Objective Long-chain omega-3 polyunsaturated fatty acids (PUFA) from fish have been inversely associated with coronary heart disease (CHD) risk. Fish may also contain methylmercury, which has been associated with higher CHD risk and may diminish the cardioprotective effect of long-chain omega-3 PUFA. We investigated the associations of serum long-chain omega-3 PUFA and hair mercury with the odds for myocardial ischaemia during exercise.
Methods A total of 2199 men from the Kuopio Ischaemic Heart Disease Risk Factor Study, aged 42–60 years were studied in 1984–89. Of the 2199 men, 342 had history of CHD. The men performed a maximal symptom-limited exercise stress test using an electrically braked bicycle ergometer. ORs for exercise-induced myocardial ischaemia were estimated with logistic regression.
Results In the multivariable analysis, those in the highest versus lowest serum long-chain omega-3 PUFA quartile had 33% lower odds of myocardial ischaemia (OR 0.67, 95% CI 0.51 to 0.87, p-trend=0.006). The association was stronger among those with CHD history (OR 0.10, 95% CI 0.03 to 0.39, p-trend <0.001), than among those without (OR 0.80, 95% CI 0.57 to 1.12, p-trend=0.17) (p-interaction=0.01). Higher hair mercury concentration was associated with increased odds for myocardial ischaemia in the entire population (OR 1.62, 95% CI 1.22 to 2.14, p-trend=0.002).
Conclusion Higher circulating concentrations of the long-chain omega-3 PUFAs, a marker for fish consumption, were associated with lower occurrence of exercise-induced myocardial ischaemia, but only among men with CHD history. Hair mercury concentration was directly associated with the occurrence of exercise-induced myocardial ischaemia in the entire study population.
- polyunsaturated fatty acids
- exercise test
- myocardial ischemia
- population study
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Coronary heart disease (CHD) is the leading cause of mortality worldwide. Substantial evidence from epidemiological studies indicates that intake of fish or the long-chain omega-3 polyunsaturated fatty acids (PUFA; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) from fish may decrease the risk of cardiac mortality, possibly through their anti-arrhythmic, anti-inflammatory and antithrombotic effects.1 However, knowledge regarding the impact of the long-chain omega-3 PUFA on myocardial ischaemia, a pathophysiological hallmark of coronary atherosclerosis,2 is still lacking.
Myocardial ischaemia is known as a well-established predictor of the future cardiac mortality, especially sudden cardiac death.3 Epidemiological studies, including the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD),4 have indicated that higher incidence of exercise-induced myocardial ischaemia is related to increased risk of future cardiac events.5
Previously in the KIHD, higher serum long-chain omega-3 PUFA concentration, an objective biomarker for the fatty acid intake, has been inversely associated with risk of CHD6 and sudden cardiac death7 and their risk factors, including high blood pressure,8 prolonged QT and JT intervals,9 low exercise cardiac power and cardiorespiratory fitness10 and high resting heart rate.11 However, very little overall is known about the association of the serum long-chain omega-3 PUFA with the risk of exercise-induced myocardial ischaemia.
Experimental studies using animal models have demonstrated that long-chain omega-3 PUFA supplementation reduced the occurrence of myocardial ischaemia and its consequences.12 In contrast, in one study, dietary supplementation with long-chain omega-3 PUFA was not associated with the parameters of myocardial ischaemia during exercise in patients with stable CHD.13 This lack of association was also observed between whole blood long-chain omega-3 PUFA and risk of inducible ischaemia among subjects with stable coronary artery disease.14
Some fish species may also contain methylmercury, an environmental contaminant.15 Methylmercury has been associated with higher risk of cardiac events and with attenuation of the cardioprotective benefits of the long-chain omega-3 PUFA, especially in the KIHD cohort.6 7 16 However, very little is known regarding the association of mercury exposure with exercise-induced myocardial ischaemia.
Therefore, we investigated the cross-sectional associations of the serum long-chain omega-3 PUFA and hair mercury concentrations with exercise-induced myocardial ischaemia among middle-aged and older men. Since knowledge regarding these associations among healthy population is lacking, we also separately investigated the associations among participants free of CHD and among those with a history of CHD.
We performed a cross-sectional analysis among the participants from the KIHD cohort, a population-based study designed to investigate risk factors for cardiovascular disease, atherosclerosis and related outcomes in men from Eastern Finland.17 A total of 2682 men (82.9% of those eligible) who were aged 42, 48, 54 or 60 years and living in the city of Kuopio or its surrounding areas were recruited to the baseline examinations in 1984–89. The baseline characteristics of the entire study population have been described previously.17 The KIHD protocol complies with Declaration of Helsinki. All the subjects signed a written informed consent.
Subjects gave hair and venous blood samples between 08:00 and 10:00 hours at the baseline examinations. The subjects were instructed to abstain from ingesting alcohol for 3 days and from smoking and eating for 12 hours before giving the sample. Comprehensive description of the determination of serum lipids and lipoproteins, assessment of medical history and medications, smoking and alcohol consumption have been reported previously.18
Physical activity was evaluated based on the 12-month leisure-time physical activity questionnaire and expressed as kcal/day.19 The most common leisure-time physical activities were recorded, including the average duration, intensity and frequency of each activity. Hypertension diagnosis was defined as systolic/diastolic blood pressure >140/90 mm Hg at study visit or use of hypertension medication. CHD at baseline was defined either a history of myocardial infarction or angina pectoris, angina pectoris on effort, the use of nitroglycerine for chest pain once a week or more frequently or chest pain as a cause of stopping exercise stress test at baseline. Dietary intakes were assessed by using 4-day food recording at the time of blood sampling. Education and annual income were assessed by using self-administered questionnaires.
Serum fatty acid and mercury measurements
Serum fatty acids from the baseline samples were determined in 1991 in one gas chromatographic run without preseparation, as described previously.20 Serum fatty acids were extracted with chloroform-methanol. Chloroform phase was evaporated and treated with sodium methoxide, which methylated esterified fatty acids. Quantification was carried out with reference standards (Check Prep, Elysian, Minnesota, USA). Each analyte had individual reference standard, and an internal standard was eicosan. Fatty acids were chromatographed in an NB-351 capillary column (HNU-Nordion, Helsinki, Finland) by a Hewlett-Packard 5890 Series II gas chromatograph (Hewlett-Packard, Avondale, Pennsylvania, USA since 1999 Agilent Technologies) with a flame ionisation detector. Results were obtained in micromoles per litter and in the data analyses proportion of the fatty acid of the total serum fatty acids was used. The coefficient of variation was 9.4% for EPA (20:5 n-3), 12.7% for docosapentaenoic acid (DPA, 22:5 n-3) and 11.9% for DHA (22:5 n-3). For the serum total long-chain omega-3 PUFA concentration, we used the sum of EPA, DPA and DHA.
Hair mercury was detected by flow injection analysis-cold vapour atomic absorption spectrometry and amalgamation.16 Repeat hair samples were collected from 21 subjects in 4–9 years (mean, 6 years) after baseline examination to survey the tracking of hair mercury values over time. Pearson’s correlation coefficient between the original and the repeat measurement was 0.91.
Exercise test and exercise electrocardiography
A maximal symptom-limited exercise stress test was assessed in the baseline study between August 1986 and December 1989. The exercise stress test was performed between 08:00 and 10:00 hours using an electrically braked cycle ergometer (Medical Fitness, Mearn, the Netherlands), as explained in detail elsewhere.19 Electrocardiography was recorded continuously with the Kone 620 electrocardiograph (Kone, Turku, Finland). The Mason-Likar lead system including Vl, V5 and aVF lead connections was used. An ECG was printed in 30 s intervals during exercise and at least 5 min during recovery while the subject was sitting on the bicycle. The criteria for ischaemia in ECG during exercise was horizontal or downsloping ST depression ≥1.0 mm at 80 ms after J point or any ST depression of >1.0 mm at 80 ms after J point.21
The univariate associations of the serum total long-chain omega-3 PUFA (EPA+DPA+DHA) concentration and hair mercury concentration with demographic, lifestyle and clinical characteristics at baseline were assessed by means and linear regression for continuous variables and χ2 test for categorical variables. Correlations between the individual long-chain omega-3 PUFAs were evaluated by Spearman’s correlation. Logistic regression models were used to estimate ORs for occurrence of exercise-induced myocardial ischaemia in exposure quartiles, with the lowest category as the reference.
Three different models were used to control for confounding factors. The model 1 was adjusted for age (years) and examination year. The model 2 included the variables in the model 1 plus body mass index (BMI) (kg/m2), type 2 diabetes (yes/no), smoking status (never smoker, previous smoker, current smoker <20 cigarettes/day and current smoker ≥20 cigarettes/day), leisure-time physical activity (kcal/day), hypertension (yes/no), energy intake (kcal/day) and alcohol intake (g/week). The model 3 was adjusted for variables in the model 2 plus hair mercury (in analyses with fatty acids) or total long-chain omega-3 PUFAs (in analyses with mercury). All quantitative variables were entered in the models as continuous variables. Additional adjustments for serum triglycerides (mmol/L), blood glucose (mmol/L), high-density lipoprotein (HDL) or low-density lipoprotein (LDL) cholesterol concentrations (mmol/L), lipid-lowering medication (yes/no), C reactive protein (mg/L), use of beta-blockers (yes/no) or maximal oxygen uptake during the exercise test (mL/kg/min) did not appreciably change the associations (<5% change in estimates). The confounders in the analyses were selected based on established risk factors for CHD, previously published associations in the KIHD study9–11 or on associations with exposures or outcomes in the present analysis.
Cohort means were used to replace missing values in covariates (<0.5%). Tests of linear trend across categories were conducted by assigning the median values for each category of exposure variable and treating those as a single continuous variable. All p values were two-sided (α=0.05). Statistical significance of the interactions with the CHD status on a multiplicative scale was assessed by stratified analysis and likelihood ratio tests with a cross-product term. Data were analysed using the SPSS software V.23 for windows (IBM, Armonk, New York, USA).
From the analyses we excluded those with missing data on the serum long-chain omega-3 PUFA (n=202) or hair mercury (n=281). After the exclusions, 2199 men were included in the analysis. Of the 2199 men, 1857 men (84.4%) were free of CHD and 342 (15.6%) had a history of CHD.
Table 1 shows the baseline characteristics of the entire KIHD population and among those with or without CHD history. Men without history of CHD were more educated and had a lower BMI, and lower serum triglyceride concentrations, lower hair mercury concentration and lower likelihood of having diabetes and higher energy intake.
Baseline characteristics of the participants according to the quartiles of the total long-chain omega-3 PUFA and hair mercury concentrations are presented in the online supplementary tables 1 and 2. Men with higher serum total long-chain omega-3 PUFA concentration were more educated, were less likely to smoke and were more physically active. They also had higher alcohol intake, serum HDL and LDL cholesterol and hair mercury concentrations and lower serum triglyceride concentrations and lower total energy intake (see online supplementary table 1). The associations were generally similar among men with or without history of CHD, although higher serum total long-chain omega-3 PUFA concentration was associated with less smoking and higher education, BMI, physical activity and lower energy intake only among those without history of CHD (see online supplementary table 1).
In the entire study population, men with higher hair mercury concentration were older, less educated and less physically active and were more likely to be smokers. They also had higher BMI, alcohol consumption and serum HDL and LDL cholesterol concentrations. These associations were relatively similar when evaluated based on history of CHD, although higher hair mercury concentration was associated with higher BMI and alcohol intake, smoking and lower physical activity and serum triglyceride concentrations only among those without history of CHD (see online supplementary table 2).
The correlation coefficients of the individual long-chain omega-3 PUFA with each other and with hair mercury were 0.71 for EPA and DHA, 0.56 for EPA and DPA, 0.50 for DHA and DPA, 0.35 for EPA and mercury, 0.30 for DHA and mercury and 0.18 for DPA and mercury (all p values <0.05).
The serum long-chain omega-3 PUFA, hair mercury concentration and odds of exercise-induced myocardial ischaemia
Exercise-induced myocardial ischaemia was found in 623 (28.3%) of the 2199 men. Of these, 328 occurred among the 1857 men without history of CHD (17.6% among these men) and 295 (86.3%) among the 342 men with CHD history. Among all men, the odds for occurrence of exercise-induced myocardial ischaemia was 33% lower (OR 0.67, 95% CI 0.51 to 0.87, p-trend across quartiles=0.006) in the highest versus the lowest serum EPA+DPA+ DHA quartile after multivariable adjustments (model 2, table 2). Each 1 SD higher serum concentration was associated with 8% (95% CI −1% to –17%) lower odds (figure 1). Generally, similar inverse associations were observed with EPA, DPA and DHA (table 2, figure 1). Further adjustment for hair mercury content slightly strengthened the associations (model 3, table 2).
Higher hair mercury concentration was associated with 62% increased odds for exercise-induced myocardial ischaemia (OR for extreme-quartile difference 1.62, 95% CI 1.22 to 2.14, p-trend=0.002) (model 2, table 2). The OR was 18% higher (95% CI 8% to 30%) for each 1 SD higher hair mercury concentration (figure 1). Further adjustment for serum EPA+DPA+ DHA concentration again slightly strengthened the association (model 3, table 2).
When we stratified the analyses based on CHD history, we found stronger associations among men with history of CHD (table 3, figure 1). For example, after multivariable adjustments (model 2), serum EPA+DPA+ DHA concentration was associated with 90% (extreme-quartile OR 0.10, 95% CI 0.03 to 0.39, p-trend <0.001) lower odds of exercise-induced myocardial ischaemia among the men with history of CHD, but no statistically significant associations were observed among the men without CHD history (OR in the highest quartile 0.80, 95% CI 0.57 to 1.12, p-trend=0.17) (p-interaction=0.01) (table 3). Also, in the case of mercury, higher hair mercury content was associated with lower odds among men with CHD history, but the association was not statistically significant (table 3, figure 1). Further adjustments for hair mercury/serum long-chain omega-3 PUFA had no appreciable impact on the associations (data not shown).
We could not find statistically significant interactions between the long-chain omega-3 PUFAs and hair mercury for exercise-induced myocardial ischaemia (p for interaction >0.16). When we stratified the analyses based on mercury content, generally similar associations were found among the participants with the higher and lower hair mercury content.
In the present study, higher serum long-chain omega-3 PUFA concentrations were associated with lower odds for exercise-induced myocardial ischaemia among middle-aged and older men from Eastern Finland. The inverse association was observed, exclusively among those with history of CHD. Furthermore, higher hair mercury concentration was associated with a higher odd for exercise-induced myocardial ischaemia in the entire study population.
Myocardial ischaemia results from an imbalance in myocardial oxygen supply and demand, and from reduction of myocardial blood flow and contractile function.22 Atherosclerosis and endothelial dysfunction, due to vessel narrowing and plaque formation in the arteries, may lead to myocardial oxygen imbalance.23 Physical exercise may also have influence on the subendocardial blood flow and myocardial oxygen consumption.24 It has been shown that exercise-induced myocardial ischaemia, as indicated in an ECG stress test, predicts the risk of atherosclerosis and the prognosis of future cardiac events.4 5
In the present study, we found that higher serum long-chain omega-3 PUFA concentrations were associated with significantly lower risk of exercise-induced myocardial ischaemia, especially among men with history of CHD. According to the result of a meta-analysis of experimental animal studies, supplementation with fish oil, a source of the long-chain omega-3 PUFA, reduced the occurrence of myocardial ischaemia and its consequences (eg, ischaemia-induced ventricular fibrillation) in dogs, rats and marmosets.12 25 In contrast, in two studies among patients with stable coronary artery disease, no association was observed between long-chain omega-3 PUFA and ischaemic response to exercise stress test.13 14 In a randomised, double-blind, crossover study among eight patients, dietary supplementation with fish oil for 12 weeks did not alter the parameters of myocardial ischaemia during exercise (eg, exercise-induced ST-segment depression and onset of angina).13 In a cross-sectional study among 992 subjects with stable coronary artery disease, the blood long-chain omega-3 PUFA level was not associated with exercise-induced ischaemia.14
Although potential mechanisms underlying the inverse association between serum long-chain omega-3 PUFAs and odds of exercise-induced myocardial ischaemia in our cohort is beyond the scope of the current study and future research is needed, it might, for example, be due to the reduction in hemostatic markers of atherosclerosis and cardiac endothelium in the population with CHD.26 Regarding the stronger inverse associations among those with existing CHD, it has been shown that the long-chain omega-3 PUFA might beneficially influence the coronary vasodilator reserve, which leads to more blood flow during exercise among patients with CHD.27 Moreover, the long-chain omega-3 PUFA have a positive impact on the vascular endothelial growth factor concentrations,28 which is increased in the patients with CHD in a response to the exercise-induced myocardial ischaemia.29
Especially some lean, long-living predatory fish species can have high concentrations of methylmercury, whereas species with higher long-chain omega-3 PUFA content, such as salmon, contain less methylmercury.15 This could potentially explain why the long-chain omega-3 PUFA and methylmercury can have opposite associations with the risk, despite that fact that fish is the major dietary source for both. We have previously shown in the KIHD cohort that the risk of CHD, especially myocardial infarction and sudden cardiac death, is increased by higher hair mercury concentration.6 7 In those studies, the inverse association of the long-chain omega-3 PUFA with these outcomes was mainly seen among the men with lower mercury exposure. In the present study, higher hair mercury concentration was associated with increased odds for exercise-induced myocardial ischaemia in the entire KIHD population. This might be due to, for example, the role of mercury in increasing oxidative stress, which seems to be related to progression of myocardial ischaemia by contributing to endothelial dysfunction and atherosclerotic plaque formation.15
The strengths of our study include the use of serum long-chain omega-3 PUFA and hair mercury, both established biomarkers for fish intake, instead of food-frequency questionnaires, which reduced the bias by misclassification. Other strengths include the extensive examination of potential confounders and a rather large study population with data on ECG parameters. A potential limitation was that the participants were middle-aged and older men from Eastern Finland, so the findings may not be generalisable to other populations or to women. Moreover, we had data only on total serum fatty acids, which reflect more recent intakes, compared with adipose tissue and erythrocyte fatty acid measurements that reflect long-term intakes.30
Higher circulating concentrations of the long-chain omega-3 PUFAs were inversely associated with the occurrence of exercise-induced myocardial ischaemia among middle-aged and older men, but only among those with history of CHD. Hair mercury concentration was associated with the higher occurrence of exercise-induced myocardial ischaemia in the entire study population. More large-scale studies in diverse populations are needed to confirm these findings.
What is already known on this subject?
Fish is a major source of long-chain omega-3 polyunsaturated fatty acids.
Some fish species also contain methylmercury.
There is little prior data regarding the impact of the long-chain omega-3 polyunsaturated fatty acids and methylmercury exposure on the occurrence of exercise-induced myocardial ischaemia.
What might this study add?
This study found that higher serum concentrations of the long-chain omega-3 polyunsaturated fatty acids were associated with lower risk of myocardial ischaemia during exercise, exclusively among those with history of coronary heart disease.
In contrast, higher methylmercury exposure was associated with higher risk.
How might this impact on clinical practice?
For the prevention of cardiovascular diseases, guidelines should continue to emphasise intake of fatty fish, which is high in long-chain omega-3 polyunsaturated fatty acids but low in methylmercury.
Contributors BT, T-PT, SK, JS and JKV acquired the data and designed and conducted the research; BT analysed the data and drafted the manuscript; JKV had primary responsibility for the final content; T-PT, SK, JS and JKV critically revised the manuscript for important intellectual content and all authors read and approved the final manuscript.
Funding This study was funded by Kuopio University Foundation, Juho Vainio Foundation, and University of Eastern Finland. The KIHD project was funded by research grants to Jukka T. Salonen and George A. Kaplan from the NIH and the Academy of Finland.
Disclaimer The funding sources had no role in study design; in the collection, analysis and interpretation of data; in the writing of the report or in the decision to submit the article for publication.
Competing interests None declared.
Ethics approval The Kuopio Ischaemic Heart Disease Risk Factor Study protocol was approved by the Research Ethics Committee of the University of Kuopio.
Provenance and peer review Not commissioned; externally peer reviewed.
Patient consent for publication Not required.
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