The prevalence of obesity, a major risk factor in the development of type 2 diabetes and increased morbidity and mortality, has increased rapidly in recent decades (7,27). Exercise represents an important intervention for weight loss through the potential to raise energy expenditure, assuming that energy intake is not increased concomitantly (5). However, the results of some exercise interventions are disappointing because some participants do not achieve the theoretical weight loss associated with the expected increase in total energy expenditure (TEE) (30,32). One possible cause of this is compensation for the energy expenditure during the exercise sessions through a reduction in energy expenditure from nonexercise activity thermogenesis (NEAT) (12). As a result of this compensation, TEE is unchanged, and thus weight loss may not occur (10).
NEAT is defined as the energy expenditure of all physical activities other than volitional exercise, such as the activities of daily living, small muscle movements, spontaneous muscle contraction, and postural maintenance (17). The role of NEAT in the development of obesity has been highlighted by the fact that a failure to increase NEAT levels may result in fat gain in susceptible individuals and the maintenance of leanness in others (17,18). The energy cost difference because of time spent in different postures such as standing, sitting, or lying may be as large as 2000 kcal·d−1 between two people of similar body size (16). The variability in fat gain has been found to be inversely related to a participant's ability to increase NEAT (17), suggesting that promoting increased NEAT along with structured exercise may be a promising strategy to increase overall TEE and thus weight loss.
Although several studies have observed that participants reduce their NEAT during the remainder of the day or on nonexercise days as a response to exercise training (6,11,15,19,22,23,25,28), the factors that may influence this compensatory response remain unknown. A possible factor that may affect how individuals respond to exercise is the intensity at which the exercise is performed. In elderly cohorts, it has been observed that an addition of structured exercise training to the day was compensated for by a significant decrease in NEAT (6,22). The proposed reason for this compensatory decline in NEAT was the high level of exercise intensity. However, because a comparison with NEAT after moderate- or low-intensity exercise was not undertaken, the influence of exercise intensity per se in elderly participants could not be confirmed. Evidence of the differential impact of exercise intensity on NEAT has been shown in a study of obese boys. Kriemler et al. (15) examined the impact of a single exercise bout on energy expenditure and NEAT in 14 obese children (10-15 yr). NEAT on the day after the exercise session was increased when exercise was of moderate intensity but decreased after an intense bout of exercise. Further, the limited duration used to measure NEAT in this study (1 d before and 1 d after the single bout of exercise) may not provide a reliable estimate of usual daily physical activity (29) or could fail to detect a delayed change in NEAT because of the brevity of the measurement period after the exercise. It is commonly recognized that delayed onset of muscle soreness (DOMS) can persist for days after an unaccustomed exercise bout (2,9). In a review of the literature, Cheung et al. (2) explain that the intensity of discomfort increases within the first 24 h after cessation of exercise, peaks between 24 and 72 h, and subsides and eventually disappears by 5-7 d postexercise. Therefore, measuring NEAT for a longer duration (e.g., across a 7-d monitoring protocol) will provide more information about the influence of different exercise intensities on NEAT and its patterns relative to time (29). The aim of this study, therefore, was to determine whether an acute laboratory-based intervention comparing moderate- and high-intensity exercise would differentially influence NEAT in overweight and obese adults. NEAT was assessed using an accelerometer for 3 d before, on the day of, and 3 d after the exercise sessions. It was hypothesized that both walking exercise bouts would decrease NEAT on the day of the exercise and 3 d after the structured exercise session. It was further hypothesized that the two walking exercise bouts would affect NEAT differentially, with the high-intensity session resulting in greater compensation.
Sixteen overweight and obese male adults (mean ± SD: age = 26.5 ± 3.0 yr, height = 170 ± 8.4 cm, body mass = 87.3 ± 22.0 kg, body mass index = 30.0 ± 5.6 kg·m−2) took part in this study. All participants were sedentary and participated in less than 1 h·wk−1 of physical activity. Written informed consent was obtained for all participants before the start of this study. Approval was obtained from the Human Research Ethics Committee of the Queensland University of Technology.
This study was conducted using a crossover repeated-measures design in which each participant served as his own control (Fig. 1). Participants took part in two different conditions: moderate-intensity walking exercise (MIE) and high-intensity walking exercise (HIE). In both conditions, NEAT was measured via accelerometry for three consecutive days before the walking exercise manipulation, on the day of, and for the following 3 d after the walking exercise bout.
A single walking exercise bout at moderate and high intensities.
Two walking exercise bouts at either moderate or high intensity were performed between 6:00 and 8:00 a.m. 1 wk apart. The MIE session consisted of walking for 60 min on a motorized treadmill at 6 km·h−1. The HIE session consisted of 60 min of interval walking at 6 km·h−1 and a 10% grade for 5 min followed by 5 min at 0% grade; the theoretical vertical displacement was 300 m. We have previously shown that this walking speed occurs between the "walking for pleasure" pace and the maximal walking speed over a 2-km trial for obese adults (8). All participants, with the exception of one individual (body mass index = 45 kg·m−2) who walked at 5 km·h−1, were able to walk for 60 min at 6 km·h−1. A Polar HR monitor (Polar Electro Oy, Kempele, Finland) was worn during both walking exercise bouts. For the MIE session, the average HR for the group was 127 ± 12.8 bpm, whereas for the HIE session the group had an average HR of 132 ± 15 bpm during the 5-min horizontal walking and 163 ± 14 bpm during the 5-min walking at a 10% incline (the overall HR for HIE was 147 ± 14 bpm). The energy costs of the exercise sessions were calculated using the American College of Sports Medicine equations for estimating the total rate of energy expenditure (gross V˙O2; mL·kg−1·min−1) (1).
The estimated gross V˙O2 was determined by the American College of Sports Medicine walking equation:
The estimated gross V˙O2 was then converted to energy expenditure (kcal·min−1) using the following equation:
The conversion of V˙O2 to energy expenditure is based on the assumption that each liter of O2 is equivalent to 5 kcal. Although it is accepted that this value overestimates the energy cost of activities characterized by a respiratory quotient < 1.0, when the respiratory quotient is not known, as was the case for the current study, the value of 5.0 kcal·L−1 is recommended (1).
The mean energy costs of the MIE and HIE sessions were 353 ± 89 and 589 ± 149 kcal, respectively. To minimize the potential confounding effect of weekends, day 1 for MIE and HIE was randomly scheduled on either Monday or Friday. Therefore, the day of the exercise manipulation (day 4) was randomly scheduled on either Thursday or Monday, and the order of the two exercise manipulations was counterbalanced. By doing so, weekends for the first half of participants were after the exercise day, and weekends were before the exercise day for the other half. The order of the conditions was counterbalanced. The day of the week of the walking exercise session was fixed within participants. All participants were familiarized with the motorized treadmill by walking slowly for 5 min.
NEAT was measured using a triaxial accelerometer: StayHealthy RT3 (StayHealthy, Inc., Monrovia, CA). The device weighs 65.2 g (7.1 × 5.6 × 2.8 cm) and measures acceleration in three individual planes (horizontal, vertical, and lateral) and then integrates accelerations to yield one value; the vector magnitude (VM). In order for the data to be collected for 7 d, the epoch duration was set at 1 min. The RT3 accelerometer was worn by all participants for a period of seven consecutive and complete days, including two weekend days (3 d before the experiment, the day of an exercise laboratory visit, and the following 3 d). Participants were instructed to clip the accelerometer to the waistband in line with their right hip and to wear the device at all times except during sleeping or when there was risk of contact with water. To interpret potential irregularities in the data and/or adjust for nonwearing times, all participants were required to complete a daily self-report log indicating the time they woke in the morning, the time they placed the RT3 on their hip, the time they removed the RT3, the time they went to bed, and the time and reason for any unusual circumstances, which caused the removal of the RT3. All participants met the criteria of having valid RT3 data as defined as having 10 h or more of movement registered (3). Sixty consecutive minutes with no movement data (zero activity) was considered to be nonwearing time, which was subtracted from 24 h. For RT3, a VM between 0 and 20 counts per minute was defined as a threshold of sedentary activity (13). Each participant used the same RT3 unit at each assessment (14).
The RT3 data of the exercise task were excluded from the data of the exercise day and then adjusted by using the following formula: total VM (counts per day) = ((total VM (counts per day)/23) × 24). Before using the RT3, it was initialized with the participant's age, gender, height, and weight. When removed from the participant, the activity data were downloaded to a computer using Microsoft Excel©.
As there are differences in biomechanical efficiency of movement between individuals, calculation of an activity-related time equivalent on the basis of accelerometry (ArteACC) was also calculated for each individual; this is defined as an index of time spent in accelerometer-measured activity equivalent to that of a reference exercise activity (26). ArteACC was calculated as follows: ArteACC (min·d−1) = (total daily activity counts (counts per day)/reference exercise activity counts (counts per minute)). The reference exercise activity counts were determined by calculating the mean of activity counts obtained during MIE session (i.e., walking at constant speed at 0% grade).
Statistical analysis and treatment of data.
Statistical analyses were carried out with the Statistical Package for the Social Sciences for Windows (Version 16; SPSS, Inc., Chicago, IL). Data were expressed as mean and SD values. To compare scores on the total daily VM (counts per minute), NEAT VM (counts per minute), and ArteACC (min·d−1) across all days (days 1-14), a one-way repeated-measures ANOVA was conducted. If there was no effect of time, the total daily VM (counts per minute) and NEAT VM (counts per minute) in days 1-3 (preexercise) and days 5-7 (postexercise) were pooled and used for later analyses.
A two-way repeated-measures ANOVA (2 (exercise intensity protocols) × 3 (times: 3 d preexercise, exercise day, and 3 d postexercise) was conducted to assess the impact of two different exercise intensity protocols (MIE and HIE) on the total daily body movements VM (counts per minute), NEAT VM (counts per minute), and ArteACC (min·d−1) across three periods (3 d preexercise, exercise day, and 3 d postexercise). Statistical significance was accepted if P < 0.05.
Table 1 shows the mean ± SD values of NEAT VM (counts per minute) and ArteACC (min·d−1) for the MIE and the HIE protocols: preexercise days (days 1-3), exercise day (day 4), and postexercise days (days 5-7). Individual data of NEAT VM (counts per minute) across 7 d for MIE and HIE are also shown [see Supplemental Digital Content Fig. 1, http://links.lww.com/MSS/A49, which shows all individual data of NEAT VM (counts per minute) for MIE across all days (days 1-7); see Supplemental Digital Content Fig. 2, http://links.lww.com/MSS/A51, which shows all individual data of NEAT VM (counts per minute) for HIE across all days (days 1-7)].
When comparing NEAT VM (counts per minute) across the 14 d of testing, a repeated-measures one-way ANOVA revealed that there was a significant effect of time (P = 0.001) (Fig. 2). Post hoc comparison indicated that there were no significant differences in NEAT VM (counts per minute) between day 1 and day 3 (preexercise period) in both the MIE and the HIE protocols. There was also no significant change in NEAT VM (counts per minute) on the day of exercise in both the MIE and the HIE protocols (113.5 ± 43.5 and 117.7 ± 39.8, respectively). NEAT VM (counts per minute) did not differ significantly between days 5 to 7, in the MIE protocol. Despite NEAT VM (counts per minute) increasing by 16% on day 7 (preexercise) compared with the exercise day in the MIE protocol, it was not statistically significant (P = 0.32). NEAT VM (counts per minute) did not differ significantly between the first 2 d (days 5 and 6) after the HIE session. However, NEAT VM (counts per minute) increased by 25% on day 7 (preexercise) compared with the exercise day (P = 0.08) and by 30%-33% compared with preexercise period (P = 0.03, 0.03, 0.02), respectively. The increase in NEAT VM (counts per minute) observed on day 7 (postexercise) in the HIE protocol was not affected by the order of the two exercise intensities performed.
When 60 min of the prescribed exercise data (counts per minute) were included in the day of exercise, total daily movement (NEAT plus 1 h exercise) on the exercise day increased significantly in both the MIE and the HIE protocols, although total daily movement on the two exercise days did not differ (204.3 ± 50 and 209.6 ± 48, respectively; see Fig. 3). For both the MIE and the HIE protocols, total daily movement (counts per minute) on the exercise days was increased significantly by 48% compared with the pooled preexercise period value (days 1-3) and by 44% compared with the pooled postexercise period value (days 5-7).
To have a reliable estimate of NEAT and although there was a significant difference in NEAT VM (counts per minute) on day 7 in the HIE protocol, days 1-3 (preexercise) and days 5-7 (postexercise) were pooled for both exercise intensities (Fig. 4). A two-way repeated-measures ANOVA was conducted and revealed no significant effect for an exercise intensity × time interaction, F(2,14) = 0.06, P = 0.94. The ANOVA also revealed no significant main effect of time (F(2,14) = 1.09, P = 0.33), with no changes in NEAT VM (counts per minute). There was no significant change in NEAT VM (counts per minute) between MIE and HIE, F(1,15) = 0.34, P = 0.56.
When ArteACC index was calculated, the main effect for exercise intensity and time (preexercise, the same day, and postexercise) and the interaction effect between exercise intensity and time remained statistically nonsignificant; F(1,15) = 0.68, P = 0.42, F(2,14) = 1.67, P = 0.22, and F(2,14) = 0.16, P = 0.85, respectively (Fig. 5).
Although our results provide further support for the importance of exercise to increase the net daily movement of overweight and obese participants, the primary finding from this study was that the exercise sessions had no immediate debilitative effect on NEAT on the day of the exercise or on the following 2 d. However, there was a delayed increase in NEAT 48 h after performing a 60-min HIE session. No previous study has investigated the acute effect of exercise intensity on NEAT in obese adults. Our results are contrary to the findings of Goran and Poehlman (6) in the elderly, of Kempen et al. (11) in obese women, and of Kriemler et al. (15) in obese boys. The reduction in NEAT after intense exercise session was partially explained by the possibility that NEAT may decline because of fatigue and discomfort. This notion was supported by Goran and Poehlman (6), who found a significant average decrease (62% (571 ± 386 to 340 ± 452 kcal·d−1)) in NEAT in response to endurance training in elderly men. The compensatory decline in NEAT during the remainder of the day may have occurred because of the high intensity of the exercise (85% V˙O2max). Consequently, elderly people fatigued during the remainder of the day. Results are also consistent in obese women who participated in moderate aerobic training (50%-60% V˙O2max). The combination of exercise-induced energy expenditure with an energy-restricted diet did not accentuate weight loss because the training was partly compensated for by a decrease in NEAT outside the training sessions (11). In obese children, NEAT was increased when exercise was of moderate-intensity but decreased after an intense bout of exercise. This finding supports the contention that moderate-intensity exercise is better tolerated than high-intensity exercise, especially for obese individuals. Previous research suggests that time spent in moderate-intensity activities is potentially more effective for increasing overall levels of energy expenditure compared with high-intensity activities (31). The findings of the current study did not concur with results of earlier studies.
The difference in duration of imposed exercise could explain the different outcome between our study and those of Kriemler et al. (15) and Goran and Poehlman (6). Furthermore, the mode of exercise (i.e., cycle ergometer) used in both earlier studies was different to the treadmill walking in our study. Cycling could cause more localized muscle fatigue than treadmill walking (4); however, this comparison has not been undertaken. Most importantly, the choice of method and period measured to detect change in NEAT is critical. In our study, accelerometry with time interval was chosen to allow us to detect changes in NEAT, especially during the period that follows exercise. This would be impossible to determine using the doubly labeled water technique, as in the Goran and Poehlman study (6), because the technique cannot discriminate between activity types and exercise time intervals. Also, in our study, NEAT was measured for 3 d after each exercise session, which provided the opportunity to detect changes in NEAT 48 h after the exercise session. If the choice of measurement period was limited to 24 h after the exercise session as in the study of Kriemler et al. (15), any changes in NEAT beyond 24 h would go undetected.
Although we detected changes in NEAT 48 h after the exercise session, it is not clear why there was a delayed increase in NEAT on day 7. One explanation could be the difference in physical activity between weekdays and weekend days (20). However, this is not possible in this study because for half the participants, day 7 was a weekend day, and for the other half it was a week day. Therefore, the phenomenon of delayed increase in NEAT needs further examination.
The effect of exercise training on NEAT has been reported in several studies, but the findings are inconsistent. For instance, Meijer et al. (24) investigated the effect of a 20-wk endurance training program on NEAT using accelerometer measured average daily metabolic rates in healthy people who were preparing for a half marathon. Although the accelerometer output increased significantly during the training program compared with baseline, the authors suggested that the increase was due to the extra activities comprising the training program and suggested that NEAT was not affected by the endurance training. Another recent study reported no compensatory effect of a short-term (8-d) exercise program on NEAT in a lean, moderately active group of men and women. Therefore, a reduction in body mass, observed in women only, was not explained by a change in NEAT but rather by a compensatory decrease in food intake (21). In contrast, a more recent study reported that overweight and obese women compensated by being less active outside exercise sessions and achieved lower than predicted fat loss as result of the 8 wk of exercise intervention, although NEAT was not measured (19). However, each of these studies only measured NEAT at two time points, namely, at baseline and at the end of the exercise intervention. The acute effect of exercise on NEAT was not investigated. It would be of interest to know what temporal changes in NEAT were evident during the intervention.
In conclusion, the results of our study indicate no adverse effect of exercise on NEAT in the days after a 60-min walking session. Further, we found a differential effect of exercise intensity on NEAT 48 h after exercise in overweight/obese adults. Contrary to our hypothesis, the walking exercise resulted in an increase in NEAT in the third day after the exercise session, and this was most pronounced in the HIE condition. However, because the average value for both NEAT and ArtACC did not confirm the delayed increase in NEAT, this finding needs to be interpreted with caution. Moreover, because the aim of this study was to explore the acute effect of exercise intensity on NEAT in response to a single exercise session, the effects of accumulated exercise sessions over a week on NEAT still needs to be determined. These data have positive implications for the role of exercise in weight management. Despite several claims, imposed acute exercise does not appear to induce a compensatory reduction in NEAT in obese individuals. Therefore, a longer-term study in this population is needed to determine the effect on NEAT after several exercise sessions.
This study was funded by the Institute of Health and Biomedical Innovation, Queensland University of Technology, and King Saud University. The authors state that the results of the present study do not constitute endorsement by the American College of Sports Medicine.
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