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Leisure time physical activity, cardiorespiratory fitness, and plasma fibrinogen concentrations in nonsmoking middle-aged men


Medicine & Science in Sports & Exercise: March 2000 - Volume 32 - Issue 3 - p 620-626

CARROLL, S., C. B. COOKE, and R. J. BUTTERLY. Leisure time physical activity, cardiorespiratory fitness, and plasma fibrinogen concentrations in nonsmoking middle-aged men. Med. Sci. Sports Exerc., Vol. 32, No. 3, pp. 620–626, 2000.

Purpose: The relationship of both leisure time physical activity and predicted maximum oxygen consumption (O2max) with plasma fibrinogen concentration was examined within a cohort of employed middle-aged men.

Methods: Analyses were performed on a subsample of 635 nonsmoking men (46.7 ± 7.7 yr) who completed a preventive medical assessment between 1992 and 1996.

Results: Among nonsmokers, mean age-adjusted fibrinogen concentration decreased significantly with higher physical activity index (PAI) categories and quartiles of predicted O2max (mL · kg−1 · min−1) (both P = 0.001). Mean age-adjusted plasma fibrinogen concentrations were significantly different (P < 0.05) between inactive and vigorous PAI groups and extreme quartiles of predicted O2max (mL · kg−1 · min−1). These relationships were no longer significant after adjustment for the confounding effect of other ischemic heart disease risk factors. Stepwise multiple regression analyses showed that age, sum of skinfolds, and blood leukocyte count were the strongest predictors of plasma fibrinogen concentration.

Conclusion: These data do not confirm a significant independent association of both physical activity and predicted O2max (mL · kg−1 · min−1) with fibrinogen concentrations among nonsmoking middle-aged men of similar high social class.

School of Leisure and Sports Studies, Leeds Metropolitan University, Leeds LS6 3QS UNITED KINGDOM

Submitted for publication May 1998.

Accepted for publication January 1999.

Address for correspondence: Sean Carroll, School of Leisure and Sports Studies, Leeds Metropolitan University, Beckett Park Campus, Leeds LS6 3QS United Kingdom.

Prospectiveinvestigations have established that high plasma fibrinogen concentrations are predictive of the incidence of ischemic heart disease (IHD) events (17). There is also considerable evidence that higher levels of leisure time physical activity and cardiorespiratory fitness are associated with lowered incidence rates of IHD (5,24). Within British studies, vigorous physical activity has been associated with the lowest risk (33), although moderate levels of physical activity are associated with a decreased risk (39). “Habitual, continuing, and current” physical activity appears essential, suggesting an effect on the acute thrombotic component of IHD (33).

Plasma fibrinogen is the component of the hemostatic system most frequently studied for the effects of physical activity (30). Several, although not all (6,12), cross-sectional studies have shown inverse associations with high levels of overall leisure time physical activity and plasma fibrinogen concentrations (16,19). Other investigations have indicated only vigorous exercise to be associated with lower fibrinogen concentrations in middle-aged (8,25) and elderly men (41).

Aging, smoking, and low social class have also been positively associated with fibrinogen (4,18,21,27) and may confound the relationship between physical activity and fibrinogen (26,27). Limited research has evaluated the relationship between cardiorespiratory fitness and plasma fibrinogen (25,26,32,35,46). Additional epidemiological investigations of physical activity habits, cardiorespiratory fitness, and plasma fibrinogen concentrations are required (15). We have examined the relationship between leisure time physical activity, predicted maximum oxygen consumption (O2max) and plasma fibrinogen concentrations within a cohort of nonsmoking middle-aged men of similar high social class.

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Study participants.

The cross-sectional sample in this study consisted of 740 Caucasian male asymptomatic subjects. Data collection was conducted between 1992 and 1996 on individuals who presented themselves for a preventive assessment at a Leeds-based private hospital. All subjects were in current employment, within socioeconomic groups I or II. All subjects were free of diagnosed IHD, and subjects with possible asymptomatic IHD had been excluded by exercise electrocardiography using sensitive diagnostic criteria (≥1 mm ST segment depression). The clinical examinations were performed after written informed consent was obtained from each participant.

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Clinical examination.

Subjects stature (meters) and body mass (to the nearest 0.2 kg) were measured in light clothing without shoes, using Seca balance scales and metal stadiometer (model 713, Seca Corporation, Birmingham, UK). Body mass index (BMI) was calculated as the ratio of the weight to height squared (kg · m−2). Subcutaneous skinfold thicknesses (to the nearest 0.2 mm) were measured by the same investigator at four anthropometric sites (13) (biceps, triceps, suprailiac, and subscapular) with Harpenden skinfold calipers (British Indicators Ltd., Burgess Hill, West Sussex, UK). Systolic and diastolic (fifth-phase) blood pressure (BP) were determined (mean of two manual measurements separated by ten minutes) from the right arm of each participant in the supine position with a standard mercury sphygmomanometer (Accoson, A. C. Cosson & Son (Surgical Ltd.), London, UK). Hypertension was defined as men with a systolic BP > 140 mm Hg or a diasolic BP > 90 mm Hg.

Subjects were instructed to follow an overnight fast (minimum 12 h), and venous blood was sampled between 9:00 a.m. and 1:00 p.m. Blood was drawn with minimal venous stasis into 3.0 ml Monovette 0.3 g · L−1 citrate solution tubes (Sarstedt, Numbrecht, Germany) for fibrinogen assay. The majority of previous studies have reported discrete 2–3 h sampling times commencing early morning in relation to fibrinogen assays (2,25,35,36). Methodological variability for fibrinogen (inclusive of sampling methods and assay techniques), however, appears to be low (11), with no evidence for a significant effect of sampling time (1).

Plasma fibrinogen concentration was determined on fresh plasma samples by the Clauss (nephelometric) method adapted for an ACL coagulometer (Instrumentation Laboratory, UK Limited) (38). Analyses were subject to continuous internal and external quality control consistent with an accredited laboratory. The coefficient of variation for plasma fibrinogen was 5.3%. Serum lipids were determined with standard enzymatic methods on a Greiner G450 automated analyzer. Serum high-density lipoprotein was measured using the heparin manganese-chloride method. Routine haematological analyses were performed on a Coulter counter (Coulter Instruments, Luton, UK).

Subjects were interviewed by a physician using a standard questionnaire for medical and social history. Smoking questions were based on standard epidemiological definitions and from this respondents were classified as current or nonsmokers. Recording of weekly alcohol units was based on self-assessment of usual frequency and quantity of drinking. Alcohol data were excluded in 21 subjects due to imprecision.

Self-reported leisure time physical activity habits (4-wk reference period) were classified using a modification of the Physical Activity Index (PAI) developed for the British Regional Heart Study (39). An individual’s overall PAI was a composite score of all reported activities. Full details of the derivation of the physical activity scores for various types and duration/frequency of leisure time activities based on their intensity have been described (39). A total of 628 of the nonsmokers (99%) provided a history sufficiently detailed to derive an overall physical activity score. For the purpose of this analysis subjects were grouped into a four-category PAI based on their total activity score; i) Sedentary (score 0–2) inactive in leisure time; ii) Occasional/light (score 3–8) regular walking/more or less frequent recreational activities, or very occasional vigorous exercise; iii) Moderate/moderately vigorous (score 9–20) frequent cycling/very frequent recreational activities in isolation or with vigorous (sporting) activity at least once weekly; or iv) Vigorous (score ≥ 21) very frequent sporting exercise (up to 4 times weekly) or regular sporting exercise (2–3 times weekly) plus other recreational activities.

Maximum oxygen consumption (O2max) is generally accepted as a valid and reliable measure of cardiorespiratory fitness (3). O2max (L · min−1) and weight-adjusted O2max (mL · kg−1 · min−1) were indirectly predicted with the Astrand-Rhyming method (3) from a modified submaximal cycle ergometer (Monark 818, Varberg, Sweden) protocol recommended for use in epidemiological surveys (40). The upper limit of submaximal work was defined as 85% of age-predicted maximum heart rate based on the 220-age equation. The cycling protocol consisted of a cadence of 50 rpm and an initial workload of 75–100 W (set on the basis of body mass). Thereafter, successive workload increments of 25 W were applied, after the attainment of steady-state heart rate response (steady state defined as a heart rate increase <5 beats · min−1) or after 3 min. Heart rates during the exercise test were determined from a continuous electrocardiogram (CR7, Cardiorater, London, UK), and a 12-lead ECG tracing (Cardioscript CD-6000, Picker, International Ltd., Wembley, Middlesex, UK) was taken at the end of each exercise stage. The test suffers from the well-established limitations of submaximal testing in determining O2max (10). Standard errors of estimates associated with O2max prediction using this method (typically 10–15% with middle-aged males) have been suggested, with some exception (43) as being sufficiently accurate to discriminate cardiorespiratory fitness levels within epidemiological investigations (40). This O2max prediction method has continued applicability in assessing the influence of cardiorespiratory fitness on IHD risk factors (29,32).

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Data analysis.

Descriptive statistics were computed for all variables. Distributions of variables were assessed for normality. Six potential outliers (standardized residual score >3.0) for fibrinogen were identified among the nonsmokers (5.1, 5.2, 5.3, 5.4, 5.9, 6.4 g · L−1) and removed from all relevant analyses. The distributions of physical activity score, alcohol consumption, and serum triglycerides were analyzed after logarithmic transformation to improve skewness and kurtosis (henceforth labeled log-PAI score, log-alcohol consumption, and log-triglycerides). Pearson’s product moment correlation coefficients and partial correlations (controlling for age) were calculated to determine the association between variables. Stepwise multiple linear regression analysis was utilized to assess predictor variables of plasma fibrinogen concentration. In the regression models, log physical activity was entered with and without predicted O2max (mL · kg−1 · min−1) along with other variables significantly associated with fibrinogen on univariate analyses. Diastolic rather than systolic BP was utilized in multivariate regression models due to its stronger age-adjusted partial correlation with plasma fibrinogen. Data for variables were compared between PAI categories and predicted O2max quartiles using one-way analysis of variance. The influence of potential confounding variables were evaluated with analysis of covariance. Post hoc comparisons were made using the Scheffe procedure for multiple comparisons. Student’s t-test for independent samples was utilized to examine group differences between subjects with and without fibrinogen determinations and between smoking and nonsmoking groups. Data are presented as mean ± standard deviation (mean ± SD), 95% confidence intervals for mean (95% CI). For all statistical tests, the alpha level adopted for significance was P < 0.05. Statistical analyses were performed on SPSS for Windows statistical package (SPSS Inc., Chicago, IL), version 6.1.

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Plasma fibrinogen concentrations were available in 710 men (95.9% of the study sample). Thirty missing cases were attributable to technical difficulties with the venepuncture or coagulation assay. There were no significant differences evident in the general characteristics of the 30 subjects with missing fibrinogen data (data not shown). The mean plasma fibrinogen concentration of the sample (N = 704) was 3.12 ± 0.62 g · L−1 (95% CI 3.07, 3.16 g · L−1). The prevalence of smokers within this high social class cohort was 9.8%. Smokers (N = 69) had significantly higher plasma fibrinogen concentrations than nonsmokers (3.64 ± 0.69 vs 3.06 ± 0.58 g · L−1;P < 0.00005).

The general characteristics of the nonsmoking and smoking groups are outlined in Table 1. Mean log-PAI score and predicted O2max (mL · kg−1 · min−1) score were significantly lower among smokers compared with nonsmokers. Smokers also exhibited significantly higher mean total cholesterol, log-triglyceride, and leukocyte counts compared with nonsmokers (Table 1). The low variability of physical activity scores within the smokers (31.1% inactive, 59% occasional/light PAI score) would be likely to obscure the association between plasma fibrinogen and physical activity. Plasma fibrinogen concentrations across PAI and predicted O2max categories were therefore subsequently analysed only among nonsmokers.

Table 1

Table 1

Of the nonsmoking subjects (N = 635), 25.9% had a total cholesterol concentration ≤5.2 mmol · L−1 and 73.0% ≥ 6.5 mmol · L−1. Fasting hypertriglyceridemia (triglycerides >1.70 mmol · L−1) was evident in 29.7%; 57.2% of the subjects were overweight (BMI > 25.0 kg · m−2) and 6.9% were obese (BMI > 30.0 kg · m−2); 3.1% of these subjects exhibited fasting hyperglycemia (fasting serum glucose >6.5 mmol · L−1). Systolic and diastolic hypertension was evident in 11.8% and 12.3%, respectively, of the subjects.

The proportion of subjects within the inactive, occasional/light, moderate/moderately vigorous, and vigorous PAI categories were 16.5%, 39.2%, 33.2%, and 11.1%, respectively. The more physically active subjects were younger (P < 0.0005) and had lower age-adjusted anthropometric estimates of obesity (body mass, BMI, and sum of skinfolds:P = 0.0015, P = 0.007, and P < 0.00005, respectively) (Table 2). Significant correlation coefficients were found between log-PAI score with predicted O2max (mL · kg−1 · min−1) and resting heart rate (beats · min−1) (r = 0.591 and r = −0.413, respectively; both P < 0.00005). Mean predicted O2max scores increased, and resting heart rates decreased significantly with each level of higher physical activity (P < 0.05). Higher predicted mean O2max scores and lower resting heart rate trends with higher PAI categories remained highly statistically significant after adjustment for age (all P < 0.00005) (Table 3).

Table 2

Table 2

Table 3

Table 3

Univariate correlation and partial correlation (adjusting for age) between fibrinogen concentration and other variables are shown in Table 4. Stepwise multiple linear regression analysis showed that age, sum of skinfolds and predicted O2max (mL · kg−1 · min−1) were the strongest determinants of plasma fibrinogen concentration when entered with body mass, BMI, total cholesterol, diastolic BP, and log-PAI score. On excluding predicted O2max (mL · kg−1 · min−1) from the model, there was also a statistically significant (P = 0.0242) inverse relationship between plasma fibrinogen concentration and log-PAI score (Table 5, Model 2).

Table 4

Table 4

Table 5

Table 5

Among nonsmokers, (N = 630), plasma fibrinogen concentrations decreased significantly across PAI groups (P = 0.0001). The unadjusted means (95% CI) for the inactive, occasional/light, moderate/moderately vigorous, and vigorous physical activity groups were 3.24 (3.12, 3.36) 3.07 (3.00, 3.15) 3.025 (2.95, 3.10), and 2.83 g · L−1 (2.70, 2.95 g · L−1) respectively. This trend remained statistically significant after age-adjustment (P = 0.001). Scheffe post hoc tests showed that the mean age-adjusted fibrinogen concentration was significantly lower in the vigorous PAI compared with the inactive category (P < 0.05). The trend of reduced fibrinogen concentrations with higher physical activity remained significant after adjustment for age and sum of skinfolds (P = 0.009). The adjusted (age, sum skinfolds) relative difference in mean fibrinogen concentration from the sedentary to the vigorous PAI was 0.295 g · L−1 or 10.0%.

Significant differences (P < 0.00005) in plasma fibrinogen concentrations between quartiles of predicted O2max (mL · kg−1 · min−1) were found among nonsmokers (N = 635). The unadjusted mean plasma fibrinogen concentrations across quartiles of predicted O2max (<28.1, 28.1–31.9, 32.0–37.19, ≥37.2 mL · kg−1 · min−1) were 3.25 (3.16, 3.35) 3.07 (2.98, 3.17) 3.03 (2.94, 3.12), and 2.89 g · L−1 (2.81, 2.97 g · L−1), respectively. After adjustment for age, differences in mean fibrinogen concentrations across predicted O2max (mL · kg−1 · min−1) quartiles remained statistically significant (P = 0.001). Scheffe post hoc tests showed significantly lower (P < 0.05) mean fibrinogen concentrations in the highest quartile of predicted O2max (≥37.2 mL · kg−1 · min−1) compared with the lowest quartile (<28.1 mL · kg−1 · min−1). There were no significant differences in plasma fibrinogen across predicted O2max (L · min−1) categories after age-adjustment.

In these nonsmokers (N = 633) the trend toward lower fibrinogen concentrations with higher PAI categories was nonsignificant after adjustment for age and sum of skinfolds (P = 0.104). The adjusted relative difference in mean fibrinogen concentration between lower and upper quartiles of O2max (mL · kg−1 · min−1) among nonsmokers was 6%.

Significant correlation coefficients were found between both log-PAI score and predicted O2max (mL · kg−1 · min−1) with blood leukocyte count (r = −0.0998, P = 0.014 and r = −0.1514, P < 0.00005, respectively). These inverse associations remained after age-adjustment (r = −0.0856, P = 0.036, and r = −0.1307, P = 0.001, respectively).

In a final stepwise multiple regression model including all significant age-adjusted correlates of plasma fibrinogen—age, sum of skinfolds, and blood leukocytes were the only significant predictors of plasma fibrinogen (P < 0.00005, P = 0.0002, and P = 0.0229 respectively) (Table 5, Model 3). The trends of lower fibrinogen concentrations with higher PAI categories and quartiles of predicted O2max (mL · kg−1 · min−1) were no longer significant after adjustment for age, sum of skinfolds, and leukocyte count.

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Prospective studies have shown plasma fibrinogen concentration to be an independent predictor for acute myocardial infarction (17). A reduction in fibrinogen concentrations may be an important mechanism through which physical activity protects against IHD (8).

The present study has examined cross-sectionally the effects of leisure time physical activity on plasma fibrinogen concentrations. The results showed that among nonsmoking middle-aged men of high social class, higher levels of overall physical activity were associated with lower plasma fibrinogen concentrations after adjustment for the confounding effects of age and subcutaneous adiposity. Total leisure time physical activity classifications have been previously associated with lower fibrinogen concentrations within more representative studies of middle-aged men (16,19). These investigations also showed beneficial effects of high intensity/sports activity. Certain studies have reported only vigorous exercise or the highest category of exercise intensity to be associated with reduced fibrinogen concentrations in middle-aged (8,25) and elderly men (41).

Within several British studies the relationships between physical activity/leisure-time energy expenditure and plasma fibrinogen have been complicated by other social characteristics associated with physical inactivity. Physically inactive groups (particularly those refraining from higher intensity activity) have tended to constitute older men, including a larger prevalance of current smokers, fewer lifelong nonsmokers, fewer nonmanual occupations, and less unemployed (8,16). In the Caerphilly Prospective Heart Disease Study plasma fibrinogen concentration (after standardization for age, smoking, and preexisting IHD) was only significantly lower within the unemployed men (47% of the cohort), who showed greater variation in physical activity habits (16). Within the Scottish Heart Health Survey and Northern Ireland health and activity survey, the inverse relationship between plasma fibrinogen and physical activity was largely explained by age, smoking, and social class (26,27). In contrast to these investigations, vigorous exercise has been associated with reduced fibrinogen concentrations in middle-aged men after adjustment for age, BMI, smoking, and social class variables (8).

The present data showed the inverse association between predicted O2max (mL · kg−1 · min−1) and plasma fibrinogen became nonsignificant after controlling for confounding variables (age, subcutaneous skinfolds). Previous studies have shown an independent inverse relationship between cardiorespiratory fitness, as determined by directly measured or predicted O2max and fibrinogen concentrations in healthy middle-aged men (25,32). These studies considered the confounding influence of age, smoking, obesity (BMI or waist-hip ratio), and social class.

The present study is consistent with all epidemiological studies to date reporting the highest plasma fibrinogen concentrations among smokers (28). The prevalence of cigarette smoking within this socioeconomic cohort (9.8%) was substantially lower than that reported in representative British studies (approximately 35–40%) (4,8,16,27,39,47). Social class differences in fibrinogen concentrations have been shown to be almost entirely due to the large social class differences in smoking habits (4). Social network characteristics have also been associated with fibrinogen concentrations within a representative sample of middle-aged men (21). In the above study, men reporting inadequate social participation and low social support had higher fibrinogen levels, mediated by cardiorespiratory fitness and smoking, respectively.

There have been inconsistent findings on the effect of physical activity on fibrinogen concentrations within smokers. The relationship between fibrinogen concentration and social factors (including active physical pursuits) has previously been reported to be evident only in nonsmokers (37), whereas other investigations have confirmed associations between higher levels of physical activity and reduced fibrinogen concentrations in smokers (8). An inverse association between plasma fibrinogen and directly determined O2max has also previously been reported to be stronger for smokers than for nonsmokers (25).

The present study confirms various associations of plasma fibrinogen concentration with other risk indicators for IHD. There is general agreement that plasma fibrinogen concentrations increase with age (23). We found adiposity, as determined by subcutaneous skinfold measurements, to be significantly related to plasma fibrinogen concentrations. Plasma fibrinogen has been previously related to skinfold measurements (31) in addition to other anthropometric measures of obesity (7,18). In middle-aged men, the relation has been reported to be more dependent on abdominal obesity, as estimated by the waist-to-hips circumference ratio, than stature-adjusted weight (14,23,46). Physically active men have been demonstrated to have lower levels of directly determined intra-abdominal adipose fat (22). Studies in middle-aged women have shown that anthropometric estimates of obesity substantially effect the relationship between physical activity and fibrinogen concentrations (20).

In our final multivariate model, the strongest determinants (age, sum of skinfolds, and blood leukocyte count) together explained approximately 8.0% of the variation in plasma fibrinogen concentrations among nonsmokers. Within a random sample of 50- to 60-year-old nonsmoking men (45), blood leukocyte count, and abdominal adiposity (as evaluated by the waist-to-hips circumferences ratio) were also the strongest predictors of fibrinogen levels. In the above study, energy expenditure contributed an additional 2.5% of the fibrinogen variance at baseline examination.

Our findings are consistent with others indicating a significant association between blood leukocytes and plasma fibrinogen among nonsmoking men (19,36,45). Both fibrinogen and blood leukocytes are well-documented indicators of a general acute-phase response and numerous epidemiological studies have reported on associations between “inflammatory” factors and IHD (9). Blood leukocyte count was significantly inversely associated with log-PAI score and predicted O2max (mL · kg−1 · min−1) in the present study. Blood leukocyte count was also inversely related to directly determined O2max in the Kuppio IHD Risk Factor Study (24). In contrast to our findings, the ARIC study showed sporting physical activity to be significantly associated with fibrinogen concentrations after adjustment for BMI and numerous biochemical confounders, including leukocyte count (19). The interrelationships between physical activity/cardiorespiratory fitness, plasma fibrinogen, and acute-phase markers require further investigation.

Despite strong epidemiological associations, there remains no convincing evidence from controlled clinical trials that exercise training reduces plasma fibrinogen concentrations (2,34), except in patients with established IHD (42,44).

In summary, the present study confirms the relationship between both physical activity and predicted O2max (mL · kg−1 · min−1) with age-adjusted plasma fibrinogen concentrations among middle-aged, nonsmoking men of similar social class. These relationships were no longer statistically significant after adjustment for all confounding IHD risk indicators.

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