Introduction

Cerebrovascular diseases primarily include ischemic stroke (i.e., thrombotic infarction or lacunar infarction), hemorrhagic stroke (i.e., subarachnoid or intraparenchymal hemorrhage induced by hypertension or arteriosclerosis) and transient ischemic attack (TIA; i.e., transient ischemia induced by arteriosclerosis or microthrombi). In 2010, stroke-related deaths totaled 5.9 million1 with stroke survivors experiencing higher frequencies of disability2. Therefore, developing a better understanding of the factors that drive stroke risk can improve these adverse outcomes.

Long-chain n-3 PUFAs, including eicosapentaenoic acid (EPA, 20:5 ω −3), docosapentaenoic acid (DPA, 22:5 ω −3) and docosahexaenoic acid (DHA, 22:6 ω −3), which are almost exclusively derived from marine sources3, have been shown to enhance atherosclerotic plaque stability while lowering blood pressure and blood triglyceride levels4,5. Although one recent meta-analysis6 revealed that long chain n-3 PUFA intake shows no association with coronary disease risk, one previous meta-analysis7 found no significant association between long-chain n-3 PUFA intake and total stroke risk but did detect a significant inverse association with ischemic stroke. Another meta-analysis8 demonstrated an inverse association between long chain n-3 PUFA intake and risk of cerebrovascular accident.

Giving this conflicting evidence and four recent prospective studies9,10,11,12, the role of long chain n-3 PUFA intake in stroke risk remains uncertain. Moreover, the influence of body mass index (BMI) upon the effects of long chain n-3 PUFA intake on stroke risk remains unknown. Thus, here we performed a meta-analysis of prospective studies to clarify the association between long chain n-3 PUFA intake and the risk of stroke morbidity and mortality.

Results

Literature search results

The detailed flowchart of study selection is shown in Fig. 1. A total of 1532 records were initially identified; of these, 1370 records were excluded by title/abstract screening. Of the 162 potentially relevant records, 104 records were excluded because they were not associated with stroke risk and 44 additional records were further excluded for the following reasons: 16 were experimental studies, 15 were non-prospective studies, 11 were reviews and two were conference abstracts. Thus, 14 prospective studies9,10,11,12,13,14,15,16,17,18,19,20,21,22 were finally included in this meta-analysis.

Figure 1
figure 1

Flow chart of study selection.

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Study characteristics

A summary of the characteristics of the included studies is shown in Table 1. The fourteen prospective studies9,10,11,12,13,14,15,16,17,18,19,20,21,22 were published from 1995–2014 and included 514,483 individuals ranging from 34 to 84 years of age and a total of 9,065 stroke events. The total number of subjects included in each study ranged from 2,710 to 134,296 subjects and the total number of stroke events included in each study ranged from 69 to 1,680. The average follow-up duration ranged from 4 to 28 years. Eleven studies9,10,12,13,14,16,18,19,20,21,22 used food frequency questionnaire (FFQ), two studies15,17 used 24-hour recall and one study11 used both FFQ and 2-hour recall for long chain n-3 PUFA intake assessment. Study quality was assessed using the Newcastle-Ottawa Scale (NOS)23; seven studies9,10,12,15,16,19,22 were classified as high-quality (>8 stars), while the remaining seven studies11,13,14,17,18,20,21 were deemed low-quality (≤8 stars).

Table 1 Baseline Characteristics of Included Studies.
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Primary outcome

Higher long chain n-3 PUFA intake was associated with reduced overall stroke risk [relative risk (RR) = 0.87; 95% confidence interval (CI), 0.79–0.95] (Fig. 2).

Figure 2
figure 2

Forest plot of relative risk for long chain n-3 PUFA intake and overall stroke risk.

RR, relative risk.

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Secondary outcome

The stratification analysis showed that higher long chain n-3 PUFA intake was associated with reduced fatal stroke risk (RR = 0.84; 95% CI, 0.73–0.97) (Fig. 3), reduced stroke risk for BMI < 24 (RR = 0.86; 95% CI, 0.75–0.98) (Fig. 4), reduced stroke risk for follow-up duration ≤14 years (RR = 0.87; 95% CI, 0.78–0.98) and >14 years (RR = 0.87; 95% CI, 0.76–0.99), reduced stroke risk for non-East Asians (RR = 0.86; 95% CI, 0.75–0.97) and East Asians (RR = 0.86; 95% CI, 0.75–0.98), reduced stroke risk for females (RR = 0.81; 95% CI, 0.71–0.92) (Fig. 5), reduced stroke risk for ischemic stroke (RR = 0.87; 95% CI, 0.76–0.99) and hemorrhagic stroke (RR = 0.82; 95% CI, 0.68–0.99), reduced stroke risk for maximum multivariates (RR = 0.88; 95% CI, 0.82–0.96) and reduced stroke risk for quality score ≤8 (RR = 0.79; 95% CI, 0.66–0.94) and >8 (RR = 0.9; 95% CI, 0.82–0.98). However, higher long chain n-3 PUFA intake was not significantly associated with stroke risk for BMI ≥ 24 (RR = 0.83; 95% CI, 0.68–1.02) and men (RR = 0.96; 95% CI, 0.84–1.11) (Table 2).

Table 2 Long Chain N-3 Polyunsaturated Fatty Acids Intake and Stroke Risk.
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Figure 3
figure 3

Forest plot of relative risk for long chain n-3 PUFA intake and fatal stroke risk.

RR, relative risk. Both analysis includes mixed fatal and non-fatal stroke risk.

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Figure 4
figure 4

Forest plot of relative risk for BMI effects in long chain n-3 PUFA intake and stroke risk.

NA, not available; RR, relative risk; BMI, body mass index.

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Figure 5
figure 5

Forest plot of relative risk for long chain n-3 PUFA intake and stroke risk for sex subgroups.

RR, relative risk.

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Meta-regression analysis

Although we used meta-regression analysis to explore the potential sources of heterogeneity, we did not determine the sources of heterogeneity.

Sensitivity analysis

Sensitivity analysis demonstrated that the relationship between higher long chain n-3 PUFA intake and reduction of stroke risk remained persistent after applying the leave-one-out method (Fig. 6).

Figure 6
figure 6

Sensitivity analysis of relative risk for long chain n-3 PUFA intake and stroke risk.

The results remained persistent after applying the leave-one-out method.

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Publication bias

The funnel plot was symmetrical by visual inspection (Fig. 7) and no significant publication bias was statistically detected by Egger’s test (p = 0.28).

Figure 7
figure 7

Funnel plot showing association of long chain n-3 PUFA intake with stroke risk.

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Discussion

This meta-analysis consisting of 14 prospective studies of 514,483 individuals and 9,065 stroke events reveals that higher long chain n-3 PUFA intake is associated with a reduced overall stroke risk, which confirms the findings from a previous meta-analysis8. In contrast, four recent prospective studies found inconsistent findings regarding the relationship between long chain n-3 PUFA intake and stroke risk. For instance, one study9 found no association between long chain n-3 PUFA intake and stroke risk in either men or women and another study10 found no association between long chain n-3 PUFA intake and stroke mortality. In contrast, another study11 demonstrated an inverse association between long chain n-3 PUFA intake and ischemic (but not hemorrhagic) stroke risk, while the most recent prospective study12 reported a significant reduction in stroke mortality for only the highest quartile of long chain n-3 PUFA intake.

The mechanism(s) by which higher long chain n-3 PUFA consumption contributes to decreased stroke risk remain unknown. Some lines of evidence4,5 show that dietary long chain n-3 PUFA enhances the stability of atherosclerotic plaques, lowers blood pressure, decreased blood triglyceride concentrations, decreases inflammation and improves vascular function. There is evidence24 demonstrating that consumption of processed meats is associated with higher incidence of coronary heart disease and diabetes mellitus, while a recent meta-analysis study25 indicates that consumption of fresh or processed red meat as well as total red meat is positively associated with increased risk of total stroke and ischemic stroke.

Red meat is a source of heme iron. Higher iron mediates damage to tissues by catalyzing the production of reactive oxygen species (ROS), which leads to lipid peroxidation, protein modification and DNA damage26,27,28,29. One meta-analysis30 showed that higher intake of heme iron is associated with an increased risk of cardiovascular disease. Moreover, red meat is also a source of saturated fat and cholesterol; accordingly, a higher intake of saturated fat from meat or high absorption of cholesterol is associated with greater risk of cardiovascular disease31,32. Moreover, higher sodium intake from processed meat can also contribute to elevated blood pressure33, reduced arterial compliance and augmented vascular stiffness34. Furthermore, nitrates and their products may facilitate vascular dysfunction and atherogenesis35. Thus, based upon the above findings and our results, the reduction of stroke risk may be an effect from higher long chain n-3 PUFA intake through greater ingestion of fish combined with a lower intake of red meat and processed meat intake.

Of these fourteen prospective studies, one study13 reported that fish oil supplements were not included in the assessment of dietary intake long chain n-3 PUFAs and the use of fish oil supplements had little effects on the results. Another study14 conducted from 1980 to 1994 reported a fish soil supplement consumption rate of only 1.6% in 1990. Another study16 that found no association between fish oil intake and stroke risk reported that approximately 2.7% of participants used fish oil supplements. Two studies12,19 reported no baseline data on fish oil supplementation, but fish oil supplement use was not common among the participants. The remaining nine studies9,10,11,15,17,18,20,21,22 failed to report any information on fish oil supplement use among the participants. However, eleven studies10,11,12,13,14,15,16,18,19,20,21 included in this meta-analysis indicated that fish and seafood were the main sources of long chain n-3 PUFAs, while three studies9,17,22 did not specify the sources of long chain n-3 PUFAs. Thus, based upon the above findings, fish and seafood (as opposed to fish oil supplements) were the primary sources of long chain n-3 PUFAs for the participants in the included studies in this meta-analysis.

The stratification results demonstrated that higher long chain n-3 PUFA intake is inversely associated with fatal stroke risk but failed to demonstrate any relationship between ethnicity, stroke type, follow-up duration, or study quality and reduced stroke risk with higher dietary long chain n-3 PUFAs intake. Sensitivity analysis demonstrated a persistent relationship between higher long chain n-3 PUFA intake and reduced stroke risk. Moreover, there was no publication bias detected between the included studies.

Although there is some evidence36 suggesting that BMI is a risk factor for stroke, BMI’s influence on the relationship between long chain n-3 PUFA intake and stroke risk remains unknown. In the current stratification analysis, there was a lack of canonical standards for defining BMI subgroups, because this meta-analysis included both East Asian and non-East Asian studies. Specifically, the upper limit for normal BMI in East Asian populations should be 23 kg/m2, while the 1997 World Health Organization (WHO) guidelines specify an upper normal BMI limit of 25 kg/m2.37 Thus, in the stratification analysis for BMI, we applied the midpoint of 24 kg/m2 as the cut-off point for BMI (24). Our results showed that individuals with a low BMI (<24 kg/m2) demonstrated reduced stroke risk through a higher intake of long chain n-3 PUFAs.

Sex has previously been shown as a factor influencing stroke risk38; based on our results, it seems that females benefit more from the increased intake of long chain n-3 PUFAs. As platelets play a pivotal role in development of cardiovascular disease39 and platelet aggregation is an early event in the induction of thrombosis and arteriosclerosis40, one possible explanation for this sex-based phenomenon may be the differential sex-based effects of anti-platelet aggregation produced by different categories of long chain n-3 PUFAs. There is evidence41 showing that DHA, DPA and EPA are all equally effective in platelet aggregation in females, while both DHA and DPA are significantly less effective in reducing platelet aggregation in males as compared with females (EPA is equally effective in reducing platelet aggregation in both sexes).

There are several notable limitations to this study. First, dietary long chain n-3 PUFA intake tends to be associated with other nutrients that may prevent stroke, such as potassium42, magnesium43, fiber44 and protein45. However, the association between long chain n-3 PUFA intake and stroke risk was persistent when we confined the analysis to studies that adjusted for these risk factors. Second, there existed heterogeneity between the included studies, although we are unable to determine the sources of heterogeneity. Third, a healthy diet for the primary prevention of cardiovascular and cerebrovascular disease should include adequate intake of vegetables and fruits46,47,48,49 as well as whole grains50,51 and olive oil52.

In conclusion, this meta-analysis reveals that higher long chain n-3 PUFA intake is inversely associated with risk of stroke morbidity and mortality with BMI and sex as key factors influencing this risk. Individuals should be encouraged to manage their body weight while increasing their intake of long chain n-3 PUFAs.

Methods

Data sources and searches

PubMed, Embase, Web of Knowledge and Google Scholar were searched without language restrictions as follows: (“fat” OR “fatty acids”) AND (“stroke” OR “cerebrovascular disease” OR “cerebrovascular disorder” OR “cerebrovascular accident” OR “TIA” OR “transient ischemic attack”). Other potential studies were identified by consulting previous reviews and reference lists of retrieved records.

Inclusion and exclusion criteria

The inclusion criteria were as follows: (i) a prospective cohort design; (ii) reported RRs and their corresponding 95% CIs for long chain n-3 PUFA intake and stroke risk; (iii) multivariates (such as age, smoking, etc.) were controlled; and (iv) only the most recent publication, or the one with the longest follow-up period, was included when duplicate reports based on the same cohort were used.

The exclusion criteria were as follows: (i) case-control or non-prospective cohort study design; (ii) reviews; (iii) experimental studies; and (iv) conference abstracts.

Data extraction and quality assessment

Data were extracted independently by two investigators (P.F.C. and W.H.) and any differences were resolved by discussion with a third investigator (X.F.Z.) We retrieved the following parameters from each included study: first author’s name, publication year, country of study population, age range or mean age, sex (%), number of participants, fat intake assessment, follow-up duration, number of stroke events, outcome assessment, RRs of stroke and corresponding 95% CIs for specific fat intake and covariates adjusted in the statistical analysis. We used the Newcastle-Ottawa Scale (NOS)23 to assess the study quality in this meta-analysis with a high-quality study defined as a study with >8 awarded stars.

Statistical analysis

Log RRs of the highest versus the lowest for cohort studies were weighed by the inverse variance method to obtain pooled RRs. For calculating more robust RRs of stroke from long chain n-3 PUFAs intake, we retrieved all supplemental files of the included studies for RRs of specific types of stroke or specific sex of stroke patient if available. However, 7 of 14 of the included studies did not provide results for males and females separately. For these studies, we contacted the authors to ask them to provide these data. Authors of one study10 provided us with separate results for females and males, authors of another study11 claimed that they did not have these data, authors of three studies17,18,19 did not respond us, authors of two studies13,20 were failed to be contacted because of unsuccessful emails and missing contact information; thus, these six studies11,13,17,18,19,20 were not included in the sex subgroup analysis. Stratification analyses were based on BMI (<24 versus ≥24), follow-up duration (<14 versus ≥14 years), ethnicity (non-East Asians versus East Asians), sex (males versus females), stroke type (ischemic versus hemorrhagic), fatal stroke risk, maximum multivariates (pooling RRs of included studies with hypertension, diabetes and smoking controlled simultaneously), study quality score (≤8 versus >8). For the purpose of obtaining more conservative results, we used a random-effects model for pooling RRs. Smoking, hypertension and diabetes could not be simultaneously adjusted in one11 of the included studies; therefore, we did not include this study in the maximum multivariates adjusted analysis. A meta-regression model was used to detect potential heterogeneity between the included studies. A sensitivity analysis was conducted using the leave-one-out method. Furthermore, publication bias was assessed using Egger’s test. Data obtained from the included studies were analyzed using Stata, version 12.0 (Stata Corp, College Station, Texas).

Additional Information

How to cite this article: Cheng, P. et al. BMI Affects the Relationship between Long Chain N-3 Polyunsaturated Fatty Acid Intake and Stroke Risk: a Meta-Analysis. Sci. Rep. 5, 14161; doi: 10.1038/srep14161 (2015).