The efficacy of fish oil in preventing coronary heart disease: A systematic review and meta-analysis

To evaluate the efficacy of fish oil for protection against coronary heart disease (CHD), we conducted a systematic review and meta-analysis of randomized controlled trials (RCTs) evaluating the use of fish oil for protection against CHD. We retrieved relevant articles published from January 1966 to January 2020 by searching the PubMed, EMBASE, Cochrane CENTRAL, and Web of Science databases. RCTs of fish oil in preventing CHD were selected. The study quality was evaluated using the Cochrane Risk of Bias tool with RevMan 5.3 software. The first selection involved 350 citations. After screening and evaluation of suitability, 19 RCTs adjusted for clustering were included in the meta-analysis. All selected manuscripts considered that fish oil was effective in preventing CHD, secondary outcome measures included angina, sepsis and death. Compared with the control group, fish oil may confer significant protection against CHD (odds ratio = 0.84; 95% confidence interval: 0.72–0.98). There was no significant difference in the incidence of secondary outcomes between the observation group and the control group (P > 0.05). The above results show that fish oil plays an important role in reducing CHD and cardiovascular events. However, because of the suboptimal quality of the studies included into the meta-analysis, these results do not justify adding fish oils systematically to the heavy pharmaceutical assortment already recommended in CHD patients.


Introduction
Coronary heart disease (CHD) is one of the leading causes of death and disease burden in the world. While age-adjusted mortality rates appear to be falling globally, they remain high in low-and middle-income regions and countries, where there is a particularly high prevalence of risk factors [1]. In developing countries such as China, the rising momentum of CHD burden has not been effectively curbed [2]. Control of risk factors such as hypertension, diabetes, hyperlipidemia, obesity and smoking can reduce incidence and mortality rates of CHD [3]. The use of antiplatelet drugs, statins, renin angiote ns in aldosterone system blockers and β receptor blockers can also improve the prognosis of patients with CHD [4]. Although efforts such as these have yielded some improvement in ameliorating CHD burden, a number of potential therapies that could be of further benefit remain to be fully explored. Recent years have seen growing interest in the notion that fish oil and ω-3 polyunsaturated fatty acids (PUFA) may be beneficial in the prevention and treatment of CHD, with A considerable number of studies have been carried out on the roles of fish oil and ω-3 PUFA in the clinical application of primary prevention and secondary prevention of CHD, including large-scale randomized controlled trials, multi center and single center cohort studies and observational studies. In this study, we conducted a comprehensive meta-analysis of the efficacy of fish oil for the prevention of CHD in order to establish its scientific basis, for the purposes of informing policies related to the use of fish oil.

Application protocol and website recording data
A protocol including the investigation methods and the inclusion criteria for the current study was submitted in advance and documented on the Center for Review and Dissemination (CRD) York website PROSPERO, an international prospective register of systematic reviews. The parameters and the analytic structure of the present work can be viewed using the CRD identification code: CRD42020183719.
This systematic investigation was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) protocols [5].

Search strategy
Articles published in English from January 1994 to January 2020 that explored the relationship between fish oil and protection against CHD were retrieved from PubMed, EMBASE, Cochrane CENTRAL, and Web of Science databases. The following search terms were used: "CHD", "coronary heart disease", "cardiovascular diseases", "preventio n" and "fish oil". Logical operators (OR, NOT, AND) were used to combine keywords and subject words (Table 1).

Inclusion criteria
Articles that met the following criteria were selected: study design was randomized controlled trial (RCT), and the study was peer reviewed; the study population was human; the exposure of interest was using fish oil; the outcome of interest was the proportion of fish oil use in the experimental and control groups; the main outcome measure was CHD.
Secondary outcome measures included angina, sepsis and death. Studies took place in healthcare settings worldwide.

Exclusion criteria
The following exclusion criteria were applied: Trials in which patients were being treated with blood pressure disease, virus infected patients, osteoporosis, immunolo gic disorders, uncontrolled diabetes mellitus, or other surgical risk related systemic conditio ns; not enough information regarding the selected topic; trials that were not RCTs; no access to the title and abstract number in the English language.

Data extraction
Data extraction was conducted in two stages. First, literature was screened by two researchers according to inclusion criteria. The screened literature was then searched and evaluated by two other researchers according to inclusion and exclusion criteria. To avoid errors, a pre-designed form was used to select the study characteristics, baseline patient characteristics and outcomes and definitions included in the literature. Any inconsistenc ies in recommendations were resolved through consultation. The main data extracted were as follows: the number of people who were assigned to using fish oil and those who were not assigned to using fish oil.

Literature quality assessment
The quality of the methodology in the included studies was evaluated by using the Cochrane Risk of Bias tool [6]. The quality of RCTs was evaluated using RevMan 5.3 software. The risk of bias was evaluated from six perspectives: choice bias, performance bias, measurement bias, attrition bias, reporting bias, other biases ( Table 2). According to the criteria for low, unclear and high risk, the quality of the methodology of the included studies was divided into three levels: mild bias, where four or more of the above six items are low risk; moderate bias, where two or three of the above six items are low risk; and severe bias, where none or only one of the above six items is low risk.

RevMan 5.3 software provided by the Cochrane Collaboration was used to conduct
this meta-analysis of the proportions of fish oil use between the experimental and control groups. Q and I 2 tests were used to evaluate the heterogeneity of the included studies (Q tests is the traditional method in the heterogeneity test of meta-analysis; I 2 tests can measure the degree of difference among multiple research effects, and can describe the percentage of inter-research variation as a proportion of the total variation). When I 2 ≤ 50% and P > 0.1, a fixed effect model was used to merge the data; when I 2 > 50% or P < 0.1, a random effect model was used to merge the data. The odds ratio (OR) and 95% confidence interval (CI) were used to express the enumeration data. P < 0.05 was considered to indicate statistical significance. (Figure 1)

Literature search results
After searching 350 papers from four databases, 19 articles were included in the final screening ( Figure 1). Of the 350 papers identified through database queries, we screened out 120, then searched the full texts of the remaining 230 articles, excluding 209 that did not meet our inclusion criteria, leaving 19 RCTs ( Table 3). All of these RCTs analyzed the effectiveness of fish oil for protection against CHD. Moreover, they all analyzed the effect of fish oil on death, sepsis and angina. There was no real evidence to suggest publicatio n bias (Figure 2A, 2B, 2C, 2D)

Randomized controlled trials
Assessment of the risk bias of 19 RCTs using RevMan 5.3 software showed moderate overall bias ( Figure 3A, 3B).

Fish oil use versus no fish oil use for protection against CHD
19 RCTs compared CHD risk in people using fish oil to that of controls using no fish oil. Using no fish oil conferred significantly greater protection against CHD (OR = 0.84; 95% CI: 0.72-0.98; P < 0.05) ( Figure 4A). Because of heterogeneity, the data were divided for subgroup analysis according to the following: single-blind and double-blind; and Europe, North America and Asia. Subgroup analysis showed that heterogeneity of singleblind data was I 2 = 67% (P = 0.0005) and the heterogeneity for double-blind was I 2 = 6% (P = 0.38). This indicated that the heterogeneity of the double-blind data was much less than that of the single-blind data. Subgroup analysis by region showed that data heterogeneity for Europe was I 2 = 32% (P = 0.14), for North America I 2 = 79% (P = 0.0007), and for Asia I 2 = 26% (P = 0.25), indicating that the North American data was far more heterogeneous than those of Europe and Asia.. Therefore, the possibility that the heterogeneity of the data in the included studies was related to the type of blinding and continent could not be excluded ( Figure 4B, 4C).

The effect of fish oil on angina, sepsis and death
Secondary outcome measures included angina, sepsis and death. Meta-analys is showed that there was no significant difference in the incidence of secondary outcomes between the observation group and the control group (P > 0.05) ( Figure 5A, 5B, 5C).

Discussion
The clinical application of fish oil has attracted much research attention in recent years.
In the present meta-analysis we sought to evaluate the evidence base on the efficacy of the use of fish oil in CHD. Fish oil is rich in ω-3 PUFA, the effects of which on cardiovascular metabolism are still under extensive study. Preliminary studies have shown that ω-3 PUFA can reduce blood pressure, improve arterial elasticity, improve endothelial functio n, increase arrhythmia threshold, reduce platelet aggregation and improve autonomic nervous tension [26].
The results of the present meta-analysis of 19 RCTs show that the use of fish oil has a significant protective effect against CHD incidence when compared to no fish oil intervention [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] that, compared with no fish oil consumption, eating fish oil can reduce mortality rate of patients with CHD [31]. A subsequent retrospective analysis of physician-initiated health studies in 2002 found that basal plasma long chain ω-3 PUFA was significantly negative ly correlated with sudden death [32]. This study was divided into four groups according to plasma ω-3 PUFA levels. Compared with the lowest level group (group 1), the relative risk of sudden death in group 3 was 0.28, and that in group 4 was 0.19 (81% less). These findings are somewhat consistent with those of the present meta-analysis. However, we remain cautious in our interpretation due to the suboptimal quality of the studies included.
The results we have obtained are too weak to justify adding fish oils systematically to the already heavy burden of pharmaceuticals prescribed to CHD patients. Moreover, no significant differences in death, sepsis and angina were found to be associated with fish oil use. There were limitations to this meta-analysis. First, the number of included studies was small, which may have resulted in distribution bias. Analysis of a greater number of studies would be required to reduce the risk of distribution bias. Second, there may have been measurement bias, publication bias and selection bias in the included articles. Third, heterogeneity among the data in the included studies was identified, which may be related to the research population, region, and CHD subtypes. Although the subgroup analysis of the use of fish oil was conducted for some indicators in this study, it was not conducted for different populations or CHD subtypes. Therefore, more detailed subgroup analysis would be required to provide a more convincing basis for our conclusions. Finally, the source of CHD was not identified in all trials and some subjects may have been sicker than others before trial commencement.

Conclusions
Here, we conducted a literature review and meta-analysis of RCTs of the protective effects of fish oil against CHD. Our analysis provides some evidence to support the universal use of fish oil in the high risk CHD patient group. However, the evidence is not sufficiently strong to support the addition of fish oils to the already heavy pharmaceutica l assortment given to CHD patients.   The method of blinding used to prevent the evaluators of the research results from knowing the intervention measures is described in detail. This provides information that can be used to judge whether the blinding method is effective.

Attrition bias
Integrity of result data The data for each major outcome indicator, including those of subjects who were lost or withdrew from the study, are reported completely. Including subjects who were lost or withdrew, the total number of people in each group (compared with the total number of randomly enrolled people), and the reasons for the loss of interview/withdrawal are clearly reported, so as to facilitate assessment of the relevant treatment by the system evaluator. research results relevant information.

Other biases
Other sources of bias In addition to the above biases, the information provided can be used to assess the existence of other bias factors. If a question or factor is mentioned in the plan, corresponding answers are required.