models. The Mann-Whitney test was used to compare continuous data between 2 groups. The Kruskal-Wallis test was used to compare continuous data between >2 groups and p-values for post-hoc pairwise comparisons were adjusted using a nonparametric multiple-comparison procedure. All statistical tests were two-sided and a p-value < 0.05 was considered statistically significant. Analyses were done using SAS Version 9.4.
[Table 2 available in download]
*Individuals who smoke require 35 mg/day more Vitamin C than nonsmokers
Recommended Dietary Allowances (RDAs) in bold type, Adequate Intakes (AIs) in ordinary type followed by an
*** Acceptable Macronutrient Distribution Range (AMDR) is the range of intake for a particular energy source
that is associated with reduced risk of chronic disease while providing intakes of essential nutrients.
A total of 123 FFQs were mailed out with a response rate of 33%. There were no differences between responders and non-responders with respect to demographics including age, sex, race, and FEV1/FVC ratio. All participants were white males with a mean age of 66.5 years and a mean BMI of 29.2kg/m2. Baseline characteristics of the respondent population are summarised in Table 1.
Nutrient Intakes. A summary of the intake of anti-oxidant and anti-inflammatory nutrients assessed by the FFQ are compared to existing recommendations as shown in Table 2. Veterans with COPD had suboptimal intakes of vitamin D, fibre, and omega-3 fatty acids, with an intake of omega-6 fatty acids above the recommended level. No DRIs for carotenes, β-cryptoxanthin, lycopene or lutein+zeaxanthin have been established; however, intake values are reported due to prior associations of these nutrients with lung disease.
Intake of nutrients and lung function. In the univariate analysis, we found intake of omega-3 fatty acids were inversely associated with FEV1 (r= -0.39, p=0.02) and FEV1 % predicted (r= -0.42, p=0.01). After adjustment for the covariates energy intake, age, BMI, and smoking status, the correlation between omega-3 fatty acid intake and lung function was attenuated (FEV1, β= -0.21, p=0.07; FEV1% predicted, β=-5.5, p=0.07). Intake of omega-3 fatty acids was not associated with FVC or FEV1/FVC ratio (data not shown). None of the other nutrients were significantly associated with FEV1, FVC and FEV1/FVC ratio (data not shown).
Serum Nutrient Levels. Serum antioxidant nutrient levels of the cohort are summarised in Table 3. When we evaluated the correlation between dietary intake and serum levels, β-cryptoxanthin intake was significantly associated with serum levels of β-cryptoxanthin (r=0.35, p=0.03).
Serum nutrient levels and lung function. In the univariate analysis, we found statistically significant correlations between serum levels of trans-β carotene and FVC % predicted (r=0.39, p=0.02) and cis-β carotene and FVC % predicted (r=0.39, p=0.02). After adjustment for age, BMI, and smoking status in the multivariable model, these relationships were no longer statistically significant. The remaining nutrients assayed in serum were not associated with lung function (data not shown). There was no association between serum nutrient levels and COPD stage.
[Table 3 available in download]
Nutrient levels and smoking status. There was no association between intake of nutrients (listed in Table 2) and smoking status (current, former, never) (data not shown). Importantly, serum levels of anti-oxidant nutrients (α-carotene, trans β-carotene, cis β-carotene, total β-carotene, β-cryptoxanthin) differed by smoking status (Table 4). There was a dose response of these serum nutrients with smoking status, where current smokers had fewer of these serum nutrients than never- smokers.
Participants in our study did not meet the minimum guidelines for intake of three important nutrients that typically have been associated with improved lung health: vitamin D, fibre, and omega-3 fatty acids. Additionally, intake of omega-6 fatty acids, which may have pro-inflammatory properties, were above the Acceptable Macronutrient Distribution Range.
Intakes of nutrients previously associated with lung function Our results are consistent with other studies that show vitamin D intake in populations of patients with COPD are below current recommendations.38 Because COPD patients spend less time outdoors,39 they would be expected to have decreased sun exposure, which places them at high risk for vitamin D deficiency. Intake of vitamin D has been shown to be associated positively with FEV1 (difference in FEV1 between top and bottom quintiles of intake, 0.079 L; 95% CI, 0.02–0.14; p for trend= 0.007), ratio of FEV1 to FVC (p=0.008), and associated negatively with COPD (odds ratio [OR] comparing top and bottom quintiles, 0.57; 95% CI, 0.38–0.87; p=0.02).40 Intake of dairy products, but not specifically vitamin D intake, has been associated with less severe measures of emphysema (defined by computed tomographic lung density) in 3,271 subjects enrolled in the Multi- Ethnic Study of Atherosclerosis (MESA) study (p=0.02 and p=0.01 for alpha, a measure of emphysema, and apical vs basilar distribution of emphysema, respectively).41This association was observed primarily for low-fat dairy intake, not high-fat dairy intake, and may have been confounded by other lifestyle factors, because those who choose low-fat dairy products may make different lifestyle choices compared with those who choose high-fat dairy products. The usefulness of evaluating the effect of vitamin D intake from diet may have limited value, given the overall minor contribution of dietary intake to vitamin D status, because the major source for vitamin D is outdoor sun exposure.
[Table 4 available in download]
The mean intake of fibre in our population was 33% below current recommendations. In large prospective studies, high fibre diets reduced mortality from infectious disease, respiratory disease, smoking-related cancers and cardiovascular disease.42,43 With regard to lung function, four large prospective studies have shown diets high in fibre are associated with better lung function and decreased risk of COPD.16,44-46 A recent study using mouse models demonstrated high amounts of dietary fibre regulated the immune system in the lungs, which included an increase in the haematopoiesis of dendritic cells that could not reactivate T helper type 2 effector cells, resulting in lower airway inflammation.47 These results provide evidence that dietary fibre can shape the immunological environment in the lung. One proposed mechanism underlying this beneficial effect comes from studies showing that fibre stimulates the growth of beneficial bacteria in the gut, which synthesse, through fermentation, high quantities of Short Chain Fatty Acids (SCFAs).47-53 SCFAs have been detected in the portal circulation where they are active in the liver and bone marrow to decrease innate immune activation.47,50,52-56
Intake of omega-3 fatty acids was extremely low and omega-6 fatty acids was notably high in our cohort. A balance between omega-6 and omega-3 fatty acids is thought to be important in the prevention and treatment of coronary artery disease, diabetes, osteoporosis and other inflammatory diseases.36,57 In patients with asthma, an omega 6:3 ratio of 5:1 decreased leukotriene production and improved respiratory symptoms.58 In the secondary prevention of cardiovascular disease, a leading cause of death in patients with COPD, a ratio of 4:1 has been associated with a 70% decrease in total mortality.36 Our cohort was from a Midwestern state where fish intake is commonly low, and this may have contributed to the poor intake of omega-3 fatty acids resulting in an extremely high omega 6:3 ratio of over 100:1. Fish and seafood are the main source of omega-3 fatty acids, contributing up to 71% of total intake.59 Fats and oils, meat and poultry, and cereal-based products and cereals, are the primary sources of omega-6 fatty acids in a Westernised diet,59 therefore it may be difficult for even populations living in an area with an ample seafood supply to achieve a desired ratio.
In the univariate analysis, we found an inverse association between omega-3 fatty acid intake and lung function. However, other studies conflict with this finding. A recent systemic review identified 11 observational studies evaluating the relationship between COPD and intake or serum levels of omega-3-fatty acids. Of these 11 studies 6 found a significant relationship and 5 of those relationships were inverse (protective against COPD).60 The remaining 5 studies found no relationship.60 Fish is one of the primary sources of omega-3 fatty acids in the diet, and dietary intake patterns with an increased consumption of fish have been associated with a decreased development of COPD in smokers and nonsmokers, increased FEV1 and decreased long-term COPD mortality.61-63 Analysis of the Seven Countries Study, a population-based cohort of 12,763 men, calculated that fruit and fish intake together explained about 67% of the variation in COPD mortality rates after 25 years.61 Our study did not find such an association between omega-3 fatty acid intake and lung function, however it has been shown that the omega 6:3 ratio is critical in achieving respiratory benefits,58 and given the high ratio present in our cohort, any findings may have been obscured.
Serum levels of anti-oxidant nutrients A large body of observational epidemiological evidence suggests that higher serum concentrations of β-carotene and other carotenoids obtained from foods are associated with lower risk of several chronic diseases,35 and serum level of carotenes have been associated with improved lung function in other studies.12-14,64,65 Although our study did not find this association, we did find a correlation between dietary beta-cryptoxanthin and the serum level of this compound, which suggest that diet is an effective intervention for increasing serum levels of beta-cryptoxanthin in subjects with COPD. This may be especially important in smokers, who exhibited decreased levels compared to non-smokers. Blood concentrations of carotenoids are the best biological markers for consumption of fruits and vegetables.35 Although no DRIs are proposed for β-carotene or other carotenoids at the present time, existing recommendations for increased consumption of carotenoid-rich fruits and vegetables in patients with COPD would appear to be supported.
Our study also documents lower serum level of β-cryptoxanthin and carotenes in smokers. A dose-response appeared to be present, with serum levels decreasing as the level of smoking increased. Several studies have reported that smokers have lower plasma carotenoid concentrations compared to non-smokers,66,67 and higher numbers of cigarettes per day have been shown to lead to a corresponding decrease in serum carotenoids in a dose-dependent manner.66 Whether this is due completely to a decreased intake of carotenoids is unknown; tobacco smoke is highly oxidative, and has been shown to destroy beta carotene and other carotenoids in human plasma in vitro,68 which may contribute to a reduction in serum levels. The association of serum β- carotene and FEV1 has been shown to be weaker in smokers when compared to non-smokers, and the strength of the relationship decreased as the amount of smoking increased.8
Supplementation of micronutrients Should clinicians consider supplementation of micronutrients in their veteran patients who smoke? In one study, adults at high risk for cardiovascular events received antioxidant vitamins for 5 years (600 mg vitamin E, 250 mg vitamin C, 20 mg beta-carotene). That study failed to identify any effect on lung function, although it was measured only at the last visit, or hospitalisation for non-neoplastic respiratory causes. Individuals with severe COPD, however, were excluded from that study.69 Another large trial showed a significantly higher incidence of lung cancer in current smokers supplemented with alpha tocopherol and beta-carotene for 5-8 years.70 However, a high intake of beta carotene and vitamin E from food was associated with an improvement in respiratory symptoms.71 The latter study seems to suggest that prevention campaigns should stress the importance on nutrient intake from food, especially in smokers.
Limitations Our study has several limitations. Our sample size was small, and it is always difficult to separate the direct effect of a nutrient on an outcome from the overall effects of associated lifestyle factors. Our population is a Midwestern population with agriculture backgrounds, which may limit generalisability to other populations. Some significant results were attenuated after adjustment for confounders, and some potential covariates, such as corticosteroid use, for which no adjustment was made.. We assessed nutrient intake using FFQ methodology, which is subject to various limitations, including measurement error in the estimated portion sizes of foods, and inaccuracies that result from incomplete listings of all possible foods. However, we do provide both serum levels and nutrient intake levels, which are often not studied together. We do not, however, report serum levels of vitamin D, which would be relevant in this population. In addition, very little data exists on nutrients that may be of concern to a population of veterans who may be more likely than the general population to be current or former smokers.
Our study shows that veterans with COPD have low intakes of nutrients associated with lung function and COPD, and that smokers have lower serum levels of anti-oxidant nutrients than non-smokers. Given the importance of inflammation and oxidant stress in lung function, a direct effect of nutrients that have anti-oxidant and anti-inflammatory properties is readily plausible. As measures of lung function are an independent predictor of mortality in the general population as well as in people with lung disease,72,73 public health initiatives to improve lung function through dietary interventions in veterans could potentially have a profound impact. In the future, larger and well-designed interventional trials are needed to confirm these associations, and to establish whether dietary interventions are effective in the prevention or treatment of lung impairment. Populations of subjects susceptible to a decline in lung function, such as smokers, may provide a target population that would incur the most benefit from the results of such trials. The low cost and safety of dietary interventions such as counselling to increase fruit and vegetable intake to maintain or improve lung function make this a very attractive intervention for researchers, as well as clinicians working with populations of patients either with, or at risk for, lung impairment.