Dietary intake of n-3 polyunsaturated fatty acids is associated with a lower incidence of cardiovascular events. Mechanisms underlying this association are poorly understood but may include beneficial effects on physical conditioning and vagal tone. We investigated the association of n-3 fatty acid levels to exercise parameters in 992 subjects with stable coronary artery disease. Cross-sectional associations of heart rate recovery time, treadmill exercise capacity, and exercise time with docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) levels were evaluated in multivariable linear and logistic regression models adjusted for demographics, cardiovascular risk factors, co-morbidities, self-reported physical activity, medication use, and left ventricular function. After multivariable adjustment, n-3 fatty acid levels (DHA + EPA) were strongly associated with heart rate recovery (beta 2.1, p = 0.003), exercise capacity (beta 0.8, p <0.0001), and exercise time (beta 0.9, p <0.0001). Increasing levels of (DHA + EPA) were also associated with decreased risk of impaired heart rate recovery (odds ratio 0.8, p = 0.004) and exercise time (odds ratio 0.7, p = 0.01) and trended toward significance for exercise capacity (odds ratio 0.8, p = 0.07). These associations were not modified by demographics, body mass index, smoking, co-morbid conditions, statin use, or β-blocker use (p for interaction >0.1 for all comparisons). In conclusion, an independent association exists between n-3 fatty acid levels and important exercise parameters in patients with stable coronary artery disease. These findings support the hypothesis that n-3 fatty acids may increase vagal tone and physical conditioning.
The primary aim of this study was to examine the relation between whole blood levels of 2 n-3 fatty acids (docosahexaenoic acid [DHA] and eicosapentaenoic acid [EPA]) and 3 key exercise parameters (treadmill exercise capacity, exercise time, and heart rate recovery [HRR] time) in patients with stable coronary artery disease (CAD).
Methods
The Heart and Soul Study is a prospective cohort study investigating the influence of psychosocial factors on cardiovascular events in outpatients with stable CAD. The enrollment process for the Heart and Soul Study has been previously described. Eligible participants were recruited from outpatient clinics in the San Francisco Bay Area if they met ≥1 of the following inclusion criteria: (1) history of myocardial infarction, (2) angiographic evidence of ≥50% stenosis by area in ≥1 coronary artery, (3) evidence of exercise-induced ischemia by treadmill electrocardiogram or stress nuclear perfusion imaging, or (4) history of coronary revascularization. Subjects were excluded if they had a history of myocardial infarction in the previous 6 months, deemed themselves unable to walk 1 block, or were planning to move out of the local area within 3 years.
The study protocol was approved by institutional review boards of the University of California at San Francisco committee on human research, the research and development committee at the San Francisco VA Medical Center, the medical human subjects committee at Stanford University, the human subjects committee at the VA Palo Alto Health Care System, and the data governance board of the Community Health Network of San Francisco. All participants provided written informed consent. From September 2000 through December 2002, 1,024 participants enrolled in the study. Of these, blood samples were available for assessment of n-3 fatty acid levels from 992 participants. There were no demographic differences between those who did and did not provide blood.
Baseline demographics, age, gender, income, and self-reported ethnicity were recorded. Cardiovascular co-morbidities including hypertension, diabetes, previous myocardial infarction, and smoking status were determined by self-report. Participants were weighed and measured without shoes. Body mass index (BMI) was calculated as the ratio of mass (kilograms) to the square of height (meters). All participants were instructed to bring their medication bottles to the study appointment where study personnel recorded all current medications. Medications were categorized using Epocrates Rx (San Mateo, California). To assess self-reported physical activity, we asked, “Which of the following statements best describes how physically active you have been during the last month, that is, done activities such as 15 to 20 minutes of brisk walking, swimming, general conditioning, or recreational sports?” Participants chose from 1 of the following 6 categories: not at all active, somewhat active (1 time to 2 times/month), fairly active (3 to 4 times/month), quite active (1 times to 2 times/week), very active (3 to 4 times/week), or extremely active (≥5 times/week). Participants who reported that they were not at all or somewhat active were considered physically inactive. Hemoglobin was measured from fasting venous blood samples.
Levels of DHA and EPA were measured from fasting venous whole blood samples. Upon enrollment, study participants refrained from smoking for 5 hours and completed an overnight 12-hour fast (except for prescribed medications taken with water). Whole blood fatty acid composition was assessed by capillary gas chromatography using a GC2010 gas chromatograph (Shimadzu Corporation, Columbia, Maryland) equipped with a SP2560 100-m column (Supelco, Bellefonte, Pennsylvania) after generation of fatty acid methyl esters by treatment with boron trifluoride-methanol. Fatty acid methyl esters were identified by comparison with a weighed standard mixture consisting of 22 fatty acids characteristic of erythrocytes (GLC-727, Nuchek Prep, Elysian, Minnesota). Blood levels of EPA and DHA were determined as percent composition of total fatty acid methyl esters. Two erythrocyte control pools were included with each batch to monitor analytic performance. Acceptable runs were those in which the 2 controls decreased within 2.5 SDs. The coefficient of variation for EPA + DHA was 5% to 6%. Laboratory measurement of fatty acids was blinded to patient characteristics and exercise parameters.
Each participant underwent a symptom-limited graded exercise treadmill test according to standard Bruce protocol. To achieve maximum heart rate, participants who were unable to complete the standard Bruce protocol were switched to slower settings on the treadmill and encouraged to exercise for as long as possible. Continuous 12-lead surface electrocardiographic monitoring was performed throughout testing, and exercise capacity was calculated as total METs achieved at peak exercise. For participants who required modification of the standard Bruce protocol, exercise capacity was determined as total METs achieved at peak exercise. These participants were included in all analyses. We defined poor exercise capacity (<5 METs) according to previously published criteria. After achievement of maximal workload, patients were immediately placed supine. Heart rate was measured exactly 1 minute after termination of exercise. HRR was calculated as the difference between maximal heart rate during exercise and heart rate 1 minute into recovery. Poor HRR was defined as values decreasing within the lowest quartile of participants (≤16 beats/min). Two-dimensional echocardiograms were obtained immediately before and after exercise treadmill testing to assess for wall motion abnormalities using an Acuson Sequoia ultrasound system (Siemens Medical Solutions, Mountain View, California) with a 3.5-MHz transducer and Doppler ultrasound examination. We defined inducible ischemia as the presence of a new wall motion abnormality at peak exercise that was not present at rest.
For descriptive purposes, we categorized participants a priori by tertile of n-3 fatty acid levels. Differences in baseline characteristics were compared using analysis of variance for continuous variables and chi-square test for dichotomous variables. Because the distribution of DHA + EPA levels was right-skewed, we performed logarithmic transformation of the predictor variable before further analysis.
We then used multivariable linear regression models to determine the relation, expressed as beta coefficients, between n-3 fatty acid levels (DHA + EPA, the n-3 index) and exercise capacity, exercise time, and HRR time (continuous dependent variables). Multivariate adjustment was made for demographic characteristics including socioeconomic status and covariates known to influence heart rate and exercise capacity: hypertension, congestive heart failure, diabetes, current smoking, BMI, statin use, and β-blocker use. Covariates were selected on their potential for confounding or mediating effects. Model assumptions were checked by visual inspection of directed acyclic graphs.
In a parallel analysis using similar adjustment models, we performed multivariate logistic regression to determine the association between n-3 fatty acid levels and exercise parameters defined as dichotomous variables (exercise capacity <5 METs, exercise time <3 minutes, and HRR ≤16 beats/min).
To explore potential modifiers, we tested for statistical interactions among age, gender, ethnicity, socioeconomic status, physical activity, hypertension, congestive heart failure, diabetes, current smoking, BMI, statin use, β-blocker use, and n-3 fatty acid levels for the outcome of exercise parameters in multivariable-adjusted models as described earlier.
Statistical analysis was performed using SAS 9.1 (SAS Institute, Cary, North Carolina). The authors take full responsibility for the integrity of the data.
Results
Baseline characteristics of the study population categorized by tertiles of n-3 fatty acid levels are presented in Table 1 . Lower levels of n-3 fatty acids were significantly associated with younger age, nonwhite ancestry, low income, low socioeconomic status, hypertension, previous myocardial infarction, diabetes, current smoking, and higher BMI. Participants in the lowest tertile were also less likely to be taking statins or to define themselves as “physically active.”
| Variable | Lowest Tertile (n = 325) | Middle Tertile (n = 327) | Highest Tertile (n = 335) | p Value |
|---|---|---|---|---|
| Age (years) | 64 ± 11 | 67 ± 10 | 69 ± 10 | <0.0001 |
| Men | 268 (82%) | 263 (80%) | 273 (82%) | 0.77 |
| Ethnicity | <0.0001 | |||
| Hispanic | 45 (14%) | 30 (9%) | 11 (3%) | |
| White | 190 (59%) | 189 (58%) | 217 (65%) | |
| Black | 62 (19%) | 70 (22%) | 27 (8%) | |
| Asian | 15 (5%) | 28 (9%) | 70 (21%) | |
| Other | 12 (4%) | 10 (3%) | 10 (3%) | |
| Income <$20,000 | 194 (60%) | 158 (48%) | 123 (37%) | <0.0001 |
| History | ||||
| Hypertension | 239 (74%) | 245 (75%) | 211 (63%) | 0.002 |
| Myocardial infarction | 193 (60%) | 172 (53%) | 162 (49%) | 0.01 |
| Congestive heart failure | 61 (19%) | 65 (20%) | 47 (14%) | 0.11 |
| Stroke | 47 (15%) | 45 (14%) | 48 (14%) | 0.97 |
| Diabetes mellitus | 107 (33%) | 87 (27%) | 65 (20%) | 0.0004 |
| Coronary bypass | 106 (33%) | 131 (41%) | 114 (34%) | 0.08 |
| Coronary angioplasty | 118 (37%) | 137 (42%) | 130 (39%) | 0.34 |
| Current smoker | 99 (31%) | 74 (23%) | 23 (7%) | <0.0001 |
| Physically active | 195 (60%) | 189 (58%) | 237 (71%) | 0.0008 |
| Body mass index (kg/m 2 ) | 28.5 ± 5.9 | 29.0 ± 5.6 | 27.8 ± 4.6 | 0.01 |
| Medication use | ||||
| β Blocker | 203 (62%) | 181 (55%) | 184 (55%) | 0.09 |
| Statin | 183 (56%) | 220 (67%) | 231 (69%) | 0.001 |
| Angiotensin-converting enzyme inhibitor | 153 (47%) | 174 (53%) | 179 (53%) | 0.18 |
| Aspirin | 252 (78%) | 264 (81%) | 246 (73%) | 0.08 |
| Hemoglobin (g/dl) | 13.9 ± 1.4 | 13.7 ± 1.4 | 14.0 ± 1.3 | 0.09 |
| Heart rate recovery | 23.8 ± 12.5 | 24.7 ± 11.9 | 27.2 ± 13.6 | 0.003 |
| Exercise capacity (METs) | 6.9 ± 3.1 | 7.1 ± 3.3 | 8.0 ± 3.6 | <0.0001 |
| Exercise time (minutes) | 5.8 ± 3.1 | 6.1 ± 3.1 | 7.0 ± 3.4 | <0.0001 |
| Left ventricular ejection fraction (%) | 61.1 ± 10.0 | 61.9 ± 8.9 | 61.8 ± 10.4 | 0.56 |
| Inducible ischemia | 67 (23%) | 75 (25%) | 76 (24%) | 0.90 |
The relation between n-3 fatty acid levels and exercise parameters is depicted graphically in Figures 1 through 3 . In multivariable-adjusted linear regression models, there was a direct association between n-3 fatty acid levels and HRR, exercise capacity, and exercise time ( Table 2 ). After sequential adjustment for demographics, cardiovascular risk factors, co-morbidities, medication use, and left ventricular function, n-3 fatty acid levels remained positively associated with HRR, exercise capacity, and exercise time.
