Effects of Conjugated Linoleic Acid Intake in the Form of Dietary Supplement or Enriched Food on C-Reactive Protein and Lipoprotein (a) Levels in Humans: A Literature Review and Meta-Analysis

Abstract

Background: C-reactive protein (CRP) and lipoprotein (a) (Lp[a]) play essential roles in cardiovascular disease incidence. This study aimed to review the association between the intake of conjugated linoleic acid (CLA) in the form of dietary supplement or enriched food with different treatment durations and the levels of Lp(a) and CRP in human studies.

Methods: All the articles published in Cochrane Library, ProQuest, Scopus, and Google Scholar from November 2014 to October 2015 were searched and the clinical trials on the effects of CLA on Lp(a) and CRP levels were assessed. Of the 2249 articles initially retrieved, 21 eligible randomized clinical trials were enrolled in this systematic review. The publication dates of the eligible articles ranged from 2005 to 2013. The mean difference and the standard deviation of changes in CRP and Lp(a) levels in intervention and control groups were used as effect-size measures for meta-analysis. The obtained data from the eligible randomized controlled trials were meta-analyzed using Stata, version 13.

Results: The intake of CLA as a dietary supplement led to a significant increase in CRP levels (standardized mean difference [SMD]=0.41, 95% CI: 0.28 to 0.54; P=0.001). Subgroup analysis based on the duration of CLA consumption showed that CLA consumption more than 24 weeks resulted in a significant increase in the levels of CRP (SMD=0.52, 95% CI: 0.36 to 0.68; P=0.001) and Lp(a) (SMD=0.24, 95% CI: 0.01 to 0.47; P=0.04).

Conclusion: The current systematic review and meta-analysis showed that the long-term consumption of CLA increases the levels of CRP and Lp(a).

Keywords: C-reactive protein; Lipoprotein (a); Meta-analysis; Conjugated linoleic acid.

Figure1

PRISMA flowchart shows the literature search and review.

Figure2

Meta-analysis shows a significant increase in C-reactive protein following conjugated linoleic acid supplementation. The black squares show study-specific standardized differences (Std diff) in means, and the horizontal lines show 95% CIs. The area of the black squares is proportional to the specific-study weight in the overall meta-analysis. The center of the black diamonds indicates the pooled standardized difference in means, and their width represents the pooled 95% CI.

Figure3

Funnel plot for the included studies on the effects of conjugated linoleic acid on C-reactive protein demonstrates outlier studies.

Figure4

Meta-analysis shows a significant increase in C-reactive protein following conjugated linoleic acid supplementation after dropping the outlier studies. The black squares show study-specific standardized differences (Std diff) in means, and the horizontal lines show 95% CIs. The area of the black squares is proportional to the specific-study weight in the overall meta-analysis. The center of the black diamonds indicates the pooled standardized difference in means, and their width represents the pooled 95% CI.

Figure5

Meta-analysis shows no significant change in lipoprotein (a) following conjugated linoleic acid supplementation. The black squares show study-specific standardized differences (Std diff) in means, and the horizontal lines show 95% CIs. The area of the black squares is proportional to the specific-study weight in the overall meta-analysis. The center of the black diamonds indicates the pooled standardized difference in means, and their width represents the pooled 95% CI.

Figure6

Meta-analysis shows a significant increase in C-reactive protein after conjugated linoleic acid supplementation for more than 24 weeks. The black squares show study-specific standardized differences (Std diff) in means, and the horizontal lines show 95% CIs. The area of the black squares is proportional to the specific-study weight in the overall meta-analysis. The center of the black diamonds indicates the pooled standardized difference in means, and their width represents the pooled 95% CI.

Figure7

Meta-analysis shows a significant increase in lipoprotein (a) after conjugated linoleic acid supplementation for more than 24 weeks. The black squares show study-specific standardized differences (Std diff) in means, and the horizontal lines show 95% CIs. The area of the black squares is proportional to the specific-study weight in the overall meta-analysis. The center of the black diamonds indicates the pooled standardized difference in means, and their width represents the pooled 95% CI.