OBJECTIVES: To compare the effects of 18 months of continuous vs intermittent exercise on aerobic capacity, body weight and composition, and metabolic fitness in previously sedentary, moderately obese females.
DESIGN: Randomized, prospective, long-term cohort study. Subjects performed continuous exercise at 60–75% of maximum aerobic capacity, 3 days per week, 30 min per session, or exercised intermittently using brisk walking for two, 15 min sessions, 5 days per week.
MEASURES: Aerobic capacity, body weight, body composition, and metabolic fitness (blood pressure, lipids, glucose and insulin).
RESULTS: Significant improvements for aerobic capacity of 8% and 6% were shown for the continuous and intermittent exercise groups, respectively. Weight loss for the continuous exercise group was significant at 2.1% from baseline weight and the intermittent group was essentially unchanged. The continuous group showed a significant decrease in percentage of body fat and fat weight while the intermittent group did not. HDL cholesterol and insulin were significantly improved for both groups.
CONCLUSIONS: In previously sedentary, moderately obese females, continuous or intermittent exercise performed long-term may be effective for preventing weight gain and for improving some measures of metabolic fitness.
Obesity is a multi-component, chronic disease affecting approximately 35% of the adult population.1 Obesity is associated with comorbidities including cardiovascular disease, hypertension, diabetes, orthopedic and gait abnormalities, and some cancers.2,3 Additionally, obese individuals appear to suffer from prejudice and discrimination in the workplace and in social relationships.4,5
Obese individuals who reduce weight generally improve risk factors.6,7 Treatment for weight reduction varies; however, ultimately a negative energy balance must be established if weight is to decrease. Energy restriction may be successful for weight loss in the short term; however, weight maintenance is not sustained by the majority.8,9 Energy expenditure from exercise may be capable of inducing a state of negative energy balance although results from published meta-analytical studies are not impressive with only ∼0.3 kg of weight loss for females and 1.3 kg for males during 16 weeks of exercise.10 It is possible that studies which have used exercise to induce negative energy balance are too short in duration. Bouchard has suggested that it may take up to 2 years for a previously sedentary, moderately obese individual to be able to attain enough volume of exercise to be effective as a treatment for obesity.11
In addition to the short duration of most exercise studies, verification of exercise is frequently lacking. Many studies using exercise to produce weight loss have relied on unverified, out-patient methodology. It is possible that subjects do not participate in the prescribed amount of exercise and therefore do not achieve the desired amount of weight loss. For example, Jakicic et al found over-reporting of exercise in moderately obese females when records were compared to accelerometers.12 Those who lost the least weight over-reported to the greatest extent.
Exercise of sufficient volume to potentially alter body weight and that can be sustained by the previously sedentary, moderately overweight adult has generally been delivered in measured doses using the prescription criteria of frequency, intensity, and duration.13 A newer alternate recommendation is to have the individual accumulate exercise throughout the day by substituting physically active behaviors for sedentary behaviors.14 A frequently cited example is taking the stairs rather than the elevator. In addition, it is thought that accumulating exercise intermittently many convey some advantage of convenience compared to obtaining exercise in one continuous session; however, a direct test of this concept has not occurred.
Due to the scarcity of long-term studies of exercise, the effects on weight loss, body composition, and metabolic fitness are largely unknown. The purpose of this investigation was to compare 18 months of a traditional, prescribed program of continuous exercise, to a program of intermittent exercise. We hypothesized that both programs would be sufficient to alter body weight, body composition and measures of metabolic fitness (lipids, insulin, glucose, blood pressure) in previously sedentary, moderately overweight females.
Twenty-two subjects gave written informed consent for this study which was approved by the University of Nebraska at Kearney Institutional Review Board. We chose subjects with a body mass index above 25 as these subjects were likely to have low aerobic capacity and represented a population at risk for continued weight gain.1,15 No subject was currently engaged in a regular exercise program as determined by physical activity recall questionnaire16 or had an oxygen consumption above the ‘Fair Category’ according to the American Heart Association Fitness classifications.17
Subjects were randomized to continuous (CON, n=11) or intermittent exercise (INT, n=11). CON exercised for 30 min, at 60% to 75% of maximal aerobic capacity, 3 times per week, with direct supervision in the Human Performance Laboratory, at the University of Nebraska-Kearney. INT was instructed to walk briskly, yet comfortably at approximately 50–65% HRR, 2 times per day, 15 min per session, 5 days per week, at their home or work site. A minimum of 2 hours elapsed between exercise sessions. By design, the two programs did not provide the same amount of exercise. Rather, they were structured to provide exercise according to the traditional ACSM recommendation and the newer recommendation issued jointly by the CDC and ACSM. Thus, these moderately obese females received 2 different clinical doses of exercise, delivered as they might normally occur.
Adherence to exercise
For each exercise session, distance walked, heart rate at the end of exercise, duration of exercise, and RPE were recorded for both exercise groups. Research assistants recorded these values after each session completed in the Human Performance Laboratory for CON. For INT, supervision was provided at the subject's exercise site on a random schedule, 2 times per week to ascertain compliance. The schedule for visits from a research assistant was unknown to the subject prior to arrival. This method for supervision of outpatient exercise studies has been described previously.18
A physical exam and health history questionnaire were completed at baseline. All laboratory and behavioral measurements were obtained at baseline, 9 months, and 18 months. Laboratory tests included body composition analysis, exercise tolerance test, and blood chemistry. Following the laboratory measures, on a separate day, a 3-day diet record was obtained.
To measure aerobic capacity, the subject walked on a motor-driven treadmill for 5 min to provide acclimation to the treadmill. The subject then sat quietly until the heart rate was within 10 bpm of the resting value. Subsequently, the subject walked to volitional exhaustion. Maximal oxygen consumption was considered as the highest observed value19,20 using a modified Balke protocol with 3 min stages.21 Heart rates were recorded at the end of each stage and at maximal exertion. Prior to exercise, with the subjects seated for 5 min, blood pressure was measured until two systolic measures were within 6 mmHg and two diastolic measures were within 4 mmHg.22 Blood pressures were recorded during the last 30 seconds of each stage. Expired air was measured for oxygen and carbon dioxide at one minute intervals using a Sensormedics MMC Horizon system calibrated before each test according to the specifications of the manufacturer (Sensormedics Corp., Yorba Linda, CA).
Body composition and regional adiposity
Hydrostatic weighing (HW) at residual volume was used to estimate percent body fat.23 Residual volume was measured immediately before body density measurement by the method of Wilmore et al.24 Body density was calculated by using the equation of Goldman and Buskirk,25 and percent body fat was calculated with equation by Brozek et al.26 To estimate regional adiposity, circumferences were taken in triplicate and the closest 2 measures were averaged for subsequent calculations. Circumference measures were taken at the widest girth of the hip and the smallest girth of the waist.27
All blood samples were analyzed by a laboratory using procedures standardized by the Center for Disease Control. Blood samples were obtained following an overnight, 12 hour fast. For the oral glucose tolerance test (OGTT), blood glucose and insulin samples (5 ml each) were drawn from an indwelling catheter just before ingesting a 75 gm glucose drink (fasting), and subsequently 30, 60, 90, 120, 150 and 180 min after ingestion. During the 9 and 18-month testing periods, the OGTT was standardized to 14–18 hours post exercise. Area under the curve analysis was calculated by the trapezoidal method as described by Allison.28 Blood samples for lipid analysis (10 ml) were drawn only at baseline (fasting). After the sample was drawn, the blood was spun in a centrifuge for 5 min. Subsequently, the serum was drawn off and placed in a small tube and then kept on ice until frozen at −70° Celsius. Serum cholesterol and triglyceride concentrations were measured with an automated analyzer (Du Pont Co.), using standard enzymatic techniques. HDL was measured after removal of VLDL and LDL from samples by precipitation with phosphotungstate.29 Glucose was measured using an autoanalyzer (Beckman) and insulin was measured using a double-label antibody technique.30
Energy and macronutrient intakes were measured with 3-day food records (2 weekdays and 1 weekend day) at baseline, 9 months and 18 months. Twenty-four hour recalls were completed at 3, 6, 12, and 15 months. Subjects were instructed in recording brand name, portion size, method of preparation and ingredients and were instructed not to purposefully reduce energy intake. Analysis of energy and macronutrient consumption was completed using The Food Processor® computer program (version 4.0).31
Descriptive statistics, mean, minimum, maximum, and standard deviation were calculated for all dependent variables. Descriptive statistics were also calculated for demographic variables such as age. The basic study design was a two factor (time by treatment group) repeated measures (time) ANOVA. In the absence of a significant interaction term, a significant main effect for time required a post-hoc analysis (Duncans) whereas a main effect for treatment group did not since there were only two groups.
Descriptive characteristics for CON and INT for body weight and composition at baseline, 9 months, and 18 months are shown in Table 1. There were no significant differences between groups for body weight or body composition at any period. Body weight and fat (%fat and fat weight) decreased significantly for the continuous group across 18 months of exercise. For INT, body weight and fat (%fat and fat weight) decreased at 9 months and then returned to baseline values at 18 months. Fat-free weight was unchanged for both groups. Results for regional body fat as measured by circumferences is shown in Table 2. Small, non-significant decreases were shown at 18 months for CON for both waist and hip circumferences with essentially no change for INT. Waist-to-hip ratio showed no significant differences after 18 months of exercise.
Table 3 shows differences in the exercise prescriptions. Both groups participated in over 90% of the scheduled exercise sessions. By design, INT walked a greater distance (819±128 km) and time (8529±862 min) compared to CON (527±46 km; 5138±222 min). INT had a lower exercise heart rate of 127±13 bpm compared to CON of 142±11 bpm. The exercise heart rate difference was probably due to the exercise prescription. That is, INT was instructed to walk briskly yet comfortably while CON exercised in the Human Performance Laboratory on treadmills, usually with elevated grade. The average speed of walking for the INT group was 3.6±0.6 mph while the CON group walked at an average speed of 3.8±0.7 mph and 1.1±1.5% incline. The ACSM equation was used to estimate energy expenditure of exercise. The INT group expended 3235 kJ per week from exercise while the CON group extended 2235 kJ per week. Thus, the INT group expended approximately 31% more energy each week than the CON group, by design, due to the greater number of sessions and the greater time spent exercising per week.
Table 4 shows that both CON and INT had low VO2 max at baseline of 23.6±2.8 vs 22.9±4.2 ml·kg−1·min−1, respectively. Likewise, both groups showed modest but significant increases (∼8% for CON and ∼6% for INT) in maximal oxygen consumption across 18 months of exercise. In response to exercise, resting heart rates showed a decrease of 7 bpm for CON (P<0.05) and 3 bpm for INT (NS). Systolic blood pressure decreased 4 mmHg for CON (NS) and 14 mmHg for INT at 18 months (P<0.05).
Energy and macronutrient values from the 3-day records are shown in Table 5. Total energy did not change significantly during the duration of this study. Likewise, macronutrient composition remained unchanged with the exception of fat intake for INT which was significantly lower at 9 and 18 months compared to baseline. Data for the 24 hour recalls was similar to the 3-day food records (not shown).
Blood chemistry results are shown in Table 6. There was a significant improvement in both CON (18%) and INT (9%) for HDL-cholesterol from baseline to 18 months. Fasting values, and values using area under the curve analysis remained unchanged for both groups for glucose (mg·dl−1). However, fasting insulin (uU·ml−1) decreased from baseline to 18-months for INT (15.81±14.95; 11.90±9.38). Insulin values for area under the curve analysis (µU·L−1×10−3) decreased significantly from baseline to 18-months for both CON (14.82±5.23; 9.85±2.73) and INT (14.86±7.87; 10.79±6.90).
We evaluated the effects of two different exercise programs without energy restriction on aerobic capacity, body composition, and metabolic fitness subsequent to 18 months of intervention. One group received a traditional activity program (continuous) where an individual exercise prescription was performed in one continuous bout under supervision in the Human Performance Laboratory at the University of Nebraska-Kearney. The other group received exercise accumulated in 2, 15 min sessions with at least 2 hours between sessions (intermittent). The intermittent group performed brisk walking at home or at the workplace under supervision of research personnel according to a random schedule.
We did not include a control group in this study. There is ample literature regarding the risks of sendentary behavior.2,32 Likewise, there is ample literature which shows little change in control groups when used in exercise intervention trials.33,34,35,36 There is no reason to expect a sudden change in the general population exercise habits in a single 18-month period. Unfortunately, sedentary individuals tend to remain sedentary. Additionally, it was our purpose to compare two different exercise programs and this comparison does not call for a control group. That is, we were not comparing the results of individual receiving the proposed intermittent or continuous exercise programs to sedentary individuals. Rather, we were attempting to determine if the changes found in the intermittent exercise group using the newer CDC/ACSM recommendations would be similar to those found in the continuous exercise group using the traditional ACSM recommendations.
We are aware that the total exercise time and energy expenditure was different between groups; however, the primary question was the comparison of 2 different potential intervention strategies, not the comparison of 2 different (structure) but equal (dose) exercise programs. By design, the INT group was asked to perform more absolute work, but at a lower intensity, thus, both programs represented interventions as we envisioned how they would be used in the general adult population.
We chose not to study subjects that were below 25 BMI or above 40 BMI. Those subjects below 25 BMI may not need to reduce body weight. In our experience, subjects over 40 BMI are more likely to be on medications and have difficulty with exercise and therefore, would not be good candidates for this study.21,37
There were no differences between INT and CON for measures of weight and body composition (Table 1). Although CON lost more weight and fat weight, and reduced BMI compared to INT, the differences did not reach statistical significance. Table 1 shows that INT had improved measures of weight and fat weight at 9 months; however, showed a return to baseline by 18 months. The reason for this is uncertain since both INT and CON reported energy intake as unchange. It is likely that energy intake was under-reported as under-reporting of energy intake is notorious, especially in females.38
The magnitude of the weight loss from baseline is consistent with other investigations that used exercise without energy restriction10,39 and represents a 2.1% decrease for CON and 1.0% decrease for INT. This amount of weight loss is below the 5% to 10% which has been recommended and is thought to be associated with improvements in risk factors, especially lipids, glucose and insulin.40 Interestingly, both INT and CON showed improvements in HDL cholesterol and insulin measured by area under the curve analysis. This may indicate that exercise conveys benefits for decreased risk for cardiovascular disease and diabetes without reaching the 5% to 10% decrease in weight from baseline values.
The amount of weight loss for CON and INt was not great; however, neither group gained weight. Data from NHANES III shows weight gain for adults as they increase in age at least through the 5th decade.1 Since weight loss is difficult to maintain, prevention of weight gain may be an extremely useful strategy. In a review of weight maintenance literature, Pronk et al concluded a common element of studies showing weight maintenance was the use of exercise.41 From our results, it appears that either 90 min per week of continuous exercise at 60 to 75% of aerobic capacity, or 150 min per week of intermittent exercise using brisk walking, is capable of preventing weight gain in previously sedentary, moderately obese females.
The change in measures of metabolic fitness is noteworthy. Treatment of obesity should be targeted at improved health, not physical appearance. It could be argued that weight loss is a secondary objective and that improvement in health risk is paramount. Both groups had significant improvements in aerobic capacity. Blair et al have shown increased aerobic capacity to be an independent predictor of mortality and may have protective effects for certain comorbidities.42
Systolic blood pressure decreased for both groups and the decrease reached statistical significance for INT. HDL cholesterol increased by 15% for INT and by 9% for CON. These results exceed those reported by Bray et al where a 5% to 10% weight loss was induced by drug therapy and HDL cholesterol increased 4%. Insulin measured by area under the curve analysis showed significant decreases from baseline for both CON and INT of 34% and 28%, respectively. This is in agreement with the findings of Leon et al where insulin was reduced by 43% in males after 16 weeks of walking which resulted in a 6% decrease in body weight.43 The changes in the present study and in the study of Leon are not remarkably different from the findings of Franssila-Kallunki et al, where an 11% decrease in weight was shown in response to VLED and fasting insulin decreased 43% and insulin area under the curve analysis in response to a test meal decreased 21%. Thus, exercise in the current study was associated with changes in metabolic fitness which compare to studies using more aggressive therapies and in which weight loss was greater.
It appears that exercise may provide improvements in aerobic capacity and metabolic fitness similar to those found with more aggressive therapies and greater amounts of weight loss. Moderate exercise is infrequently associated with side effects and does not require regular medical supervision. Except for the cost of good walking shoes, exercise is cost efficient compared to many alternative therapies. These results suggest that long-term, intermittent or continuous exercise may be effective to prevent weight gain and improve some measures of metabolic fitness with previously sedentary, moderately overweight females.
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This project was supported by a grant from the American Heart Association #9507837S.
About this article
The effect of brisk walking on postural stability, bone mineral density, body weight and composition in women over 50 years with a sedentary occupation: a randomized controlled trial
BMC Women's Health (2016)
Current Obesity Reports (2013)
Annals of Behavioral Medicine (2011)
BMC Genetics (2009)
European Journal of Clinical Nutrition (2008)