Volume 81, Issue 5 p. 807-818
Systematic Review
Free Access

Statin therapy and plasma free fatty acids: a systematic review and meta-analysis of controlled clinical trials

Amirhossein Sahebkar

Amirhossein Sahebkar

Biotechnology Research Center, Mashhad University of Medical Sciences, Mashhad, 9177948564 Iran

Metabolic Research Centre, Royal Perth Hospital, School of Medicine and Pharmacology, University of Western Australia, Perth, Australia

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Luis E. Simental-Mendía

Corresponding Author

Luis E. Simental-Mendía

Biomedical Research Unit, Mexican Social Security Institute, Durango

Universidad Autónoma España de Durango, Durango, Dgo., México

Correspondence

Dr Gerald F. Watts, MD, DSc, School of Medicine and Pharmacology University of Western Australia, GPO Box X2213, Perth, WA6847, Australia.

E-mail: [email protected]Tel.: +61 8 9224 0252Fax: +61 8 9224 0246

Dr Luis E. Simental-Mendía, MD, PhD, Predio Canoas 100, Col. Los Angeles 34067, Durango, Dgo., Mexico.

E-mail: [email protected]Tel.: (+52) (618) 812-0997Fax: (+52) (618) 813-2014

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Claudio Pedone

Claudio Pedone

Area di Geriatria, Università Campus Biomedico di Roma, Via Alvaro del Portillo 21, 00128 Roma

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Gianna Ferretti

Gianna Ferretti

Dipartimento di Scienze Cliniche Specialistiche ed Odontostomatologiche (DISCO), Università Politecnica Delle Marche, Italy

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Petr Nachtigal

Petr Nachtigal

Faculty of Pharmacy in Hradec Kralove, Department of Biological and Medical Sciences, Charles University in Prague, Hradec Kralove, Czech Republic

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Simona Bo

Simona Bo

Department of Medical Sciences, University of Turin, Turin

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Giuseppe Derosa

Giuseppe Derosa

Center for the Study of Endocrine-Metabolic Pathophysiology and Clinical Research, University of Pavia, Pavia

Department of Internal Medicine and Therapeutics, University of Pavia and Fondazione IRCCS Policlinico S. Matteo, Pavia, Pavia, Italy

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Pamela Maffioli

Pamela Maffioli

Department of Internal Medicine and Therapeutics, University of Pavia and Fondazione IRCCS Policlinico S. Matteo, Pavia, Pavia, Italy

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Gerald F. Watts

Corresponding Author

Gerald F. Watts

Lipid Disorders Clinic, Cardiovascular Medicine, Royal Perth Hospital, School of Medicine and Pharmacology, University of Western Australia, Perth, Western Australia, Australia

Correspondence

Dr Gerald F. Watts, MD, DSc, School of Medicine and Pharmacology University of Western Australia, GPO Box X2213, Perth, WA6847, Australia.

E-mail: [email protected]Tel.: +61 8 9224 0252Fax: +61 8 9224 0246

Dr Luis E. Simental-Mendía, MD, PhD, Predio Canoas 100, Col. Los Angeles 34067, Durango, Dgo., Mexico.

E-mail: [email protected]Tel.: (+52) (618) 812-0997Fax: (+52) (618) 813-2014

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First published: 01 December 2015
Citations: 34

Abstract

AIM

The aim of this meta-analysis was to evaluate the effect of statin therapy on plasma FFA concentrations in a systematic review and meta-analysis of controlled clinical trials.

Methods

PubMed-Medline, SCOPUS, Web of Science and Google Scholar databases were searched (from inception to February 16 2015) to identify controlled trials evaluating the impact of statins on plasma FFA concentrations. A systematic assessment of bias in the included studies was performed using the Cochrane criteria. A random effects model and generic inverse variance method were used for quantitative data synthesis. Sensitivity analysis was conducted using the leave-one-out method. Random effects meta-regression was performed using unrestricted maximum likelihood method to evaluate the impact of potential moderators.

Results

Meta-analysis of data from 14 treatment arms indicated a significant reduction in plasma FFA concentrations following treatment with statins (weighted mean difference (WMD) –19.42%, 95% CI –23.19, −–15.64, P < 0.001). Subgroup analysis confirmed the significance of the effect with both atorvastatin (WMD –20.56%, 95% CI –24.51, –16.61, P < 0.01) and simvastatin (WMD –18.05%, 95% CI –28.12, –7.99, P < 0.001). Changes in plasma FFA concentrations were independent of treatment duration (slope –0.10, 95% CI –0.30, 0.11, P = 0.354) and magnitude of reduction in plasma low density lipoprotein cholesterol concentrations (slope 0.55, 95% CI –0.17, 1.27, P = 0.133) by statins.

Conclusions

The results of the present study suggest that statin therapy may lower plasma FFA concentrations. The cardiovascular and metabolic significance of this finding requires further investigation.

What is Already Known about this Subject

  • The main effect of statins is to decrease plasma low-density lipoprotein cholesterol (LDL-C) and triglycerides and increase high-density lipoprotein cholesterol levels.
  • Previous reports have suggested that free fatty acids (FFAs) may contribute to coronary heart disease by either promoting atherogenesis or thrombogenesis.
  • Studies evaluating the effects of statins on serum fatty acid metabolism in humans are lacking and published data contradictory.

What this Subject Adds

  • Results of this meta-analysis revealed a significant reduction in plasma FFAs following treatment with statins.
  • Changes in plasma FFA concentrations were independent of statin type, treatment duration, and magnitude of LDL-C reduction.

Introduction

Inhibitors of 3-hydroxy-3methyl-glutaryl coenzyme A (HMG-CoA), are known for their established efficacy in decreasing cardiovascular outcomes and mortality in both primary and secondary prevention 1-4. The main effect of statins is reducing plasma low density lipoprotein cholesterol (LDL-C) concentrations 5. In addition, statin therapy can reduce triglycerides and increase high density lipoprotein (HDL) cholesterol concentrations plus a plethora of non-lipid pleiotropic actions 6-12.

FFAs are non-esterified fatty acids that are released from adipocyte triglyceride stores following lipolysis, and from phospholipids after hydrolysis by phospholipases 13. FFAs promote the formation and release of triglycerides by the liver leading to an overproduction of very low density lipoprotein (VLDL) 14, 15 and consequent development of atherogenic dyslipidaemia. On the other hand, overproduction of VLDL and LDL can increase the flux of plasma FFAs to the liver causing hepatic insulin resistance and inflammation 16, 17.

Fatty acid metabolism was once considered to be unchanged by statin therapy 18-20. However, mild elevations in fatty acid synthesis were subsequently reported in animal models 21 cultured cells 22, and after statin treatment in mice 23. Moreover, statins may also inhibit hepatic synthesis of apolipoprotein B-100 and decrease the synthesis and secretion of triglyceride-rich lipoproteins 24, 25. The molecular mechanism by which statins reduce triglycerides levels is not known with certainty. In this regard, statins could affect hepatic FFA metabolism 26. However, studies evaluating the effects of statins on serum fatty acid metabolism in humans are lacking and published data are contradictory 27. Therefore, the aim of this study was to evaluate the effect of statin therapy on plasma FFA concentrations and calculate the size of this effect using a systematic review and meta-analysis of controlled clinical trials.

Methods

Search strategy

This study was designed according to the guidelines of the 2009 preferred reporting items for systematic reviews and meta-analysis (PRISMA) statement 28. PubMed-Medline, SCOPUS, Web of Science and Google Scholar databases were searched using the following search terms in titles and abstracts (also in combination with MESH terms): (atorvastatin OR simvastatin OR rosuvastatin OR fluvastatin OR pravastatin OR pitavastatin OR lovastatin OR cerivastatin OR ‘statin therapy’ OR statins) AND (‘free fatty acid’ OR ‘free fatty acids’ OR FFA OR FFAs). The wild-card term ‘*’ was used to increase the sensitivity of the search strategy. No language restriction was used in the literature search. The search was limited to studies in humans. The literature was searched from inception to February 16 2015.

Study selection

Original studies were included if they met the following inclusion criteria: (i) being a controlled trial with either parallel or crossover design, (ii) investigating the impact of statin therapy, either as monotherapy or combination therapy, on plasma/serum concentrations of FFAs, (iii) treatment duration of at least 2 weeks and (iv) presentation of sufficient information on FFA concentrations at baseline and at the end of follow-up in each group or providing the net change values. Exclusion criteria were (i) non-interventional trials, (ii) lack of an appropriate control group for statin therapy, (iii) observational studies with case–control, cross-sectional or cohort design and (iv) lack of sufficient information on baseline or follow-up FFA concentrations.

Data extraction

Eligible studies were reviewed and the following data were abstracted: 1) first author's name, 2) year of publication, 3) study location, 4) study design, 5) number of participants in the statin and control groups, 5) age, gender and body mass index (BMI) of study participants, 6) baseline concentrations of total cholesterol, low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C), and triglycerides, 7) systolic and diastolic blood pressures and 8) data regarding baseline and follow-up concentrations of FFAs.

Quality assessment

A systematic assessment of bias in the included studies was performed using the Cochrane criteria 29. The items used for the assessment of each study were as follows: adequacy of sequence generation, allocation concealment, blinding, addressing of dropouts (incomplete outcome data), selective outcome reporting and other potential sources of bias. According to the recommendations of the Cochrane Handbook, a judgment of ‘yes’ indicated low risk of bias, while ‘no’ indicated high risk of bias. Labelling an item as ‘unclear’ indicated an unclear or unknown risk of bias.

Quantitative data synthesis

Meta-analysis was conducted using Comprehensive Meta-Analysis (CMA) V2 software (Biostat, NJ) 30. Net changes in measurements (change scores) were calculated as follows: measure at end of follow-up − measure at baseline. For single arm crossover trials, net change in plasma concentrations of FFA were calculated by subtracting the value after control intervention from that reported after treatment. All values were collated as percent change from baseline in each group, or percent change in the statin group relative to control group. Standard deviations (SDs) of the mean difference were calculated using the following formula: SD = square root [(SDpre-treatment)2 + (SDpost-treatment)2 – (2R × SDpre-treatment × SDpost-treatment)], assuming a correlation coefficient (r) = 0.5. If the outcome measures were reported in median and range (or 95% confidence interval [CI]), mean and standard SD values were estimated using the method described by Hozo et al. 31. Where standard error of the mean (SEM) was only reported, standard deviation (SD) was estimated using the following formula: SD = SEM × square root (n), where n is the number of subjects. To avoid the problem of double-counting in randomized controlled trials with multiple treatment arms and a common control group, the number of subjects in the control group was divided by the required comparisons.

A random effects model (using the DerSimonian–Laird method) and the generic inverse variance method were used to compensate for the heterogeneity of studies in terms of demographic characteristics of populations being studied and also differences in study design and type of statin being studied 32. Heterogeneity was quantitatively assessed using I2 index. Effect sizes were expressed as weighed mean difference (WMD) and 95% confidence interval (CI). Subgroup analyses were carried out to explore the impact of statin type and treatment duration (< 12 weeks vs. ≥12 weeks) on plasma FFA concentrations. In order to evaluate the influence of each study on the overall effect size, sensitivity analysis was conducted using the leave-one-out method, i.e. removing one study each time and repeating the analysis [33-35].

Meta-regression

Random effects meta-regression was performed using the unrestricted maximum likelihood method to evaluate the association between calculated WMD and potential moderators including duration of treatment with statins, dose of treatment (expressed as equivalent dose of atorvastatin) and magnitude of LDL-C reduction by statin therapy.

Publication bias

Potential publication bias was explored using visual inspection of Begg's funnel plot asymmetry, Begg's rank correlation and Egger's weighted regression tests. Duval & Tweedie ‘trim and fill’ and ‘fail-safe N’ methods were used to adjust the analysis for the effects of publication bias [36].

Results

Flow and characteristics of included studies

Firstly, 189 published studies were identified following the database search. After reviewing the titles and abstracts, 170 studies did not meet the inclusion criteria and were excluded. Then, 19 full text articles were carefully assessed and reviewed. Ten of these studies were excluded for the following reasons: non-interventional design (n = 1), being non-original (n = 1), presenting incomplete data (n = 2), not measuring plasma/serum FFA concentrations (n = 4) and not being controlled for statin therapy (n = 2). Finally, nine eligible studies with 14 treatment arms were included in the systematic review and meta-analysis. The study selection process is shown in Figure 1.

Details are in the caption following the image
Flow chart of the number of studies identified and included into the meta-analysis

A total of 764 individuals were incorporated in the nine eligible controlled trials, including 462 and 302 subjects in the statin and control groups (participants from the crossover trials were counted in both treatment and control groups), respectively. Included studies were published between 1991 and 2014. The clinical trials used only atorvastatin and simvastatin but with different doses. Two studies used atorvastatin 10 mg day–1 37, 38, one study atorvastatin 20 mg day–1 39, one study atorvastatin 80 mg day–1 38, one study simvastatin 10 mg day–1 40, one study simvastatin 20 mg day–1 41, one study simvastatin 30 mg day–1 42, three studies simvastatin 40 mg day–1 41, 43, 44 and one study simvastatin 80 mg day–1 45. The range of intervention periods was from 3 weeks 42 to 2 years 41. Study designs of included studies were parallel 38-41, 43-45 and crossover 37, 42. Selected trials enrolled subjects with primary hypercholesterolaemia 41, 43, 44, type 2 diabetes 38, 42, 45, metabolic syndrome 37, 40 and mixed dyslipidaemia 39 (Table 1). Finally, most of the included studies measured FFA concentrations using an enzymatic assay method 38, 40, 41, 43-45, while three trials did not specify the method used 37, 39, 42.

Table 1. Demographic characteristics of the included studies
Author Study design Target population Treatment duration n Study groups Age (years)s Female (n, %) BMI, (kg m2) Systolic blood pressure (mmHg) Diastolic blood pressure (mmHg) Total cholesterol (mgdl–1) LDL cholesterol (mg dl–1) HDL cholesterol (mg dl–1) Triglycerides

(mg dl–1)

FFA

(mmol l–1)

Bays et al. 39* Randomized, double-blind, placebo-controlled

Parallel group

Overweight and mixed dyslipidaemia 8 weeks 31 28 32 31 30 29 Control MBX-8025, 50 mg day–1 MBX-8025, 100 mg day–1

Atorvastatin 20 mg day–1

MBX-8025, 50 mg day–1 + atorvastatin 20 mg day–1

MBX-8025, 100 mg day–1 + atorvastatin 20 mg day–1

54.9 ± 1.5

54.3 ± 2.2

54.6 ± 1.9

54.3 ± 1.9

54.6 ± 1.7

52.6 ± 1.7

15 (48.0)

11 (39.0)

17 (53.0)

18 (58.0)

18 (60.0)

16 (55.0)

33.4 ± .08

33.5 ± 1.3

31.8 ± 0.6

34.0 ± 1.2

33.8 ± 1.1

35.2 ± 1.1

ND ND ND ND ND ND ND ND ND ND ND ND 251.7 ± 5.8

254.6 ± 6.1

250.6 ± 6.4

245.4 ± 5.5

258.4 ± 6.6

239.8 ± 4.8

160.7 ± 4.3

169.3 ± 5.0

163.6 ± 6.3

165.1 ± 5.0

175.0 ± 5.3

157.6 ± 3.1

43.6 ± 1.3

44.6 ± 1.2

44.3 ± 1.7

42.7 ± 1.5

44.8 ± 1.5

42.6 ± 1.2

254.6 ± 22.5

214.8 ± 14.0

236.3 ± 16.3

199.8 ± 11.6

196.2 ± 13.5

211.6 ± 13.9

0.6 ± 0.0

0.6 ± 0.0

0.6 ± 0.1

0.6 ± 0.0

0.5 ± 0.0

0.6 ± 0.1

Huptas et al. 37 Randomized, double-blind, placebo-controlled

Crossover

Metabolic syndrome 6 weeks 10 Control Atorvastatin 10 mg day–1 40 ± 12 0 (0.0) 33.6 ± 5.2 ND ND 208 ± 46

164 ± 41

125 ± 43

85 ± 30

45.0 ± 11.4

47.6 ± 10.5

198 ± 122

192 ± 188

0.96 ± 0.49

0.52 ± 0.17

Krysiak et al. 43 Placebo-controlled

Parallel

Primary hypercholesterolaemia 12 weeks 21 23 21 Control Simvastatin 40 mg day–1

Simvastatin 40 mg day–1+ ezetimibe 10 mg day–1

51.1 ± 2.6

51.9 ± 2.7

52.5 ± 3.5

9 (43)

9 (39)

9 (43)

26.8 ± 2.7

26.7 ± 2.3

26.5 ± 2.4

ND ND ND ND ND ND 250 ± 15

192 ± 13

156 ± 12

175 ± 13

128 ± 11

98 ± 9

47 ± 4

49 ± 5

54 ± 4

112 ± 13

107 ± 14

105 ± 18

0.34 ± 0.04

0.24 ± 0.02

0.20 ± 0.02

Krysiak et al. 44 Placebo-controlled

Parallel

Primary hypercholesterolemia 30 days 17 22 Control Simvastatin 40 mg day–1 50.7 ± 2.6

51.8 ± 2.6

8 (47)

9 (41)

27.2 ± 2.6

26.3 ± 2.5

ND ND ND ND 246 ± 14

191 ± 13

175 ± 14

128 ± 11

48 ± 5

49 ± 5

119 ± 12

113 ± 15

0.32 ± 0.05

0.30 ± 0.03

Mitropoulos et al. 41 Randomized, placebo-controlled Primary hypercholesterolaemia 2 years 54 57 51 Simvastatin 40 mg/day Simvastatin 20 mg day–1

Control

62.6 ± 6.7

62.5 ± 7.6

64.0 ± 6.2

6 (11.1)

8 (14.0)

10 (19.6)

26.0 ± 2.7

25.8 ± 3.5

26.7 ± 4.2

ND ND ND ND ND ND 194.9 ± 34.8

233.6 ± 64.2

273.0 ± 45.2

95.5 ± 25.1 ND

162.8 ± 35.2

62.6 ± 15.1 ND

58.4 ± 14.7

146.1 ± 71.7

131.1 ± 64.7

185.1 ± 94.8

0.32 ± 0.21

0.34 ± 0.21

0.41 ± 0.23

Paolisso et al. 42 Randomized, double-blind, placebo-controlled cross-over Type 2 diabetes and hypercholesterolaemia 3 weeks 12 Control

Simvastatin 30 mg day–1

72.3 ± 1.4 6 (50.0) 27.2 ± 0.6 ND ND 305.5 ± 11.6

204.9 ± 7.7

278.4 ± 15.5

166.3 ± 11.6

23.2 ± 7.7

34.8 ± 11.6

256.9 ± 35.4

186.0 ± 17.7

1.10 ± 0.21

0.81 ± 0.10

Plat et al. 40* Randomized, double-blind, placebo-controlled

parallel

Metabolic syndrome 8 weeks 9 10 9 8 Control Simvastatin 10 mg day–1Plant stanols Simvastatin 10 mg day–1 + plant stanols 60 ± 7

61 ± 8

60 ± 4

60 ± 8

4 (44.4)

4 (40.0)

2 (22.2)

3 (37.5)

30.2 ± 1.9

29.2 ± 3.3

28.1 ± 2.6

29.7 ± 7.5

142 ± 14

139 ± 7

138 ± 11

150 ± 30

92 ± 10

89 ± 7

94 ± 8

96 ± 21

250.2 ± 57.6

247.5 ± 43.7

288.1 ± 49.1

280.4 ± 47.6

ND ND ND ND 45.6 ± 7.7

43.3 ± 8.9

44.9 ± 7.3

45.2 ± 16.2

174.5 ± 80.6

203.7 ± 75.3

128.4 ± 44.3

177.1 ± 81.5

0.30 ± 0.10

0.39 ± 0.14

0.29 ± 0.10

0.29 ± 0.08

DALI Study Group 38 Randomized, double-blind, placebo-controlled parallel Type 2 diabetes and dyslipidaemia 30 weeks 72 73 72 Control Atorvastatin 10 mg day–1

Atorvastatin 80 mg day–1

58.5 ± 7.5

59.7 ± 7.6

60.1 ± 7.7

26 (36.1)

13 (17.8)

32.2 ± 6.0

30.0 ± 3.8

30.4 ± 4.5

144 ± 19

146 ± 17

145 ± 17

85 ± 9

86 ± 10

85 ± 9

232.0 ± 3.9

158.5 ± 3.9

139.2 ± 3.9

139.2 ± 3.9

85.1 ± 3.9

65.7 ± 3.9

40.2 ± 1.2

42.5 ± 1.5

42.2 ± 1.5

255.1 ± 19.5

163.0 ± 8.9

157.7 ± 14.2

0.72 ± 0.04

0.57 ± 0.03

0.61 ± 0.03

Szendroedi et al. 45 Randomized, double-blind, placebo-controlled parallel Type 2 diabetes and hypercholesterolaemia 2 months 10 10 Simvastatin 80 mg day–1

Control

55 ± 6

58 ± 8

3 (30.0)

5 (50.0)

28.9 ± 3.5

27.3 ± 3.7

ND

ND

ND

ND

197.2 ± 38.7

255.2 ± 30.9

108.3 ± 34.8

162.4 ± 19.3

54.1 ± 11.6

54.1 ± 11.6

132.9 ± 35.4

186.0 ± 70.9

0.39 ± 0.13

0.60 ± 0.23

  • a Values are expressed as mean ± SD. *Only basal conditions. ND, no data; BMI, body mass index; FFA, free fatty acids; DALI, Diabetes Atorvastatin Lipid Intervention.

Risk of bias assessment

Seven included studies were characterized by lack of information about the sequence generation, allocation concealment and blinding of participants, personnel and outcome assessors 37, 38, 41-45. In addition, two studies had high risk of bias for sequence generation and allocation concealment 43, 44. However, six evaluated studies showed low risk of bias with respect to selective outcome reporting and other sources of bias 38-41, 43, 45. Finally, all trials had low risk of bias for incomplete outcome data. Details of the quality of bias assessment are shown in Table 2.

Table 2. Quality of bias assessment of the included studies according to the Cochrane guidelines
Study Sequence

generation

Allocation

concealment

Blinding of participants, personnel and outcome assessors Incomplete

outcome data

Selective outcome reporting Other sources of bias
Bays et al. 39 L L U L L L
Huptas et al. 37 U U U L U L
Krysiak et al. 43 H H L L L L
Krysiak et al. 44 H H L L L U
Mitropoulos et al. 41 U U L L L L
Paolisso et al. 42 U U U L U L
Plat et al. 40 U L L L L L
DALI Study Group 38 U U U L L L
Szendroedi et al. 45 U U U L L L
  • L, low risk of bias; H, high risk of bias; U, unclear risk of bias.

Effect of statin therapy on plasma FFA concentrations

Meta-analysis of data from 14 treatment arms revealed a significant reduction in plasma FFAs following treatment with statins (WMD –19.42%, 95% CI –23.19, –15.64, P < 0.001; I2 65.27%). This effect was robust in the sensitivity analysis (Figure 2). In subgroup analysis, reductions in plasma FFA concentrations were observed in both subsets of trials with treatment durations <12 weeks (WMD –7.38%, 95% CI –13.36, –1.40, P = 0.016; I2 0%) and ≥12 weeks (WMD –23.51%, 95% CI –25.88, –21.14, P < 0.001; I2 52.41%), though the effect size was numerically greater in the latter group (Figure 3). With respect to the type of statin, there were significant reductions in plasma FFA concentrations with both atorvastatin (WMD –20.56%, 95% CI –24.51, –16.61, P < 0.01; I2 71.91%) and simvastatin (WMD –18.05%, 95% CI –28.12, –7.99, P < 0.001; I2 61.81%) (Figure 4).

Details are in the caption following the image
Forest plot displaying weighted mean difference and 95% confidence intervals for the impact of statin therapy on plasma FFA concentrations (I2 65.27%). Lower plot shows leave-one-out sensitivity analysis
Details are in the caption following the image
Forest plot displaying weighted mean difference and 95% confidence intervals for the impact of statin therapy on plasma FFA concentrations in trials with treatment durations of <12 weeks (upper plot) (I2 0%) and ≥12 weeks (lower plot) (I2 52.41%)
Details are in the caption following the image
Forest plot displaying weighted mean difference and 95% confidence intervals for the impact of atorvastatin (upper plot) (I2 71.91%) and simvastatin (lower plot) (I2 61.81%) on plasma FFA concentrations

Meta-regression

Random effects meta-regression was performed to evaluate the impact of potential moderators on the estimated effect size. Changes in plasma FFA concentrations were independent of treatment duration (slope –0.10, 95% CI –0.30, 0.11, P = 0.354), statin dose (slope –0.08, 95% CI –0.33, 0.16, P = 0.514) and magnitude of LDL-C reduction (slope 0.55, 95% CI –0.17, 1.27, P = 0.133) by statins (Figure 5).

Details are in the caption following the image
Meta-regression plots of the association between mean changes in plasma FFA concentrations with duration of statin therapy (A), statin dose (expressed as equivalent dose of atorvastatin; B) and magnitude of LDL-C reduction (C)

Publication bias

The funnel plot of standard error vs. effect size (mean difference) was asymmetric and suggested potential publication bias. Presence of publication bias was also suggested by Egger's linear regression (intercept =1.05, standard error = 0.48; 95% CI = 0.004, 2.10, t = 2.19, df = 12.00, two-tailed P = 0.049) but not Begg's rank correlation test (Kendall's Tau with continuity correction =0, z = 0, two-tailed P value =1.000). After adjustment of effect size for potential publication bias using the ‘trim and fill’ correction, five potentially missing studies on the left side of the funnel plot were imputed leading to a corrected effect size that was greater than the initial estimate (WMD –21.54%, 95% CI –25.44, –17.64) (Figure 6). The ‘fail-safe N’ test showed that 1417 studies would be needed to bring the WMD down to a non-significant (P > 0.05) value.

Details are in the caption following the image
Funnel plot displaying publication bias in the studies reporting the impact of statin therapy on plasma FFA concentrations. Open diamond represents observed effect size; closed diamond represents imputed effect size

Discussion

The findings of the present meta-analysis suggest that statins are associated with a significant reduction in the plasma concentrations of FFAs. This effect that was independent of statin type, treatment duration and dose and magnitude of changes in plasma LDL-C concentrations. It has been reported that high concentrations of FFAs induce oxidative stress, inflammation and insulin resistance by activating the nuclear factor-kappa B pathway and promoting increased reactive oxidant species 17. Therefore, the decrease in FFA concentrations after statin therapy may have important clinical implications slowing the progression of atherogenesis and consequently a lower cardiovascular risk. However, these potential effects of statins as antioxidant and anti-inflammatory through the reduction of FFA concentrations are still uncertain and remain to be clarified in further studies. Although the effect of statins on FFA metabolism is unclear, there are several mechanisms which may be involved. In this regard, acetyl coenzyme A carboxylase and fatty acid synthase, two important regulatory enzymes in fatty acid biosynthesis, could be regulated simultaneously by HMG-CoA reductase enzyme at the genomic level. Therefore, activity of both enzymes could be influenced by statin therapy 46, 47. Since HMG-CoA reductase and acetyl coenzyme A carboxylase are reversibly inactivated through phosphorylation by AMP-activated protein kinase 48, down regulation of this enzyme by sterol deficiency would increase acetyl coenzyme A carboxylase activity after statin therapy when regulatory sterols are absent 49. In addition, a pleiotropic effect of statins on peroxisome proliferator-activated receptor-α (PPARα) expression has been described. Statins activate PPARα, which increases the hepatic fatty acid uptake and promotes the transformation of fatty acids to acyl-coenzyme A, resulting in increased β-oxidation and reduced availability of fatty acids 50. However, available data on the effects of statins on plasma FFA concentrations in humans is still limited and is based on small scale studies. The aim of the present study was to obtain more robust evidence on the impact of statins on plasma FFA concentrations through combining individual studies and applying a random effects meta-analysis.

Circulating FFAs mainly derive from adipose tissue lipolysis by the action of lipase. Hepatic FFAs are available from de novo lipogenesis and uptake of triglyceride-rich lipoproteins, cholesteryl esters, and plasma FFAs. Fatty acid derivatives can serve as ligands for the peroxisome proliferator-activated receptors (PPARs) 51, particularly PPAR-α which is considered as the most important regulator of intra and extracellular fatty acid metabolism. In fact, PPAR-α activation decreases VLDL synthesis and secretion, increases plasma HDL, decreases triglyceride concentrations and enhances fatty acid oxidation 52. Interestingly, lipid lowering drugs have shown a reduction of serum total fatty acid concentration while simultaneously increasing the proportion of long chain polyunsaturated fatty acids and precursor fatty acids for eicosanoid production 53. Circulating total fatty acids are found in different forms: 45% in triacylglycerols, 15% in cholesteryl esters, 35% in phospholipids and about 5% as non-esterified free fatty acids 53. Between 75% and 80% of serum cholesterol is esterified with fatty acids and only 20% to 25% is non-esterified cholesterol 53. Nonetheless, statins appear to exert their effects on the metabolism and serum composition of FFAs through disturbing the biogenesis of isolated fatty acids independently of the mechanism that regulates lipoprotein synthesis and secretion. In this context, the results of meta-regression analysis revealed that changes in plasma FFA concentrations were independent of the magnitude of LDL-C reduction. This lack of dependence of FFA changes on the potency and duration of statin treatment is suggestive of a class effect that is independent of degree of inhibition of HMG-CoA reductase. As referred to above, it is likely that enhancement of PPAR-α expression and activity in liver and muscle tissues is responsible for the FFA-reducing effect through increasing FFA uptake, FFA conversion to acyl-CoA and β-oxidation of FFAs 54.

There is evidence indicating that treatment with statins is a risk factor for the development of new onset type 2 diabetes 55, though the causes of this negative effect remain unexplained. In this regard, it has been suggested that statins may contribute to insulin resistance and impair β-cell function 56. In animal models, long term treatment with statins increased insulin resistance in the adipose tissue 57. There is evidence indicating that both increased efflux of FFAs from adipose tissue and impaired insulin-mediated skeletal muscle uptake of FFAs increase fatty acid flux to the liver promoting the development of peripheral insulin resistance 58, 59. The association between plasma FFA concentrations and insulin resistance has been supported by epidemiological studies 60. There is robust evidence from published meta-analyses indicating that although statin therapy increases the risk of new-onset diabetes, the risk is not equivalent for all statins. It has been reported that the diabetogenic effect of statins depends on the type (being highest with rosuvastatin) and dose of statins (a higher risk at higher doses) 61, 62. In this context, it is important to note that none of the studies included in the present meta-analysis included a rosuvastatin arm, and except one trial, all other included studies used a mild to moderate intensity statin therapy regimen. Hence, the overall set of trials included in this meta-analysis may not fully represent the impact of a highly diabetogenic statin regimen on plasma concentrations of FFAs, and future studies are required to see if a similar or differential effect on plasma FFAs could be seen in trials with rosuvastatin and high intensity statin regimens. Moreover, in spite of some controversies, it has been shown that in diabetic subjects statin therapy is not associated with an adverse impact on glycaemic index and insulin resistance 63. Therefore, since diabetes was either the main inclusion criteria or comorbidity in most of the studies included in the present meta-analysis, it is unlikely that the diabetogenic effect of statins has occurred predominantly in the overall population studied in our analysis. Hence, further evidence is required to clarify if the FFA-reducing effect of statins can be replicated in non-diabetic subjects receiving high intensity statins, particularly rosuvastatin.

Some limitations of this study should be mentioned. The most important one was the small sample size in several of the included studies. As another limitation, changes in plasma FFA concentrations were not among the primary objectives of the included studies. Thus, further studies are warranted to evaluate the effect of statin therapy on FFA concentrations as primary outcome to obtain more robust evidence about the effects of statins on circulating FFA status. Finally, the diversity of statin types in the included studies was low and administered statins were limited to atorvastatin and simvastatin. While this may reduce the inter-study heterogeneity, it limits the generalizability of findings to other statin types.

Conclusion

In conclusion, results of this meta-analysis, being the first of its kind, showed that statin therapy significantly reduces plasma FFA concentrations. Future investigations are required to clarify if this effect of statin therapy accounts, at least in part, for the established cardiovascular benefits of these drugs. Also, the association of this effect of statins with the hepatic content of FFAs and risk of hepatic insulin resistance needs to be elucidated in future studies. Finally, the magnitude of the FFA lowering effects of statins in comparison with other conventional and novel lipid lowering therapies 64-70 remains to be clarified.

Competing Interests

All authors have completed the Unified Competing Interest form and declare no support from any organization for the submitted work. GFW has received honoraria for lectures and commentaries, outside the submitted work, from Genfit, Pfizer, Astrazeneca, MSD, Novartis, Amgen and Sanofi-Aventis in the previous 3 years. There are no other relationships or activities that could appear to have influenced the submitted work.

Author Contributions

AM conceptualized and designed the study, carried out the statistical analyses and interpretation of data, drafted the initial manuscript, and approved the final manuscript as submitted. LES-M drafted the initial manuscript, critically reviewed the manuscript and approved the final manuscript as submitted. CP critically reviewed the manuscript and approved the final manuscript as submitted. GF critically reviewed the manuscript and approved the final manuscript as submitted. PN critically reviewed the manuscript and approved the final manuscript as submitted. SB critically reviewed the manuscript and approved the final manuscript as submitted. GD critically reviewed the manuscript and approved the final manuscript as submitted. PM critically reviewed the manuscript and approved the final manuscript as submitted. GFW critically reviewed the manuscript, interpreted data and approved the final manuscript as submitted.