Improvement of Lung Function by Micronutrient Supplementation in Patients with COPD: A Systematic Review and Meta-Analysis

Background: A healthy, well-balanced diet plays an essential role in respiratory diseases. Since micronutrient deficiency is relatively common in patients with chronic obstructive pulmonary disease (COPD), micronutrient supplementation might have the beneficial health effects in those patients. This systematic review and meta-analysis aimed to demonstrate the impact of micronutrient supplementation on the lung function of patients with COPD. Methods: The PubMed, Cochrane Library, and Web of Science databases were searched from their corresponding creation until February 2024. Search terms included ‘chronic obstructive pulmonary disease’, ‘COPD’, ‘micronutrients’, ‘dietary supplements’, ‘vitamins’, ‘minerals’, and ‘randomized controlled trials’. Meta-analysis was performed to evaluate the effects of micronutrient supplementation alone or complex on lung function in patients with COPD. Results: A total of 43 RCTs fulfilled the inclusion criteria of this study. Meta-analysis revealed that vitamin D supplementation could significantly improve FEV1% (WMDdifferences between baseline and post-intervention (de): 6.39, 95% CI: 4.59, 8.18, p < 0.01; WMDpost-intervention indicators (af): 7.55, 95% CI: 5.86, 9.24, p < 0.01) and FEV1/FVC% (WMDde: 6.88, 95%CI: 2.11, 11.65, WMDaf: 7.64, 95% CI: 3.18, 12.10, p < 0.001), decrease the odds of acute exacerbations, and improve the level of T-cell subsets, including CD3+%, CD4+%, CD8+%, and CD4+/CD8+% (all p < 0.01). The effects of compound nutrients intervention were effective in improving FEV1% (WMDde: 8.38, 95%CI: 1.89, 14.87, WMDaf: 7.07, 95%CI: −0.34, 14.48) and FEV1/FVC% (WMDde: 7.58, 95% CI: 4.86, 10.29, WMDaf: 6.00, 95% CI: 3.19, 8.81). However, vitamin C and vitamin E supplementation alone had no significant effects on lung function (p > 0.05). Conclusions: Micronutrient supplementation, such as vitamin D alone and compound nutrients, has improved effect on the lung function of patients with COPD. Therefore, proper supplementation with micronutrients would be beneficial to stabilize the condition and restore ventilation function for COPD patients.


Introduction
Chronic obstructive pulmonary disease (COPD) has become the world's third leading cause of death [1].In 2019, more than 200 million patients with COPD were reported globally [2].It is a heterogeneous lung disease with persistent respiratory symptoms and airflow obstruction caused by abnormalities in the airways and/or alveoli [3].The patients will present with a series of symptoms such as cough, sputum, dyspnea, and acute exacerbation due to decreased immune function [4].Lung function is an important indicator to evaluate the degree of disease in patients with COPD [5].In other lung diseases, the number of T lymphocytes was significantly correlated with forced expiratory volume in 1 s (FEV1) [6,7].The poorer the lung function, the higher the level of inflammation and the poorer the immune function, such as IL-6, CD8 + [8].The current routine treatment of COPD includes drug therapy, smoking cessation counseling, pulmonary rehabilitation therapy [9], and nutritional supplement, which have gradually entered the clinical treatment [10,11].
Micronutrient intake is inadequate in patients with COPD.It was shown that the intake of calcium, potassium, folate, retinol, and thiamine was lower than recommended dietary allowance (RDA) in over 75% of COPD patients [12].The intake of vitamins A, C, D, E, B12, carotenoids, and magnesium all have protective effects on the loss of lung function, such as improving the average level of lung function and reducing the rate of decline of lung function indicators [13].Meanwhile, patients with decreased immune function are susceptible to bacterial and viral infections, leading to worsening clinical symptoms and an increase in the frequency of acute exacerbations, and multiple nutrients play a critical role in regulating immune function and anti-inflammatory effects, mainly reflected in regulating barrier function, inflammatory factors [14].
Although there have been few meta-analyses showing the improvement of vitamin C and D in patients with COPD [15,16], a comprehensive analysis of micronutrients and complex nutrients are currently lacking in COPD patients.Therefore, we conducted this systematic review and meta-analysis to prove the effects of micronutrients on lung function, T-cell immunity, and other indicators of COPD patients.

Search Methods
The PubMed (https://pubmed.ncbi.nlm.nih.gov/),Cochrane Library (https://www.cochranelibrary.com/),and Web of Science databases (https://clarivate.com.cn/solutions/web-of-science/) were searched from their corresponding inception until 12 February 2024.Searching was conducted using keywords including 'chronic obstructive pulmonary disease', 'COPD', 'micronutrient', 'Dietary Supplements', 'vitamin', 'mineral', and 'randomized controlled trial'.Detailed search strategies are reported in Supplementary Table S1.In addition, the reference lists of critical articles and related meta-analysis articles were also searched in this study.Articles are language-restricted by Chinese and English.This study was registered in the PROSPERO (registration number ID: CRD42023461552).

Inclusion Criteria and Exclusion Criteria
The inclusion criteria were as follows: (1) the subjects were patients diagnosed with COPD and were 18 years old or older in the eligible studies; (2) the study type was randomized controlled trials (RCTs); (3) the studies involved dietary supplementation of various micronutrients (including vitamins and minerals) in the effects on symptoms or outcomes in patients with COPD.Exclusion criteria were: (1) the studies were conducted in cell assays or animal experiments; (2) observational studies: cross-sectional studies, cohort studies, and case-control studies; (3) the studies included non-micronutrients intervention and non-dietary nutrition intervention; (4) the studies included unavailable available data; (5) narrative reviews, conference abstracts, case series, and studies not published in peer-reviewed journals.

Study Selection and Data Extraction
Study selection and data extraction were carried out independently by two different reviewers (ML, LZ).When disagreements arose, a consensus was reached through discussion or reevaluation by the third reviewer (JC).These retrieved articles were imported into EndNote X9.1.After removing duplicate articles, the titles and abstracts of the literature were initially screened according to the inclusion and exclusion criteria.Then the full text was read and screened again, and the study data was extracted for validity check.
The data extracted from the selected research articles included first author, publication year, country, study type, randomized, blinding, sample size, patient style, the characteristic of experimental and control groups, intervention duration, and outcome indicators including FEV1, the ratio of forced expiratory volume in 1 s and forced vital capacity, 6-min walk distance (6MWD), acute exacerbation, COPD assessment test (CAT) score, maximal expiratory pressure (MEP), maximal inspiratory pressure (MIP), and T-cell immunity level, such as CD3 + , CD4 + , CD8 + , and CD4 + /CD8 + .For each trial, the means and standard deviation (SDs) of biomarkers at baseline and endpoint in both the control and intervention groups were extracted, respectively.If the trial did not provide the SDs directly, they were calculated from interquartile or standard error of the mean (SEM) using the equation listed in the Cochrane Handbook.

Quality Assessment
The modified Jadad scale was used to evaluate the quality of randomized clinical trials.The modified Jadad scale consists of four parts: random sequence generation, randomization concealment, blinding, and withdrawal.The studies were scored from 0 to 7, and 1-3 signified low quality, while 4-7 signified high quality [17].

Statistical Analysis
Statistical analysis was conducted using STATA 11.0.Heterogeneity among studies was assessed with I 2 statistics.The I 2 represented the proportion of total variation, and the high degrees of heterogeneity was defined based on values of 75% as cut-off points.When heterogeneity was high, a random-effects model was chosen, otherwise a fixed-effect model was used.Data were graphically displayed using forest plots.According to different data extracted, the combination effect indicators were selected: weighted mean difference (WMD), standardized mean difference (SMD), and odds ratio (OR) with 95% confidence intervals (CIs).Subgroup analysis of FEV1% and FEV1/FVC% was performed according to geographical differences in the patients with COPD, the style of patients, supplement dosage and vitamin D supplement forms and duration of intervention.Sensitivity analysis and the Egger regression test were conducted to explore the potential sources of heterogeneity.Two sets of data were used to illustrate the effects of various micronutrients on patients with COPD, including differences from baseline and post-intervention (de), and post-intervention indicators (af).There was a statistically significant difference when p < 0.05.

Study Screening and Results
The literature search following the search strategy returned 3736 potentially relevant records, including 457 duplicate articles.The remaining 3279 papers were screened, and 3142 were preliminarily excluded according to the title and abstract.At the same time, 22 articles were manually retrieved and included in this study.Finally, after reading and analyzing the full text, 43 articles remained in the review for the synthesis of the qualitative analysis, and 36 articles entered the quantitative analysis (Figure 1).

Characteristics of Studies Included
Among all the 43 RCT articles, a total of 4094 participants were included between 1997 and 2022, of which 2225 were in the experiment group and 2069 were in the control group.In all the included studies, 3411 patients were treated alone with vitamin D (1771 in the intervention group and 1640 in the control group), 122 with vitamin C (62 in the intervention group and 60 in the control group), 50 with vitamin E (28 in the intervention group and 22 in the control group), 49 with magnesium (24 in the intervention group and 25 in the control group), and 662 with complex nutrient intervention (340 in the intervention group and 322 in the control group).There were 30 studies from Asia, 11 studies from Europe, and two studies from North America.The main characteristics of the selected studies were shown in Table 1.Abbreviations: I: Intervention group, C: control group, M: male, F: female, DB: double-blinding, SB: single-blinding, COPD: chronic obstructive pulmonary disease, AECOPD: acute exacerbations of chronic obstructive pulmonary disease, FEV1: forced expiratory volume in 1 s, FEV1/FVC: the ratio of forced expiratory volume in 1 s and forced vital capacity, 6MWD: 6-min walk distance, MEP: maximal expiratory pressure, MIP: maximal inspiratory pressure, CAT: COPD assessment test, SGRQ: St George's Respiratory Questionnaire.

Quality Assessment
The quality of each study was evaluated by two independent reviewers.Differences were resolved by the third reviewer.There were 20 articles of high quality and 23 articles of low quality.Specific scores of the included literature were displayed in Supplementary Table S2.
The effect of vitamin D supplementation on muscle strength was mentioned in three articles [20,25,44].In the literature, two papers [20,25] were included involved the indicators of the strength of the respiratory muscles for meta-analysis.After the intervention of vitamin D, the results demonstrated no heterogeneity of MEP (WMD de : 0.25, 95% CI: −0.16, 0.66, p > 0.05) and greater consistency of MIP (WMD de : −0.30, 95% CI: −1.51, 0.92, p > 0.05, Supplementary Figure S3D,E).

Magnesium
There is only one article on how single-mineral supplementation improves lung function in people with COPD [49].It was revealed that magnesium supplementation alone did not significantly improve lung function, as well as other indicators.

Discussion
This study analyzed the effects of micronutrients alone or in a compound on lung function in COPD patients through systematic review and meta-analysis.The beneficial effects of vitamin D alone and complex nutrients supplemented in patients were reflected in improving lung function and related disease severity indicators, such as the levels of T cells.In addition, the supplementation of vitamins C and E alone did not have a significant promoting effect on lung function.The improvement in patients supplemented with magnesium may be reflected in reducing the patient's level of inflammation, while other mineral interventions alone in COPD patients have been sparsely described.
This meta-analysis showed that supplementation of vitamin D alone had a certain improvement effect on lung function and immunity in COPD patients, such as FEV1 and FEV1/FVC, reduced the number of acute exacerbations, and improved the levels of T cells.It was indicated that vitamin D can be given in the general treatment of patients with COPD.The effects of vitamin D supplementation on the lung function were consistent with most of the results of a meta-analysis written by Li et al. [16], but the 6WMD suggested different findings.This may be because the previous meta-analysis did not divide into vitamin D alone or in a complex with other nutrients.The ameliorating effect of vitamin D on COPD might be due to the regulation of the inflammatory state and immunity system in vivo [58].Vitamin D relieves airway inflammation and can also improve lung function by promoting cell multiplication and reducing cell apoptosis [59,60].Vitamin D can protect the innate immunity and adaptive immune system in the epithelial mucosa, which is reflected in maintaining the integrity of the epithelial mucosa, inhibiting nuclear factor (NF)-κB in epithelial cells, and reducing the expression of cytokines [61].
There was considerable heterogeneity in the results of vitamin D on lung function index (FEV1%, FEV1/FVC%) in the included literature.Sensitivity analyses and publication bias analyses were performed, and it was demonstrated that no single article affected the results.Subsequently, multi-faceted subgroup analyses were conducted.Although the heterogeneity was reduced in all subgroups, the reduction effect on heterogeneity was not obvious.The reasons were hypothesized as manifold for the high degree of heterogeneity in vitamin D interventions to improve the COPD patients' lung function.Of course, intervention studies with a larger population and more clear experimental design are still needed to definitively prove this.
Vitamin C and vitamin E are the primary antioxidant nutrients [62].Oxidative stress is the main driving mechanism of COPD pathogenesis; oxidative stress in the lungs comes from exogenous oxidative stress caused by exposure to smoking or air pollution in and around the city, as well as endogenous oxidative stress generated by activated inflammatory cells, especially neutrophils [63,64].Therefore, treating oxidative stress with antioxidants or enhancing endogenous antioxidants should be an effective strategy to treat the underlying pathogenesis of COPD [65].Previous studies have proven that dietary vitamin C supplementation might prevent COPD, and vitamin E intake was positively correlated with FEV1 [66,67].However, after meta-analysis, we found no significant improvement in vitamin C intervention because there were fewer studies of vitamin C interventions alone.Supplementation with a single antioxidant vitamin has been less studied, possibly because of the limited effectiveness of supplementation, and the combination of antioxidant vitamins and other nutrients have been opted to improve COPD.
It was demonstrated in this study that multinutrient supplementation improved FEV1 and FEV1/FVC in COPD patients but did not significantly effect 6WMD and SGRQ.The many micronutrients contained in the complex nutrients such as vitamins (A, C, D, and E), minerals (zinc, selenium, iron, and magnesium), and a range of others all play important roles in reducing the risk of chronic lung disease and viral infections [68][69][70][71].Compared with single micronutrients, multiple nutrients are added to supplements to have a stable effect, comprehensive nutrients, and nutrients can also promote mutual absorption, such as vitamin C and vitamin E work together to exert antioxidant effects, and Gu Wenchao also uses vitamin D and calcium in combination to improve COPD patients [51,52,54].The effect of complex nutrient intervention on COPD may be multifaceted [72].Comprehensive nutrients can improve the nutritional level of patients, and the nutritional level of patients is also significantly correlated with lung function and quality of life [73,74].In addition, compound nutrients can improve body composition, and reduce inflammation levels [48,49,56].The results of our study illustrated that the intervention effect of complex nutrients is mainly focused on improving lung function, and it may be that the addition of vitamin D plays a very important role in lung function.Complex nutrients also have some effect on reducing inflammation levels, possible because the added nutrients exert some anti-inflammatory effects.Therefore, the supplementation of complex nutrients may be a better intervention for the stability of disease status and lung function in patients with COPD.
There were some certain advantages in this systematic review and meta-analysis.So far, this is a full-scale meta-analysis to systematically evaluate the clinical efficacy of micronutrients alone or in a compound supplement, including lung functions and disease severity, in COPD patients with comprehensive consideration.A comprehensive range of micronutrients were included, including supplementation of vitamin D, vitamin C, vitamin E, and magnesium alone, and complex micronutrients.In addition, the variables analyzed in this study were not just post-intervention differences between groups, but also included comparisons of pre-and post-intervention differences, which better eliminated the effects of baseline data.However, few limitations still existed.There were high heterogeneities in a few outcomes, and the sources of heterogeneity were discussed through subgroup analyses, sensitivity analyses, and publication bias.In addition, some of the included literature reviews were of low quality, suggesting that future clinical trial designs should be more rigorous to improve the quality of the literature.We were also able to minimize bias throughout the process by creating detailed protocols, independently selecting articles, and using statistical analysis and data selection.

Conclusions
In summary, the results found that micronutrient dietary supplementation has important clinical implications for COPD patients.Vitamin D supplementation alone and multinutrient supplementation resulted in significant improvements in lung function and disease severity in patients with COPD.Due to the small number of articles on vitamin C and vitamin E supplementation alone in people with COPD, our results showed no significant effect on improving lung function and need to be further confirmed by large and well-designed prospective randomized controlled trials.Consequently, it is recommended that patients with COPD can properly supplement vitamin D or complex nutrients on the basis of basic treatment.

Figure 1 .
Figure 1.The flow diagram of included and excluded studies.

Table 1 .
Characteristics of included studies.

Table 2 .
Subgroup analyses of meta-analysis of vitamin D used in patients with COPD on FEV1% and FEV1/FVC.