Habitual physical activity differentially affects acute and short-term energy intake regulation in young and older adults.

BACKGROUND: Energy intake (EI) regulation is impaired in older adults, but it is not known if habitual physical activity affects accuracy of EI regulation in older compared with young adults. OBJECTIVE: We hypothesized that the ability to compensate for a high-energy yogurt preload beverage at a subsequent ad libitum meal (i.e. acute compensation) and over the course of the testing day (i.e. short-term compensation) would decrease with age, but the magnitude of the decline would be smaller in physically active compared with sedentary older adults. DESIGN: On two occasions, young active (n=15), young sedentary (n=14), older active (n=14) and older sedentary (n=11) subjects consumed either a high-energy yogurt preload beverage (YP: 500 ml, 1988 kJ, men; 375 ml, 1507 kJ, women), or no preload (NP), 30 min before an ad libitum test meal. EI at both ad libitum meals was measured, and total daily EI was determined on both testing days. Percent EI compensation for the YP was calculated for the test meal and testing day to determine acute and short-term compensation. RESULTS: Percent EI compensation at the test meal was significantly lower in the older compared with the young subjects (65+/-4 vs 81+/-4%, P=0.005). There was no effect of habitual physical activity level on acute compensation, and no age by physical activity level interaction (P=0.60). In contrast, short-term compensation was not different with age (87+/-5 vs 93+/-6%, older vs young, P=0.45), but was more accurate in active vs sedentary subjects (100+/-5 vs 79+/-6%, P=0.01). As with acute compensation, there was no age by physical activity interaction (P=0.39). CONCLUSION: Acute EI regulation is impaired in older adults, which is not attenuated by physical activity status. However, EI regulation over the course of a day is more accurate in active vs sedentary adults, which may facilitate long-term energy balance. Future work is needed to determine if higher energy expenditure in older active vs older sedentary adults improves long-term EI regulation.

Regulation of macronutrient balance in healthy young and older men.

OBJECTIVE: To determine the influence of age on the ability to adjust macronutrient oxidation to changes in diet composition. Our hypothesis was that the ability to adjust macronutrient oxidation to changes in diet composition would be impaired with age. DESIGN: Cross-sectional, randomized to three different isocaloric diets containing a constant percentage protein but varying in percentage fat and percentage carbohydrate: mixed diet (M; 15/30/55); high-fat diet (HF; 15/60/25), and high-carbohydrate (HC; 15/15/70). SUBJECTS: Six young (YM; age=25+/-1 y) and five middle-aged and older men (OM; age=63+/-3 y). MEASUREMENTS: Each subject underwent 24 h whole-room calorimetry on day 4 of each diet to determine 24 h macronutrient oxidation rates. Macronutrient balance was calculated from the individual macronutrient oxidation rates and the corresponding macronutrient intake. RESULTS: Body mass, percentage fat, and fat-free mass were similar in the two groups. Twenty-four-hour energy expenditure (EE) and energy balance did not differ across diets or between groups; 24 h EE was approximately 7% lower (NS) in the OM. Macronutrient oxidation rates were not significantly different in YM vs OM during M. Protein oxidation was similar across diets, but higher (P<0.05) in OM. Fat oxidation contributed 28.8+/-7.0% vs 37.8+/-4.7% to 24 h EE on M (NS) in the OM vs YM, respectively. This increased to 58.4+/-6.7 vs 51.9+/-5.3% of 24 h EE (NS) in the OM vs YM, respectively, during HF and decreased to 25.4+/-9.7 vs 20.2+/-14.3% (NS) during HC (diet effect, both P<0.05). Carbohydrate oxidation contributed 54.3+/-10.5% vs 56.6+/-2.4% of 24 h EE (NS) on M in the OM vs YM, respectively. This decreased to 19.5+/-10.6 vs 29.9+/-12.6% (NS) during HF and increased to 53.6+/-12.3 vs 64.7+/-14.3% (NS) in the OM vs YM, respectively during HC (diet effect, P<0.05). CONCLUSION: Taken together, these results suggest that the ability to adjust macronutrient oxidation to changes in diet composition is maintained in OM and, thus, is unlikely to contribute to the increased susceptibility to weight gain and obesity development that accompanies aging.

Gender difference in cardiovagal baroreflex gain in humans.

We tested the hypothesis that women would demonstrate lower cardiovagal baroreflex gain compared with men. If so, we further hypothesized that the lower cardiovagal baroreflex gain in women would be associated with their lower aerobic fitness and higher body fat percentage compared with men. To accomplish this, we measured cardiovagal baroreflex gain (modified Oxford technique) in sedentary, nonobese (body mass index < 25 kg/m2) men (age = 26.0 +/- 2.1 yr, n = 11) and women (age = 26.9 +/- 1.6 yr, n = 14). Resting R-R interval and diastolic blood pressure were similar in the two groups, but systolic blood pressure was lower (P < 0.05) in the women. Cardiovagal baroreflex gain was significantly lower in the women compared with the men (13.3 +/- 1.5 vs. 20.0 +/- 2.8 ms/mmHg, P < 0.05). The lower cardiovagal baroreflex gain in the women was not related (P > 0.05) to their lower aerobic fitness and was only marginally related to their higher body fat percentage (r = -0.34, P < 0.05). There were no gender differences in the threshold and saturation, operating range, or operating point (all P > 0.05), although the operating point fell significantly to left (i.e., at a lower systolic blood pressure) compared with men. Therefore, the findings of this study suggest that the gain of the cardiovagal baroreflex is reduced whereas other parameters were similar in women compared with men. The mechanisms responsible for the reduced cardiovagal baroreflex gain remain unclear.

Blood pressure reductions with exercise and sodium restriction in postmenopausal women with elevated systolic pressure: role of arterial stiffness.

OBJECTIVES: This study determined the relative efficacy of aerobic exercise (daily walking) and moderate dietary sodium restriction (sodium intake <100 mmol/day) for reducing systolic blood pressure (SBP) and pulse pressure (PP) in postmenopausal women with elevated initial levels, and the potential role of reductions in large artery stiffness in these changes. BACKGROUND: Lifestyle behaviors are recommended for lowering blood pressure (BP) in adults with elevated baseline levels, but there is little information as to the relative efficacy of different interventions or the mechanisms underlying their potential beneficial effects. METHODS: After baseline measurements and random assignment, 35 nonmedicated healthy postmenopausal women with SBP between 130 and 159 mm Hg completed three months of either aerobic (walking) exercise (n = 18; 62 +/- 9 years, mean +/- SD) or moderate dietary sodium restriction (SR) (n = 17; 65 +/- 10 years, mean +/- SD). RESULTS: Body mass and composition, plasma volume, and fasting concentrations of metabolic coronary risk factors did not differ between the groups at baseline or change with intervention. Systolic BP and PP at rest decreased with both exercise and SR (p < 0.05); however, the reductions were three- to fourfold greater with SR (p < 0.05). Sodium restriction, but not exercise, also reduced 24-h SBP and PP (p < 0.05). Aortic pulse wave velocity (PWV) and carotid augmentation index were reduced only with SR (p < 0.05). Changes in SBP and PP at rest and over 24 h correlated with the corresponding changes in aortic PWV (r = 0.53 to 0.61, p < 0.01). CONCLUSIONS: Moderate SR lowers SBP and PP in postmenopausal women with elevated baseline levels more than does daily walking. The greater blood pressure reductions with SR may be mediated in part by a decrease in the stiffness of the large elastic arteries.

Total blood volume in endurance-trained postmenopausal females: relation to exercise mode and maximal aerobic capacity.

We have recently shown that postmenopausal female distance runners demonstrate elevated levels of blood volume compared with sedentary healthy peers. We also found a strong positive relation between blood volume and maximal oxygen consumption. In young adult males, endurance exercise training increases blood volume when performed in the upright, but not in the supine body position. Based on these observations, we hypothesized that among postmenopausal females, the elevation in blood volume would be absent or attenuated in women who train in the horizontal vs. upright body position, and that the lower blood volume in the former would be associated with lower maximal aerobic capacity. Thus, we measured supine resting plasma and total blood volumes (Evans blue dye) and maximal oxygen consumption in postmenopausal women: 10 sedentary controls, 10 swimmers and 10 runners matched for age (60 +/- 2; 59 +/- 2; 58 +/- 2 years, mean +/- SE) and hormone replacement use (5 per group). The swimmers and runners were further matched for training volume (4.5 +/- 0.2 vs. 4.8 +/- 0.6 h week-1), relative performance (78 +/- 5 vs. 75 +/- 3% of age-group world record) and fat-free mass (45.5 +/- 0. 8 vs. 44.9 +/- 1.5 kg). Total blood volume and maximal oxygen consumption were highest in the runners (81.2 +/- 4; 52.4 +/- 3 mL kg-1, respectively) and progressively lower in the swimmers (68.8 +/- 3; 44.2 +/- 2) and controls (59.2 +/- 2; 37.9 +/- 2; all P < 0. 05). In the pooled population, blood volume was positively related to maximal oxygen consumption (r = 0.72, P < 0.0001). We conclude that in endurance-trained postmenopausal females matched for training volume and competitive performance: (1) blood volume is lower in those who train in the horizontal (swimmers) compared with the upright position (runners); (2) the lower blood volume is associated with a lower maximal aerobic capacity. Nevertheless, blood volume and maximal oxygen consumption are higher in postmenopausal women who train in the horizontal position than in sedentary controls.

Influence of gender on the sympathetic neural adjustments to alterations in systemic oxygen levels in humans.

The purpose of this investigation was to determine whether gender influences the muscle sympathetic nerve activity (MSA) and systemic cardiovascular adjustments to alterations in systemic oxygen levels. To accomplish this, we performed direct (intraneural) measurements of muscle sympathetic nerve activity in 11 male and seven female young healthy adults during room air breathing, moderate isocapnic hypoxaemia and hyperoxaemia. During hypoxaemia, arterial oxygen saturation declined similarly in men and women. The magnitudes of the peak increases in MSA and stimulus-response 'gain' were not different between groups. However, the women had a shorter latency of response (P < 0.05). Women also demonstrated a greater increase in heart rate and a modest elevation in diastolic blood pressure, whereas the ventilatory responses were identical in the two groups. During normoxic recovery, MSA returned to baseline more quickly in women than in men (P < 0.05). During hyperoxaemia, muscle sympathetic nerve activity decreased only in the men (P < 0.05). Heart rate decreased slightly (P < 0.05) in both men and women, whereas blood pressure and minute ventilation were unchanged from normoxic control levels. Our findings fail to support an effect of gender on the peak muscle sympathetic nerve activity response to moderate isocapnic hypoxaemia in healthy young adult humans, although women demonstrate a shorter latency for sympathoexcitation and recovery under these conditions. In response to hyperoxaemia, women fail to demonstrate the sympathoinhibition consistently observed in men, possibly because of the low resting levels of MSA characteristic of young adult women. Thus, gender appears to contribute to the interindividual variability in sympathetic and cardiovascular responses to alterations in systemic oxygen levels.

Cardiopulmonary baroreflex inhibition of sympathetic nerve activity is preserved with age in healthy humans.

1. We tested the hypothesis that the ability of the cardiopulmonary baroreflex to produce sympathoinhibition is reduced with age in humans. Eleven young (23 +/- 1 years, mean +/- s.e.m.) and ten older (64 +/- 1) healthy adult males were studied under supine conditions (control) and in response to cardiopulmonary baroreflex stimulation evoked by acute central circulatory hypervolaemia (10 deg head-down tilt). The two groups were normotensive and free of overt cardiovascular disease. 2. Supine baseline (control) levels of efferent muscle sympathetic nerve activity (MSNA) burst frequency were twice as high in the older men (41 +/- 2 vs. 21 +/- 2 bursts min-1, P < 0.05). In both groups in response to head-down tilt arterial blood pressure and heart rate were unchanged, peripheral venous pressure (PVP) increased (P < 0.05), MSNA total activity decreased (P < 0.05), antecubital venous plasma noradrenaline concentrations did not change significantly, and forearm blood flow and vascular conductance increased (vascular resistance decreased) (all P < 0.05). The mean absolute DeltaMSNA/DeltaPVP was similar in the young and older men, although the higher control levels of MSNA in the older men resulted in a smaller percentage DeltaMSNA/DeltaPVP (P < 0.05). Per DeltaPVP, the reduction in forearm vascular resistance was smaller in the older men, but there were no age group differences when expressed as increases in forearm vascular conductance. 3. These results indicate that the ability of the cardiopulmonary baroreflex to inhibit MSNA is well preserved with age in healthy adult humans. As such, these findings are not consistent with the concept that this mechanism plays a role in the age-associated elevation in basal MSNA.

Elevated heart rate variability in physically active young and older adult women.

1. Low heart rate variability is associated with an increased risk of cardiac sudden death, coronary heart disease and all-cause mortality. We have previously shown that physically active postmenopausal women demonstrate higher levels of heart rate variability and cardiac baroreflex sensitivity compared to their sedentary peers. The purpose of the present prospective study was to test the hypothesis that heart rate variability and cardiac baroreflex sensitivity would be reduced with age in sedentary but not physically active women. To accomplish this, we measured heart rate variability (both time and frequency domain) and spontaneous cardiac baroreflex sensitivity (SBRS, sequence method) in the sitting posture in 23 sedentary women [11 premenopausal and 12 postmenopausal (age, 28 +/- 1 and 61 +/- 2 years; VO2max, 35.3 +/- 1.4 and 21.7 +/- 1.5 ml.min-1.kg-1 respectively] and in 22 physically active women [12 premenopausal and 10 postmenopausal (age, 31 +/- 1 and 59 +/- 2 years; VO2max, 52.5 +/- 1.4 and 39.7 +/- 1.8 ml.min-1.kg-1)]. 2. The S.D. of the R-R interval (time domain) was reduced (P < 0.05) with age in both sedentary (52 +/- 6 versus 33 +/- 4 ms) and physically active women (72 +/- 8 versus 49 +/- 9 ms). The high-frequency power (3740 +/- 1527 versus 915 +/- 188 and 9516 +/- 2849 versus 2803 +/- 1083 ms2/Hz), total power of heart rate variability and SBRS (11 +/- 2 versus 7 +/- 2 and 19 +/- 3 versus 13 +/- 2 ms/mmHg) also demonstrated similar age-related reductions in sedentary and physically active women, respectively (all P < 0.05). The S.D. of the R-R interval, high-frequency and total power of heart rate variability, and SBRS were higher (all P < 0.05) in the physically active compared with the sedentary women at any age. There was no significant influence of age or physical activity status on the low-frequency power of heart rate variability. In addition, no significant differences in any of the time or frequency domain measures of heart rate variability or SBRS were observed in users compared with non-users of hormone replacement therapy. 3. The results of the present study suggest that heart rate variability and cardiac baroreflex sensitivity decline similarly with age in healthy sedentary and physically active women. However, physically active women demonstrate higher levels of heart rate variability and cardiac baroreflex sensitivity compared with their sedentary peers, regardless of age.

Influence of age on arterial baroreflex inhibition of sympathetic nerve activity in healthy adult humans.

Resting levels of muscle sympathetic nerve activity (MSNA) increase markedly with age in healthy adult humans. An age-related reduction in arterial baroreflex inhibition of MSNA could contribute to these elevations. To test this hypothesis, we measured MSNA using peroneal microneurography in young (age, 25 +/- 1 yr; n = 8) and older (69 +/- 1 yr; n = 7) healthy normotensive men before (baseline control) and during graded constant infusion of phenylephrine hydrochloride (0.5-2.0 microgram . kg-1. min-1) that produced a sustained approximately 10-mmHg increase in arterial blood pressure. Central venous pressure was controlled at baseline levels (+/-1 mmHg) using lower body negative pressure. Resting MSNA was approximately 95% higher in the older compared with the young subjects (43 +/- 5 vs. 22 +/- 3 bursts/min; P < 0.05). However, arterial baroreflex MSNA inhibitory responsiveness was similar in the older compared with the young subjects (254 +/- 112 vs. 259 +/- 40 arbitrary integration units/mmHg, respectively), although the percent reduction in MSNA was smaller in the older men (8.9 +/- 0.7 vs. 5.2 +/- 1.1%/mmHg), due to their elevated baseline levels. The reflex increase in the R-R interval was not different in the two groups (13 +/- 10 vs. 16 +/- 7 ms/mmHg). In summary, our findings suggest that arterial baroreflex inhibition of MSNA is preserved with age in healthy normotensive adult humans. As such, this mechanism does not appear to contribute to the age-related rise in tonic MSNA.

Smaller differences in total and regional adiposity with age in women who regularly perform endurance exercise.

Our aim was to determine if women who regularly perform endurance exercise demonstrate age-related elevations in body mass and adiposity. Ninety-five healthy females were studied: premenopausal (n = 28; mean +/- SE age 30 +/- 1 yr) and postmenopausal (n = 31; 56 +/- 1 yr) endurance-trained runners and premenopausal (n = 17; 29 +/- 1 yr) and postmenopausal (n = 19; 61 +/- 1 yr) sedentary controls. In the runners, body mass did not differ across age, but percent fat and fat mass were higher (P < 0.05) in the postmenopausal women. The age-related difference in total body fat, however, was only approximately 50% as great (P < 0.01) as that observed in the sedentary controls due in part to smaller age-related differences in central (truncal) fat. The higher fat mass in the postmenopausal runners was modestly (inversely) related to both exercise volume (r = -0.44, P < 0.01) and maximal oxygen consumption (r = -0.41, P < 0.01). The present findings provide experimental support for the hypothesis that women who regularly engage in vigorous endurance exercise may not gain body weight, undergo only a modest increase in total body fat, and do not demonstrate a significant elevation in central adiposity with age.

Role of central circulatory factors in the fat-free mass-maximal aerobic capacity relation across age.

Fat-free mass (FFM) (primarily skeletal muscle mass) is related to maximal aerobic capacity among healthy humans across the adult age range. The basis for this physiological association is assumed to be a direct relation between skeletal muscle mass and its capacity to consume oxygen. We tested the alternative hypothesis that FFM exerts its influence on maximal aerobic capacity in part via an association with central circulatory function. To do so, we analyzed data from 103 healthy sedentary adults aged 18-75 yr. FFM was strongly and positively related to maximal oxygen consumption (r = 0.80, P < 0. 001). FFM was also strongly and positively related to supine resting levels of blood volume (r = 0.79, P < 0.001) and stroke volume (r = 0.75, P < 0.001). Statistically controlling for the collective influences of blood volume and stroke volume abolished the tight relation between FFM and maximal oxygen consumption (r = 0.12, not significant). These results indicate that 1) FFM may be an important physiological determinant of blood volume and stroke volume among healthy sedentary adult humans of varying age; and 2) this relation between FFM and central circulatory function appears to represent the primary physiological basis for the strong association between FFM and maximal aerobic capacity in this population. Our findings suggest that sarcopenia (loss of skeletal muscle mass with aging) may contribute to the age-related decline in maximal aerobic capacity primarily via reductions in blood volume and stroke volume rather than a direct effect on the oxygen-consuming potential of muscle per se.

Augmented cardiopulmonary and integrative sympathetic baroreflexes but attenuated peripheral vasoconstriction with age.

Based on observations of smaller increases in limb vascular resistance during acute incremental hypovolemia in older adults, cardiopulmonary and integrative (combined cardiopulmonary and arterial) baroreflex control of sympatho-circulatory function is thought to be impaired with aging in humans. We tested this hypothesis directly by making intraneural measurements of skeletal muscle sympathetic nerve activity (MSNA; peroneal microneurography) in groups of young (23+/-1 years; n=11) and older (64+/-1 years; n=12) healthy adult men during progressive hypovolemia produced by graded (-5 to -40 mm Hg) lower body negative pressure (LBNP). Baseline levels of MSNA and arterial blood pressure were higher and heart rate was lower in the older subjects (P<0.05). Lower levels of LBNP (-5 to -20 mm Hg) did not affect arterial blood pressure or heart rate in either group; systolic and pulse pressures declined during higher levels of LBNP (-30 and -40 mm Hg) but only in the young subjects (P<0.05). Graded LBNP evoked progressive, linear reductions in peripheral venous pressure (PVP) and increases in MSNA, plasma norepinephrine concentration (PNE), and forearm vascular resistance (FVR) in both groups (all P<0.05). DeltaMSNA/ deltaPVP was approximately 150% greater in the older versus young men during both lower and higher levels of hypovolemia (P<0.01); however, deltaFVR/deltaPVP was approximately 50% smaller in the older men (P<0.05). There was no difference in the MSNA-PNE relation with age, but deltaFVR/deltaMSNA was approximately 65% to 70% smaller in the older subjects (P<0.05). Our findings indicate that cardiopulmonary and integrative baroreflex control of central sympathetic outflow during hypovolemia is augmented, not impaired, with age in healthy humans. However, the reflex-mediated increases in limb vascular resistance during hypovolemia are smaller in older adults because of attenuated vasoconstrictor responsiveness to sympathetic stimulation.

Low-frequency arterial pressure fluctuations do not reflect sympathetic outflow: gender and age differences.

Low-frequency arterial pressure oscillations (Mayer waves) have been proposed as an index of vascular sympathetic outflow. However, cross-sectional differences in these pressure oscillations may not reflect different levels of sympathetic nervous outflow in humans. Three groups of healthy subjects with characteristically different sympathetic nervous outflow were studied: young females (n = 10, 18-28 yr), young males (n = 11, 18-29 yr), and older males (n = 13, 60-72 yr). Average R-R interval, arterial pressures, and systolic pressure variability at the Mayer wave frequency (0.05-0.15 Hz) did not differ among the three groups. Diastolic pressure Mayer wave variability was similar in young females vs. young males (39 +/- 10 vs. 34 +/- 5 mmHg2) and lower in older males vs. young males (14 +/- 2 mmHg2; P < 0.05). In contrast, muscle sympathetic activity was lowest in young females (892 +/- 249 total activity/min) and highest in older males (3,616 +/- 528 total activity/min; both P < 0.05 vs. young males: 2,505 +/- 285 total activity/min). Across the three groups, arterial pressure Mayer wave variability did not correlate with any index of sympathetic activity. Our results demonstrate that arterial pressure Mayer wave amplitude is not a surrogate measure of vascular sympathetic outflow.

Reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output in humans at rest.

The specific aims of the present study were to determine: (1) the day-to-day reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output under resting conditions; (2) the reproducibility of this technique among subjects differing in gender and age; and (3) the number of trials within a session necessary to maximize the day-to-day reproducibility of the technique. To address these aims, cardiac output was measured in 21 healthy men (n = 8) and women (n = 13) between the ages of 25 and 71 years in the supine posture on two separate days. Mean levels of cardiac output at rest were similar on day 1 vs. day 2 in the overall group. Cardiac output measured on day 1 was highly correlated (r = 0.98, P < 0.001) with cardiac output measured on day 2. The day 1 to day 2 mean difference in cardiac output for the individual subjects was < 4%. The mean levels of heart rate and stroke volume also were similar between day 1 and day 2. The relation between cardiac output measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group. The mean absolute difference among the three rebreathing trials within a day was 360 ml min-1 in the overall group, with a coefficient of variation of 7%. The variability between rebreathing trials measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group. The reliability of cardiac output measured on different days was excellent with a single rebreathing trial (r = 0.93) and improved significantly up to three trials (r = 0.98). In conclusion, the findings of the present study indicate that the acetylene rebreathing technique can be a highly reproducible method for measuring cardiac output under resting conditions. The reproducibility is consistently strong in healthy humans of varying age and in both genders, and is enhanced by the use of multiple trials.

Relations of total and abdominal adiposity to muscle sympathetic nerve activity in healthy older males.

OBJECTIVES: We recently reported that skeletal muscle sympathetic nerve activity (MSNA) is related to total body and abdominal fatness in a pooled population of young and older males. Both MSNA and adiposity increase with age. Thus, it is not clear if the relation between MSNA and adiposity exists among older adults and if the age-related increase in MSNA is explained by increases in adiposity. We therefore tested the hypotheses that: 1) among older men, those with higher total body fatness and abdominal adiposity have higher MSNA and 2) MSNA is not different in healthy young and older men with similar total body and/or abdominal fatness. DESIGN: Older healthy men (63 +/- 1 y) were separated into higher and lower groups of body fat (26.9 +/- 0.8%, n = 9 vs 21.3 +/- 1.1, n = 10; P < 0.0001) and waist circumference (96.4 +/- 3.5 cm, n = 8 vs 86.2 +/- 1.5, n = 8; P < 0.01). Younger controls (26 +/- 1 y) were then matched with those in the older-lower groups for %body fat (21 +/- 1.1%, n = 10) or waist circumference (86.2 +/- 0.8 cm, n = 10). MEASUREMENTS: Total body fat was determined by hydrodensitometry, abdominal adiposity by waist circumference and resting MSNA by microneurography. RESULTS: Among the older subjects those in the higher %body fat and waist circumference groups had higher (P < 0.02) MSNA (47 +/- 3 and 48 +/- 4 bursts/min, respectively) than those in the lower groups (37 +/- 2 and 38 +/- 3 bursts/min). MSNA was directly related to %body fat (r = 0.52, P = 0.03) and waist circumference (r = 0.64, P = 0.007) in the older groups. MSNA was greater (P < 0.001) in the older-lower groups than in the young controls matched for %body fat (23 +/- 2 bursts/min) or waist circumference (24 +/- 3 bursts/min). CONCLUSIONS: 1) among healthy older men, higher levels of total body and/or abdominal adiposity are associated with higher levels of MSNA and 2) the age-related elevation in MSNA is reduced but not abolished when differences in adiposity are eliminated.

Regular exercise and the age-related decline in resting metabolic rate in women.

A low resting metabolic rate (RMR) is a risk factor for future weight gain. We tested the hypothesis that the age-related decline in RMR in sedentary women is not observed in women who exercise regularly. Sixty-five healthy, weight-stable women, aged 21-35 or 50-72 yr, were studied: 12 premenopausal and 15 postmenopausal sedentary women, 13 premenopausal and 15 postmenopausal distance runners, and 10 endurance-trained postmenopausal swimmers. RMR was measured by indirect calorimetry (ventilated hood system) after an overnight fast, and values were adjusted for fat mass and fat-free mass (RMRadj). The RMRadj was approximately 10% lower in the postmenopausal vs. premenopausal sedentary women (52 +/- 2 vs. 57 +/- 2 Cal/h; P < 0.002). In contrast, RMRadj was not significantly different in the premenopausal (59 +/- 2 Cal/h) and postmenopausal (57 +/- 1 Cal/h) distance runners. The postmenopausal swimmers had a RMRadj (57 +/- 2 Cal/h) identical to that of the postmenopausal runners, suggesting a generalized influence of the endurance exercise-trained state in postmenopausal women. Group differences in RMRadj were not associated with differences in total energy intake or composition or with plasma concentrations of norepinephrine, T3, or T4. However, maximal oxygen consumption (aerobic fitness) accounted for 35% of the individual variance in RMRadj in the overall population (r = 0.59; P < 0.001). Our results are consistent with the concept that the age-related decline in RMR in sedentary women is not observed in women who regularly perform endurance exercise. The elevated level of RMR observed in middle-aged and older exercising women may play a role in their lower levels of body weight and fatness compared to those in sedentary women.

Plasma fibrinogen levels in healthy postmenopausal women: physical activity and hormone replacement status.

BACKGROUND: Fibrinogen is a major component of the coagulation system and a powerful independent risk factor for cardiovascular disease in postmenopausal women. Regular physical activity has been recommended as an effective clinical approach to lowering plasma fibrinogen levels; currently, however, there are little or no data to support a relationship between habitual exercise status and plasma fibrinogen levels in healthy postmenopausal women who either use or do not use hormone replacement therapy (HRT). METHODS: Plasma fibrinogen levels were measured in 20 physically active (56 +/- 1 yr) and 31 sedentary (58 +/- 1 yr) healthy postmenopausal women. Nine (45%) physically active and 15 (48%) sedentary women had been using HRT for > 1 year; the others were nonusers of HRT. RESULTS: Plasma fibrinogen levels were approximately 15% lower (p = .001) in the physically active women (2.48 +/- .08 g/L) than the sedentary controls (2.92 +/- .06 g/L) and approximately 7% lower (p = .04) in the users (2.65 +/- .08 g/L) versus nonusers (2.84 +/- .08 g/L) of HRT. Moreover, the lower (0.4 g/L) plasma fibrinogen levels associated with regular physical activity were evident in both the users (2.39 +/- .11 vs 2.80 +/- .08 g/L, p = .001) and nonusers (2.56 +/- .11 vs 3.03 +/- .08 g/L, p = .006) of HRT. Stepwise multiple regression analysis revealed that percent body fat was the primary determinant of plasma fibrinogen levels, accounting for 30% of the variability. CONCLUSIONS: Regular physical activity is associated with lower plasma fibrinogen levels in postmenopausal women; the lower plasma fibrinogen levels associated with regular physical activity are evident in both users and nonusers of HRT; and plasma fibrinogen levels are positively related to percent body fat in postmenopausal women differing in physical activity and HRT status. Lower plasma fibrinogen levels in physically active postmenopausal women may contribute to their lower risk of cardiovascular disease.

Systemic hemodynamic determinants of blood pressure in women: age, physical activity, and hormone replacement.

We tested the hypothesis that the age-related changes in systemic hemodynamic determinants of arterial blood pressure in healthy women are related to physical activity and hormone replacement status. We studied 66 healthy, normotensive premenopausal (21-35 yr) and postmenopausal (50-72 yr) sedentary and endurance-trained women under supine resting conditions. Mean blood pressure was 7 mmHg higher in sedentary post- compared with premenopausal women, which was associated with an 11-mmHg higher systolic blood pressure, a 25% lower stroke volume and cardiac output, and a 50% higher systemic vascular resistance (all P < 0.05). Absolute (ml) levels of total blood volume did not differ across age, but resting oxygen consumption was approximately 35% lower in the postmenopausal women (P < 0.05). The elevations in mean and systolic blood pressures with age were similar in endurance-trained runners, but, in contrast to the sedentary women, the elevations were not associated with significant age-related differences in cardiac output, stroke volume, or oxygen consumption, and only a modest (15%) increase in systemic vascular resistance (P = 0.06). Postmenopausal swimmers demonstrated the same systemic hemodynamic profile as that of postmenopausal runners, indicating a nonspecific influence of the endurance-trained state. Blood pressure and its systemic hemodynamic determinants did not differ in postmenopausal users compared with those of nonusers of hormone replacement therapy. Resting oxygen consumption was the strongest physiological correlate of cardiac output in the overall population (r = 0.65, P < 0.001). We conclude that 1) the increases in arterial blood pressure at rest with age in healthy normotensive women are not obviously related to habitual physical activity status; 2) the systemic hemodynamic determinants of the age-related elevations in blood pressure are fundamentally different in sedentary vs. active women, possibly due, in part, to an absence of decline in resting oxygen consumption in the latter; and 3) systemic hemodynamics at rest are not different in healthy normotensive postmenopausal users vs. nonusers of estrogen-based hormone replacement.

Influence of age on the sympathetic neural adjustments to alterations in systemic oxygen levels in humans.

We tested the hypothesis that aging influences the regulation of sympathetic nervous system activity (SNA) and arterial blood pressure during alterations in systemic O2 levels in humans. To accomplish this, we performed direct (intraneural) measurements of SNA to skeletal muscle (MSNA) in 10 young and 7 older healthy normotensive men during room air breathing (normoxic-control), moderate isocapnic hypoxemia [15 min of 10% fractional inspired O2 (FIO2)], and hyperoxemia (10 min of 50% FIO2). After hypoxemia, arterial O2 saturation (SaO2) declined similarly in the young and older men. MSNA (burst frequency and total minute activity) increased significantly (P < 0.05) in both groups. The magnitudes of the absolute increases in MSNA and the delta MSNA/delta SaO2 were not significantly different in the young and older men; however, because of higher normoxic baseline levels, the percentage increases in burst frequency were smaller (P = 0.02) and those for total minute activity tended to be smaller (P = 0.11) in the older men. Arterial blood pressure increased modestly (P < 0.05) and similarly in both groups, although the older men demonstrated a smaller increase in heart rate. After hyperoxemia [corrected], SaO2 increased and MSNA decreased (both P < 0.05) similarly in the young and older men. Arterial blood pressure did not change significantly from normoxic control levels in either group; however, a small (P < 0.05) reduction in heart rate was observed in both groups. In conclusion, aging does not obviously influence the regulation of absolute levels of MSNA or arterial blood pressure during alterations in systemic O2 levels in healthy men, although older men demonstrate a smaller percentage increase in MSNA from their elevated baseline levels, as well as an attenuated tachycardia in response to acute hypoxemia. As such, the present results are consistent with our previous findings on aging and sympathocirculatory control during other types of acute stress in humans.

Effect of regular aerobic exercise on elevated blood pressure in postmenopausal women.

The efficacy of aerobic exercise for lowering arterial blood pressure (BP) in postmenopausal women with elevations of 130 to 159/85 to 99 mm Hg has not been established. To determine this, 10 postmenopausal women with high normal resting BP or stage I essential hypertension were studied throughout a 12-week lead-in period (no exercise, n = 5) and/or 12 weeks of moderate-intensity aerobic exercise (walking, n = 9). There were no significant time effects during the lead-in period (all p >0.4). Maximal aerobic capacity (as assessed by maximal oxygen consumption) was unchanged after 12 weeks of exercise, but exercise tolerance (treadmill walking time) increased by approximately 10% (p <0.05). Body weight, dietary intake and composition, and urinary sodium excretion were unchanged before versus after exercise training. After 12 weeks of exercise, systolic and diastolic BP at rest were significantly lowered by 10/7 and 12/5 mm Hg, respectively, in the sitting and standing positions (p <0.001); some (> or = 3 to 5 mm Hg) decrease in BP was observed in every subject. On average, subjects with stage I hypertension had a reduction in BP into the high normal range, whereas subjects with high-normal initial levels had a reduction in BP into the normal range. Borderline significant (p = 0.06 to 0.07) reductions in systolic and diastolic BP were observed by the end of the second and tenth weeks of training, respectively. Ambulatory determined 24-hour levels of BP were unchanged with training, but significant reductions in BP during submaximal exercise occurred. Our results demonstrate that regular aerobic exercise can produce clinically important reductions in resting BP in Caucasian postmenopausal women with mild to moderately elevated initial levels. This effect of exercise is observed in the absence of changes in maximal aerobic capacity, body weight, or dietary intake.