Beta 1-adrenoreceptors regulate resting metabolic rate

Exploring adaptations to the modified shuttle walking test

OBJECTIVE

The 10 m modified shuttle walking test (MSWT) is recommended to determine the functional capacity in older individuals and for patients entering cardiac rehabilitation. Participants are required to negotiate around cones set 1 m from the end markers. However, consistent comments indicate that for some individuals manoeuvring around the cones can be quite difficult. Therefore, the objective of this study was to explore differences within and between non-cardiac and postmyocardial infarction (MI) males during MSWT with and without the cones.

DESIGN

Comparative study.

PARTICIPANTS

20 post-MI (64.8±6.6, range 51-74 years) and 20 non-cardiac male controls (64.1±5.7, range 52-74 years) participated.

METHODS

Participants performed MSWT with and without cones. Throughout, the participants expired air, and the heart rate (bpm) (HR) and ratings of perceived exertion (RPE) were measured. Participant protocol preference was recorded verbatim.

RESULTS

One-way analysis of variance found no significant difference in VO2 peak (cones 20.4±5.1 vs no-cones 21.9±4.8 ml/kg/min, p=0.197) or distance ambulated (cones 631.8±132.9 m vs no-cones 662.4±164.1 m, p=0.371) between protocols or groups. Analysis comparing lines of regression showed a significant trajectory difference in VO2 (ml/kg/min) (p<0.01) between protocols with higher HR (p<0.01) and respiratory exchange ratio (RER, p<0.001) values during cones. RPEs were higher for post-MIs versus controls during both protocols (p<0.05). Post-MIs taking β-blockers produce significantly lower HR values. The χ(2) analysis found no significant difference in protocol preference (no-cones: all n=25, 63%; post-MIs n=13, 65%; and controls n=12, 60%).

CONCLUSIONS

Post-MIs found both protocols subjectively harder than controls with no significant difference in the VO2 peak. However, both groups worked at a lesser percentage of their anaerobic threshold during no-cones protocol as indicated by lower RER values. Importantly, for the post-MIs, this would reduce their risk of functional impairment. Therefore, though more research is required, indicators at present are more favourable for the use of the no-cones with post-MIs.

Fat-free mass and gender influences the rapid-phase excess postexercise oxygen consumption

The purpose of this study was to determine the influence of gender dimorphism and body composition on postexercise oxygen consumption during the rapid recovery phase. We compared the rapid-phase excess postexercise oxygen consumption (EPOC) in men and women matched for age (32.1 years), physical activity status, and maximal oxygen consumption (44.7 mL·kg–1·min–1), but not for body mass or fat-free mass (FFM). All subjects exercised for 1 h at 50% of their peak capacity. Although there were differences between genders in the magnitude of the absolute oxygen consumption and EPOC during the rapid phase of recovery, there were no differences found when EPOC was corrected for FFM. We conclude that the gender differences in the absolute O2 consumption and EPOC are related to the size of the FFM.

Metabolic and physiological effects of ingesting extracts of bitter orange, green tea and guarana at rest and during treadmill walking in overweight males

OBJECTIVE

This study examined the acute effects of ingesting a widely used commercial formula containing extracts of bitter orange, green tea and guarana (Gx) on the metabolic rate and substrate utilisation in overweight, adult males at rest (study 1) and during treadmill walking (study 2).

SUBJECTS

Two different groups of 10 sedentary males with more than 20% body fat participated in studies 1 and 2.

DESIGN

In each study, subjects participated in two experimental trials during which they were given two 500 mg capsules containing either Gx or a placebo (P) in a counterbalanced double-blind manner. Doses of the main active ingredients were 6 mg of synephrine, 150 mg caffeine and 150 mg catechin polyphenols.

MEASUREMENTS

In study 1, subjects completed 7 h supine rest with baseline measures taken during the first hour, with expired gases, blood pressure, heart rate and venous blood being collected every 30 min for the remaining 6 h following ingestion of Gx or P. In study 2, subjects exercised for 60 min at 60% heart rate reserve following ingestion of Gx or P 1 h previously. Venous blood samples were collected twice at rest and at 5, 10, 15, 20, 30, 40, 50 and 60 min, with expired gas measurements taken at 4, 9, 14, 19, 29, 39, 49 and 59 min. In both studies, venous blood was analysed for NEFA, glycerol, glucose and lactate concentrations, while expired gases were used to calculate ATP production from carbohydrate and NEFA, as well as the total substrate utilised.

RESULTS AND CONCLUSION

The results did not show any significant effect of Gx ingestion on total ATP utilisation during 6 h rest or during 60 min treadmill walking. Changes were observed in the relative contributions of CHO and NEFA oxidation to ATP production in both studies, such that there was an increase in ATP production from CHO and a decrease from NEFA. The increase in CHO oxidation was shown to be as high as 30% at rest.

Effect of an aerobic exercise training program on resting metabolic rate in chronically beta-adrenergic blocked hypertensive patients

PURPOSE

There is substantial evidence that the use of beta-adrenergic receptor blockers (beta-blockers) is associated with a reduction in resting metabolic rate (RMR) and that a weight gain usually occurs in the first few months of treatment. The purpose of this study was to determine if RMR of overweight and sedentary subjects consuming beta-blockers can be increased following an aerobic exercise training program.

METHODS

Twenty-four subjects participated in the study; 11 (6 women, 5 men) were treated with beta-blockers for hypertension and 13 (9 women, 4 men) were nonmedicated (control group). Body composition, RMR, and peak oxygen uptake (VO2peak) were assessed for all subjects before and after a 12-week aerobic exercise training program. Weekly exercise energy expenditure equaled 83.68 kJ (20 kcal) per kg of body weight while exercise intensity was maintained between 60% and 70% of the VO2 reserve.

RESULTS

Body composition, RMR, and VO2peak did not differ among groups at the beginning of the study. Body weight (-1.4 kg for beta-blockers, P < .05; -2.5 kg for control, P < .05) and VO2peak (+2.2 mL x kg(-1) x min(-1) for beta-blockers, P < .05; +4.1 mL x kg(-1) x min(-1) for control, P < .001) were significantly improved in both groups, whereas RMR (-272 kJ x day(-1) for beta-blockers, NS; +573 kJ x day(-1) for control, P < .05) was increased only in the control group after the aerobic exercise training program.

CONCLUSIONS

These results suggest that beta-blockers limit the increase in RMR normally observed following an aerobic exercise training program. Consequently, obtaining a negative energy balance in an attempt to lose weight may be more difficult.

Effect of selective and nonselective beta-blockers on resting energy production rate and total body substrate utilization in chronic heart failure

BACKGROUND

In chronic heart failure (CHF) beta-blockers reduce myocardial oxygen consumption and improve myocardial efficiency by shifting myocardial substrate utilization from increased free fatty acid oxidation to increased glucose oxidation. The effect of selective and nonselective beta-blockers on total body resting energy production rate (EPR) and substrate utilization is not known.

METHODS

Twenty-six noncachectic patients with moderately severe heart failure (New York Heart Association class II or III, left ventricular ejection fraction < 0.40) were treated with carvedilol (37.5 +/- 13.5 mg/12 h) or bisoprolol (5.4 +/- 3.0 mg/d) for 6 months. Indirect calorimetry was performed before and after 6 months of treatment.

RESULTS

Resting EPR was decreased in carvedilol (5.021 +/- 0.803 to 4.552 +/- 0.615 kJ/min, P <.001) and bisoprolol group (5.230 +/- 0.828 to 4.978 +/- 0.640 kJ/min, P <.05; nonsignificant difference between groups). Lipid oxidation rate decreased in carvedilol and remained unchanged in bisoprolol group (2.4 +/- 1.4 to 1.5 +/- 0.9 mg m(2)/kg min versus 2.7 +/- 1.1 to 2.5 +/- 1.1 mg m(2)/kg min, P <.05). Glucose oxidation rate was increased only in carvedilol (2.6 +/- 1.4 to 4.4 +/- 1.6 mg m(2)/kg min, P <.05), but did not change in bisoprolol group.

CONCLUSIONS

Both selective and nonselective beta-blockers reduce total body resting EPR in noncachectic CHF patients. Carvedilol compared to bisoprolol shifts total body substrate utilization from lipid to glucose oxidation.

The major components of human energy balance during chronic beta-adrenergic blockade

PURPOSE

The authors compare the major components of energy balance--dietary food intake, resting metabolic rate, and physical energy expenditure--in patients receiving beta-blocking medications and healthy controls.

METHODS

The authors recruited subjects who were receiving beta-blocking agents (n = 22) from local hospitals and clinics. Twenty-two healthy controls were matched to each patient based on age, weight, and gender. Resting metabolic rates were determined using indirect calorimetry. Three-day dietary intakes and physical activity assessments were analyzed.

RESULTS

There were no between-group differences in calories ingested or self-reported physical activity patterns. However, resting metabolic rates were lower in the beta-blocked subjects (beta-blocked = 1,541 +/- 464 kcals/day; controls = 1,817 +/- 426 kcals/day; P < 0.05).

CONCLUSIONS

beta-adrenergic blockade causes a reduction in resting metabolic rate. Therefore, beta-blocked individuals should increase physical activity, decrease dietary intake, or both to maintain daily energy balance and prevent weight gain that accompanies a reduction in metabolic rate.

Cardio-respiratory responses to rowing ergometry and treadmill exercise soon after myocardial infarction

PURPOSE

The aim of this study was to compare the cardio-respiratory differences between rowing ergometry and treadmill exercise in beta-blocked men participating in exercise rehabilitation soon after myocardial infarction (postMI).

METHODS

Eleven males all receiving beta-blockade medication were measured for oxygen consumption (VO2), respiratory exchange ratio (RER), and rating of perceived exertion (RPE) at individualized submaximal exercise target heart rates (THR) during 6 min of exercise on each of a motorized treadmill and a rowing ergometer 2-6 wk (4.9 +/- 1.4) postMI.

RESULTS

The mean THR of the group, predetermined from an exercise ECG stress test, was 107 +/- 16 beats x min(-1). No significant difference was found between rowing versus treadmill VO2 (19.4 +/- 3.2 vs 19.7 +/- 4.2 mL x kg(-1) x min(-1); P = 0.53) or RPE (12.6 +/- 1 vs 12.7 +/- 1; P = 0.72). RER was significantly greater (P = 0.02) during rowing (0.99 +/- 0.07) compared with treadmill exercise (0.94 +/- 0.07).

CONCLUSION

Exercising at a specified submaximal THR during rowing versus treadmill exercise in beta-blocked men participating in very early cardiac rehabilitation represents the same VO2 and RPE. A significantly greater RER was, however, apparent during rowing compared with treadmill exercise; thus, agreement was shown with previous studies on healthy individuals where rowing ergometry was less metabolically efficient than treadmill exercise. The results suggest that establishing a THR from a standard treadmill stress test soon after MI is not only suitable for walking/treadmill exercise but also in setting exercise intensity for rowing ergometry.