Effects of Zinc Supplementation on Serum Lipids

A Systematic Review and Meta-Analysis

Priyanga Ranasinghe; WS Wathurapatha; MH Ishara; R. Jayawardana; P. Galappatthy; P. Katulanda; GR Constantine

Disclosures

Nutr Metab. 2015;12(26) 

In This Article

Methods

The current systematic review was conducted in accordance with the PRISMA statement (Preferred Reporting Items for Systematic reviews and Meta-Analyses) for systematic reviews of interventional studies.[29]

Literature Search

A five staged comprehensive search of the literature was conducted in the following databases; PubMed® (U.S. National Library of Medicine, USA), Web of Science® [v.5.4] (Thomson Reuters, USA) and SciVerse Scopus® (Elsevier Properties S.A, USA) for studies published before 31 st December 2014. During the first stage the above databases were searched using the following keywords; ('Zinc' or 'Zn' or 'Zinc supplementation' or 'Zn supplementation' or 'Zinc therapy' or 'Zn therapy') AND ('Lipid(s)'or 'Cholesterol' or 'LDL' or 'HDL' or 'Lipoprotein(s)' or 'Triglyceride(s)').

In the second stage the total hits from the 3 databases were pooled and duplicates were removed. This was followed by screening of the retrieved articles by reading the article 'title' in the third stage and 'abstracts' in stage four. In the fifth stage individual manuscripts were screened, and those not satisfying inclusion criteria were excluded. To obtain additional data a manual search of the reference lists of articles selected in stage five was performed. This search process was conducted independently by two reviewers (PR and RJ) and the final group of articles to be included in the review was determined after an iterative consensus process.

Inclusion and Exclusion Criteria

A study was considered eligible for data extraction if it was a controlled clinical trial in humans, that included a Zinc supplement intervention, either alone or in combination with other micronutrients and evaluated at least one of the following outcomes: TG,TC, LDL-c and HDL-c. Results were limited to studies conducted in humans, published in English, while conference proceedings, editorials, commentaries and book chapters/book reviews were excluded.

Data Extraction and Analysis

A meta-analysis of selected studies examining the effects of Zinc supplementation on serum lipid parameters was performed using the Rev Man version 5.3 (Review Manager, Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2011) statistical software package. A random effect analysis was conducted for all comparisons and in all analyses a p-value < 0.05 was considered statistically significant. Forest plots were used to illustrate the study findings and meta-analysis results. Statistical heterogeneity was assessed using the χ 2 test on Cochrane's Q statistic[30] and by calculating I 2 [31] and is considered significant if p < 0.05. TC, LDL-c, HDL-c and TG are reported as mg/dl, where studies reported as mmol/l a numerical conversion to mg/dl was done as follows; For TC, HDL-c, and LDL-c cholesterol, values presented in mmol/l multiplied by conversion factor 38.67 and for TG, conversion factor used was 88.57.[32] In a study where there was several interventions including interventions with multiple supplements, the interventions using Zinc supplementation alone was used in the meta analysis as the elemental dose of Zinc was similar in all interventions.[33] In a study done by Farvid et al. intervention with least number of additional supplements was used to compare with the placebo group.[34] In cross-over studies the pooled estimate of Zinc and placebo groups after completion of entire cross-over scheme was used in the analysis.[17,19,35] Three separate sub-group analyses were performed; a) for group of studies using Zinc supplementation alone,[17–19,21–23,26,27,33,35–43] b) for group of studies done on healthy participants[35–37,39–41,44] and c) for group of studies done on non-healthy participants.[17–23,26–28,33–35,38,42,43,45,46]

Quality Assessment

The Jaded scale was used to assess the methodological quality of the trials included in the systematic review.[47] Each study was scored from 0 ('poor' quality) to 5 ('good' quality) according to the following criteria: 1) was the study described as randomized? ;2) was the study described as double blind?; 3) was there a description of withdrawals and dropouts? 3) was the method of randomization described in the paper and appropriate?; 4) was the method of blinding described and appropriate?. Each question would score a single point if the answer is 'yes' or zero points if the answer is 'no'. Questions 4) and 5) would score −1 mark each if method of randomization was described, but inappropriate and method of blinding was described, but inappropriate respectively.

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