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Potential Causes of Elevated REE after High-Intensity Exercise

HUNTER, GARY R.1,2; MOELLERING, DOUGLAS R.2,3; CARTER, STEPHEN J.2; GOWER, BARBARA A.2; BAMMAN, MARCAS M.4; HORNBUCKLE, LYNDSEY M.5; PLAISANCE, ERIC P.1,2; FISHER, GORDON1,2

Medicine & Science in Sports & Exercise: December 2017 - Volume 49 - Issue 12 - p 2414–2421
doi: 10.1249/MSS.0000000000001386
BASIC SCIENCES
Free

Introduction Resting energy expenditure (REE) increases after an intense exercise; however, little is known concerning mechanisms.

Purpose The purpose of this study was to determine effects of a single bout of moderate-intensity continuous (MIC) aerobic exercise, or high-intensity interval (HII) exercise on REE under energy balance conditions.

Methods Thirty-three untrained premenopausal women were evaluated at baseline, after 8–16 wk of training, 22 h after either MIC (50% peak V˙O2) or HII (84% peak V˙O2). Participants were in a room calorimeter during and after the exercise challenge. Food intake was adjusted to obtain energy balance across 23 h. REE was measured after 22 h after all conditions. Twenty-three-hour urine norepinephrine concentration and serum creatine kinase activity (CrKact) were obtained. Muscle biopsies were obtained in a subset of 15 participants to examine muscle mitochondrial state 2, 3, and 4 fat oxidation.

Results REE was increased 22 h after MIC (64 ± 119 kcal) and HII (103 ± 137 kcal). Markers of muscle damage (CrKact) increased after HII (9.6 ± 25.5 U·L−1) and MIC (22.2 ± 22.8 U·L−1), whereas sympathetic tone (urine norepinephrine) increased after HII (1.1 ± 10.6 ng·mg−1). Uncoupled phosphorylation (states 2 and 4) fat oxidation were related to REE (r = 0.65 and r = 0.55, respectively); however, neither state 2 nor state 4 fat oxidation increased after MIC or HII. REE was not increased after 8 wk of aerobic training when exercise was restrained for 60 h.

Conclusions Under energy balance conditions, REE increased 22 h after both moderate-intensity and high-intensity exercise. Exercise-induced muscle damage/repair and increased sympathetic tone may contribute to increased REE, whereas uncoupled phosphorylation does not. These results suggest that moderate- to high-intensity exercise may be valuable for increasing energy expenditure for at least 22 h after the exercise.

1Department of Human Studies, University of Alabama at Birmingham, Birmingham, AL; 2Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL; 3Diabetes Research Center Bioanalytical Redox Biology (BARB) Core, University of Alabama at Birmingham, Birmingham, AL; 4Department of Cell, Developmental, and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL; and 5Department of Kinesiology, Recreation, and Sport Studies, University of Tennessee, Knoxville, TN

Address for correspondence: Gary R. Hunter, Ph.D., Department of Human Studies, University of Alabama at Birmingham, Birmingham, AL 35294; E-mail: ghunter@uab.edu.

Submitted for publication January 2017.

Accepted for publication July 2017.

Low total daily energy expenditure is predictive of weight gain (48). The largest component of total energy expenditure is resting energy expenditure (REE), so habitual increases in REE may increase ease of weight maintenance. Fat-free mass (FFM) has a large influence on REE, explaining 25%–30% of its variance, whereas fat mass (FM) normally explains less than 10% of the variation in REE (8). Other factors that may influence REE include variability in protein turnover due to muscle damage (13,15,18,22,29,41), sympathetic tone (3), and uncoupled phosphorylation in adipose tissue (6,7,14) and/or muscle (38).

We (21,45), as well as others (28,36,46), have shown that REE is increased for 18–24 h after a high-intensity aerobic exercise bout of at least 70% maximal oxygen uptake (V˙O2max). The increase in REE may be attributed to increased sympathetic tone induced by increased energy flux. Mitochondrial uncoupling proteins (UCP) may contribute to REE because their expression may drive thermogenesis by uncoupling oxidative phosphorylation. Studies in rodents demonstrate that moderate-intensity aerobic exercise increases UCP1 expression in brown (50) and subcutaneous inguinal white adipose tissue (37). The increase in adipose tissue UCP1 expression is associated with physiologically relevant increases in REE, which have been shown to attenuate weight gain regardless of dietary composition (7). However, evaluation of the effects of exercise on UCP1 in humans have been less promising, with the suggestion that exercise does not increase UCP1 expression (47). Although it is important to fully investigate the role of adipose tissue uncoupling of phosphorylation with exercise, the effects of exercise on skeletal muscle, if any, are unclear. Several studies have suggested either no change or a decrease in skeletal muscle UCP3 content or uncoupled phosphorylation after chronic exercise training (10,17,34) or a single bout of exercise (14). We know of no studies that have attempted to determine the relative contribution of muscle damage, increased sympathetic tone, or changes in uncoupled phosphorylation to the increased REE after high-intensity aerobic exercise.

In addition to muscle damage, sympathetic tone, allosteric metabolic regulators, uncoupled phosphorylation, and energy imbalance could be contributing to increased REE after aerobic exercise. Aerobic exercise may induce a temporary energy deficit after a bout of exercise. An energy imbalance could contribute to increased REE (49). Furthermore, V˙O2max is related to REE independent of FFM, FM, and 24-h urinary norepinephrine (a surrogate of sympathetic tone) (21), suggesting that a bout of exercise may affect REE. It is possible that the relationship between V˙O2max and REE is mediated by an acute energy imbalance after exercise. Although energy flux and REE assessed 22 h after exercise are both decreased (9), no one has attempted to clamp energy balance after high-intensity exercise to control for its potential confounding effects on REE after exercise.

The purpose of this study was to determine whether moderate-intensity exercise and high-intensity exercise influence REE under energy balance conditions. We also wanted to identify factors that might potentially be responsible for any increases in REE after the exercise. We hypothesize, under energy balance conditions, the following: 1) exercise training will not be associated with increased REE; 2) a bout of high-intensity interval aerobic exercise will be associated with increased REE 22 h after exercise but continuous moderate-intensity aerobic exercise will not; 3) changes in muscle damage, fat oxidation, mitochondrial uncoupling, and sympathetic tone will be associated with differences in REE 22 h after an acute bout of exercise.

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METHODS

Study participants

Thirty-three women between 20 and 40 yr of age participated in this study. Participants reported normal menstrual cycles and were not taking oral contraceptives or any medications known to influence glucose and/or lipid metabolism. Additional inclusion criteria were as follows: (i) normotensive; (ii) nonsmoker; (iii) sedentary, as defined by participating in any exercise-related activities <1× per week; and (iv) normoglycemic as evaluated by postprandial glucose response to a 75-g oral glucose tolerance test. All participants provided written informed consent. Study procedures were approved by the institutional review board at the University of Alabama at Birmingham and conformed to the guidelines set forth by the Declaration of Helsinki.

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Procedures

After initial screening and fitness assessments, all participants were evaluated four times during the follicular phase of four different menstrual cycles. Participants stayed in a room calorimeter for the 23 h before testing. The first evaluation was considered baseline. Posttraining evaluations took place after 8, 12, and 16 wk of exercise training. The three posttraining evaluations were as follows: 1) after 60 h of no exercise (NE; designed to evaluate the effects of chronic exercise training on REE), 2) ≈22 h after 1 h of continuous stationary cycle ergometry at 50% peak V˙O2 (moderate-intensity continuous (MIC) exercise), and 3) ≈22 h after 1 h of interval stationary cycle ergometry at 84% peak V˙O2 (high-intensity interval (HII) exercise).

Work intervals for the HII condition were 144 s followed by 103-s stationary rest intervals (work and rest intervals were designed so the total work was matched with MIC). HII work and rest intervals were designed to produce identical cumulative work to that of the MIC condition; therefore, the number of intervals performed was determined by this component of the experimental design. Posttraining assessments (NE, MIC exercise, and HII exercise each 1 month apart) were randomized to reduce the risk of bias by ordered effects. Forty-eight hours before testing, participants were required to abstain from any exercise or vigorous physical activity.

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Food intake

Food was provided the day before room calorimeter visits and during the days spent in the room calorimeter. Diets were prepared by the Clinical Research Unit kitchen staff and consisted of approximately 60% energy as carbohydrates, 25% as fat, and 15% as protein. Macronutrient content of the diet was held constant.

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Energy balance clamp

One goal for having provided food was to try to achieve energy balance (energy intake matching energy expenditure) during the stay in the room calorimeter. Caloric intake during room calorimeter visit was based on estimates generated from 330 doubly labeled water estimates of free-living energy expenditure of sedentary premenopausal women collected in our laboratory. The equation was as follows: equation 1 = 750 kcal + [(31.47 FFM) − (0.31 FM) − (155 × race); race coded 1 for African American and 2 for European American]. An equation for estimating the room calorimeter energy intake was developed from more than 200 room calorimeter visits of premenopausal women: equation 2 = 465 kcal + [(27.8 FFM) − (2.4 FM) – (188 × race); race coded 1 for African American and 2 for European American]. On the basis of the anticipated energy expenditure of the exercise during the MIC and HII visits, the estimated energy cost of the exercise was added to the equation 2 result: equation 3 = (equation 2 estimated energy expenditure + energy cost of MIC or HII exercise) (note that the baseline and NE conditions did not have exercise so equation 3 was not used). However, we recognized that the estimates may result in overfeeding or underfeeding individual subjects. Therefore, we developed a correction equation for the room calorimeter visit that was based on energy expenditure during the room calorimeter stay up to 5:30 PM. This equation was as follows: [equation 4 = 9(390 kcal + average energy expenditure in kilocalories per minute between 8:00 AM and 5:30 PM) × 925 kcal) − equation 3 estimate of energy expenditure]. We then adjusted the food intake of the evening meal to match the results of equation 4. The energy balance clamp was deemed affective because there were no significant differences in energy balance between the four conditions (mean values varying between −11 ± 176 and −39 ± 206) or significant difference from zero for any of the four conditions.

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Exercise training

After the baseline testing, all participants aerobically exercise trained three times per week on a cycle ergometer for the duration of the 16-wk study. Participants trained initially for 20 min at 65% of maximal heart rate and progressively increased their training until they were training continuously for 40 min at 80% of maximal heart rate by week 4. Heart rate was monitored throughout each session by a Polar Vantage XL heart rate monitor (Polar Beat, Port Washington, NY). All sessions were under the supervision of an exercise physiologist in a training facility dedicated to research.

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Peak aerobic capacity

Two to 4 d before each room calorimeter visit, peak aerobic capacity was measured. After an initial warm-up, participants completed a graded cycle ergometer test to measure peak oxygen uptake (V˙O2peak) as determined by the highest level reached in the final stage of exercise. Power output began at 25 W and increased by 25 W every 2 min until participants reached volitional exhaustion. A 60-rpm cycle cadence was maintained throughout the test. Oxygen uptake, ventilation, and respiratory exchange ratio were determined by indirect calorimetry using a MAX-II metabolic cart (Physio-Dyne Instrument Company, Quogue, NY). Heart rate was continuously monitored by Polar® Vantage XL heart rate monitors (Polar Beat). Although, we do not claim a true maximal oxygen uptake because tests were done on a cycle ergometer rather than treadmill, criteria for achieving a true maximum were heart rate within 10 bpm of estimated maximum, RER of at least 1.1, and plateauing. All subjects reached at least one criterion, and all but three subjects reached at least two criteria at each of the four test time points.

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Room calorimeter

Participants spent 23 h in a whole-room respiration calorimeter (3.38 m long, 2.11 m wide, and 2.58 m high) for measurement of total energy expenditure and REE. The design characteristics and calibration of the calorimeter were described previously (45). Oxygen consumption and carbon dioxide production were continuously measured using a magnetopneumatic differential oxygen analyzer (Magnos206; ABB, Frankfurt, Germany) and a nondispersive infrared industrial photometer differential carbon dioxide analyzer (Uras26; ABB). The calorimeter was calibrated before each participant entered the chamber. Before each test, calibration was carried out on the oxygen and carbon dioxide analyzers using standard gasses. The full scale was set for 0%–1% for the carbon dioxide analyzer and 0%–2% for the oxygen analyzer. Each participant entered the calorimeter at 8:00 AM. Metabolic data were collected throughout the 23-h stay. Each participant was awakened at 6:30 AM the next morning in the calorimeter. REE was then measured for 30 min before the subject left the calorimeter at 7:00 AM. Energy expenditure was calculated by the Weir equation (11). The REE data were extrapolated for 24 h and expressed as kilocalories per day for total energy expenditure and REE.

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Twenty-two-hour fractionated urinary norepinephrine

Unless multiple blood samples are taken during a day, 24-h urine values are considered to be much more reflective of sympathetic tone for 1 d than blood samples (31,42). Twenty-four-hour urine norepinephrine has been reported to be reliable (23) and valid for tracking chronic sympathetic activity (12). Therefore, we measured urine norepinephrine as a surrogate of sympathetic tone. Twenty-two-hour urine samples were treated with hydrochloric acid and glutathione upon collection, and norepinephrine was measured by high-performance liquid chromatography. Serum levels of norepinephrine are incredibly labile, fluctuating dramatically during a short period.

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Exercise sessions in room calorimeter

Workload on the Collins electronically braked cycle ergometer (Warren E. Collins, Braintree, MA) was based on the V˙O2peak test using the cycle ergometer metabolic equation, in accordance with the American College of Sports Medicine (1). During the MIC exercise, participants cycled continuously for 60 min at an intensity of 50% V˙O2peak. Work intervals for the HII exercise were 144 s at 84% V˙O2peak, whereas the rest intervals were 103 s. Total work was identical among the two exercise bouts. Workload was controlled outside the room calorimeter via a cable interface with the Collins electronically braked cycle ergometer.

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Body composition

Total and regional body composition (i.e., FM and lean mass) was determined by dual-energy x-ray absorptiometry (iDXA; GE Lunar, Madison, WI). Participants were evaluated after an overnight fast 3 to 4 d before the room calorimeter visit and had not exercised for at least 36 h, wore light clothing, and remained supine with arms at the side but not touching the body in compliance with manufacture-recommended testing procedures. Scans were analyzed by the same investigator using ADULT software, LUNAR-DPX-L version 1.33 (GE Medical Systems Lunar).

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Tissue biopsy and preparation of permeabilized muscle fibers

Twenty-three hours after the exercise challenge (MIC and HII conditions) or 60 h after no exercise (baseline and NE conditions), a subset of 15 women had muscle biopsies sampled (70–140 mg) from the vastus lateralus. Muscle biopsy samples were obtained from the lateral side of the vastus lateralis under local subcutaneous anesthesia (1% lidocaine) by percutaneous needle biopsy using a 5-mm Bergstrom needle under suction, as previously described (2). Each of the four biopsies were performed on the contralateral leg on the basis of the prior biopsy, and subsequent biopsies on the same leg were performed a minimum of 1-inch distance from the prior biopsy with a minimum of 8 wk in between. A portion of the biopsy sample was immediately placed and transported in an ice-cold relaxing and preservation solution BIOPS, containing (in mM) 2.77 Ca–ethyleneglycol-bis(beta-aminoethylether)-N,N′-tetraacetic acid (EGTA) buffer, 0.0001 free calcium, 50 K-MES, 7.23 K2EGTA, 20 imidazole, 0.5 dithiothreitol, 20 taurine, 5.7 adenosine triphosphate (ATP), 14.3 protein-to-creatinine ratio, and 6.56 MgCl2·6H2O (pH 7.1, 290 mOsm) (26,32) and was used to prepare permeabilized muscle fiber (PmF) bundles. Briefly, small pieces of skeletal muscle (~20–25 mg) were placed immediately in fresh ice-cold BIOPS, trimmed of fat and connective tissue on ice, and separated into four small muscle bundles (~2–6 mg wet weight). The PmF bundles were mechanically separated by gentle blunt dissection with a pair of needle-tipped, antimagnetic forceps under magnification (Zeiss, Stemi S2000-C Stereo Microscope; Diagnostic Instruments). They were then treated with 30 μg·mL−1 saponin, gently rocked (Rocker II, model 260350; Boekel Scientific) at 4°C for 30 min in BIOPS. PmF bundles were then rinsed twice by gentle rocking to wash out saponin and ATP at 4°C for at least 15 min and <30 min in MiR05 containing (in mM) 105 K-MES, 30 KCl, 1 EGTA, 10 K2HPO4, and 5 MgCl2·6H2O, with 0.5 mg·mL−1 bovine serum albumin (pH 7.1, 290 mOsm). The PmF bundles were then transferred to a fresh MiR05/creatine solution (500 μL) and blebbistatin (25 μM).

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High-resolution mitochondrial respirometry in permeabilized fibers

Mitochondrial respiration assays were performed using high-resolution respirometry by measuring oxygen consumption in 2 mL of MiR05/creatine/blebbistatin buffer, in a two-channel respirometer (Oroboros Oxygraph-2k with DatLab software; Oroboros Instruments Corp., Innsbruck, Austria) with constant stirring at 750 rpm (27) and after a modified substrate–uncoupler–inhibitor titration protocol to evaluate respiratory control in a sequence of coupling and inhibitory states induced by multiple titrations in each assay (33). Seventy percent ethanol was run in both chambers for a minimum of 30 min, rinsed three times with Milli-Q ultrapure ddH2O, and the chambers were calibrated after a stable air-saturated signal was obtained before every experiment. Reactions were conducted with PmF bundle (2–8 mg wet weight) at 37°C with hyperoxygenation to maintain oxygen concentrations above air saturation (~500–200 μM) (20) and prevent oxygen diffusion restrictions, which have been shown to limit oxygen supply to the core of the fiber bundle (16). All experiments were completed before the oxygraph chamber [O2] reached 150 μM. Respiration rates were measured using malate (2 mM; anapleurotic intermediate) and palmitoylcarnitine (40 μM) to determine mitochondrial β-oxidation per se with all electron transport chain complexes, independent of the step catalyzed by carnitine palmitoyltransferase I (CPT1B), which may differ across individuals and races (30). Polarographic oxygen measurements are expressed as picomoles per second times milligrams of wet weight. Determination of state 2, 3, and 4 respiration rates were made in the presence of substrate alone (state 2; LEAK state; low ATP), after the addition of adenosine diphosphate (ADP; 2 mM; State 3; OxPhos state), and after inhibition of ATP synthase (complex V) with oligomycin (state 4o; LEAK state; high ATP). For quality control and to ensure outer mitochondrial membrane integrity, cytochrome c (10 μM) was added to the assay after activation by ADP and only preparations with <10% increase after addition were included. Respiratory control ratios were determined as the ratio of state 3/state 4 respiration rates.

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Myofiber type distribution

All visible connective and adipose tissues were removed from the biopsy samples with the aid of a dissecting microscope. Portions used for immunohistochemistry were mounted cross sectionally on cork in optimum cutting temperature mounting medium mixed with tragacanth gum, frozen in liquid nitrogen–cooled isopentane, and stored at −80°C. The relative distribution of myofiber Types I, IIa, and IIx were determined by myosin heavy-chain immunohistochemistry using our well-established protocol (25).

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Serum creatine kinase activity

Serum samples were stored at −80°C, only thawed once when the assay was performed, and only samples without any indication of hemolysis were used (44). The assay was performed in duplicate according the manufacturer’s instructions (Creatine Kinase Activity Assay Kit-MAK116; Sigma-Aldrich), and activity is reported in units per liter. Two different samples were run in triplicate and used for quality control and for inter- and intra-assay coefficients of variation (CV). The inter-assay CV values were 4.1% and 7.0%, and the intra-assay CV values were between 2.2% and 5.8%.

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Statistics

Paired t-tests were used to evaluate baseline (pretraining) versus NE as well as ΔNE versus ΔMIC and ΔHII. Correlations were run between variables of interest and REE at baseline. SPSS Version 22 was used for all analyses, and probability was set at <0.05. Because no study has shown a decrease in state 3 fat oxidation (coupled phosphorylation) after exercise training and most studies show increased state 3 fat oxidation, a one-tailed test of significance was used for the state 3 fat oxidation test between baseline and posttraining.

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RESULTS

Descriptive data are contained in Table 1. No significant differences were found between the NE condition and either MIC or HII for any of the descriptive variables. In addition, there was no significant time effect between 8-, 12-, and 16-wk evaluations. Therefore, Table 1 contains only the contrasts between the baseline and NE. No significant changes between baseline and posttraining were observed, except for a significant increase in FFM and V˙O2peak. Table 2 contains the results of the REE, muscle fiber type, muscle mitochondrial fat oxidation, and urinary norepinephrine for baseline and posttraining (chronic training effects). With the exception of state 2 and state 3 fat oxidation, which increased significantly, no significant training-induced changes were observed. Creatine kinase activity (CrKact) is also shown for the posttraining value, but CrKact was not measured at baseline.

TABLE 1

TABLE 1

TABLE 2

TABLE 2

Table 3 shows the differences that occurred 22 h after the MIC and HII, MIC, or HII different from NE (60 h of no exercise). REE was significantly elevated compared with NE after both MIC and the HII, with a greater increase in HII compared with MIC. No significant changes in state 2, 3, or 4 mitochondrial fat oxidation occurred after either the MIC or the HII. However, CrKact was significantly increased after both MIC and HII, with a greater elevation after HII. Twenty-four-hour urinary norepinephrine concentrations were increased after only HII exercise.

TABLE 3

TABLE 3

State 2 (r = 0.65, P < 0.01) and 4 (r = 0.55, P = 0.02) mitochondrial fat oxidation rates were significantly related to REE at baseline (state 4 mitochondrial fat oxidation rates and REE relationship depicted in Fig. 1). Muscle fiber type was not related to REE or change in REE, except that Type IIA muscle fiber type was significantly related to an increase in REE after HII (r = 0.65, P < 0.04). No significant correlations were observed between percent fiber type with state 2, state 4, or the ratio of state 3 to state 4 at baseline. However, significant correlations between percent Type I muscle fiber type and the ratio of state 3 to state 4 mitochondrial fat oxidation and percent Type IIA fiber type and state 2 fat mitochondrial fat oxidation were observed after the HII. None of the changes in mitochondrial fat oxidation variables were significantly related to change in REE.

FIGURE 1

FIGURE 1

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DISCUSSION

Our observation that REE increased by more than 100 kcal 22 h after the high-intensity exercise is consistent with a number of other studies (21,28,36,45,46). The primary observation of this study that is unique is that under energy balance conditions, REE increased after both the HII and MIC exercises, although the increase after HII was larger. Because both 24-h urine norepinephrine and CrKact increased after HII, we suggest that increased sympathetic tone and protein repair after HII may be contributing to increased energy expenditure. Although mitochondrial state 2, 3, and 4 lipid oxidation rates were related to REE at baseline (Fig. 1), no 22-h postexercise changes were noted after a bout of HII or MIC. Nor were delta mitochondrial lipid oxidation rates after HII related to delta REE, suggesting that intrinsic mitochondrial β-oxidation per se may be involved in REE but not cause the increase noted after high-intensity exercise, at least 22 h after exercise.

Rolfe and Brand (39) proposed that mitochondrial protein leak uncoupled to ATP synthesis accounts for as much as 50% of resting skeletal energy expenditure, which is thought to be regulated by UCP, particularly UCP2 and UCP3 (5,40). Hence, variations in mitochondrial uncoupling may be associated with variability in total REE. Indeed, this has been shown to be the case at least in rats (38). Consistent with this, we show a relationship between state 2 and state 4 fat oxidation and REE in this study (see Fig. 1). Several studies have shown decreases in UCP3 protein or messenger RNA (6,14,34) and uncoupled respiration (14) after exercise training, whereas others have shown increases (35,43). Consistent with the studies that showed increases, we show a significant increase in state 2 fat oxidation and a trend for an increase in state 4 fat oxidation. However, similar to results reported by Jones et al. (24), the increase in state 2 and state 4 fat oxidation matched the relative increase in state 3 fat oxidation, indicating that the increase was a function of increased mitochondrial biogenesis.

We had hypothesized that uncoupled mitochondrial fat oxidation may contribute to increased REE after a bout of high intensity. Although Fernström et al. (14) found a decrease in uncoupled respiration 3 h after a bout of moderate-intensity exercise, no one had previously observed what effect high-intensity interval exercise had on fat oxidation uncoupling 22 h after the exercise. We found no significant differences in delta state 2 fat oxidation, delta state 4 fat oxidation, or the ratio of state 3 to state 4 fat oxidation with delta REE for either the contrast between the HII or MIC with NE. In addition, there was no relationship between any of the posttraining delta fat oxidation measures and delta REE. These observations suggest that changes in uncoupled mitochondrial fat oxidation did not contribute to increased REE after either moderate-intensity or high-intensity exercise.

Hesselink and colleagues (19) have previously reported that Type II muscle fibers contain more UCP3 compared with Type I muscle fibers, suggesting that Type II muscle fibers may be more prone to uncouple oxidative phosphorylation than Type I muscle fibers. However, we saw no significant relationships between fiber type, change in mitochondrial uncoupling, or change in REE. The percent muscle fiber type does not seem to contribute substantially to variations in REE.

A unique approach to our work was the implementation of an energy balance clamp. By managing energy balance, the differences observed between conditions seem to be independent of either an energy surplus or an energy deficit, thus isolating the exercise-mediated effect. Increased REE after the moderate-intensity exercise was an unexpected finding. To our knowledge, previous studies have not found an increase in REE for aerobic exercise intensities less than 70% V˙O2peak. However, this is the first study to attempt to clamp energy balance after exercise and it is known that energy deficit can decrease REE (49). Little is known about the short-term effects of energy imbalance, although it is known that longer-term energy imbalance (e.g., 2 wk) will decrease REE (49). It is possible that in some of the previous studies (19,28,35,36,46), the bout of exercise may have induced a temporary energy imbalance and induced a reduction in REE, concealing a modest increase in REE after the moderate-intensity exercise. Consistent with this being the case, we found a significant relationship between energy balance and REE at baseline, supporting the notion that even small deviations in energy balance for as little as 22 h may influence REE. It is important to note that the REE increase with MIC in the present study was less than after the HII condition (+64 vs +103 kcal·d−1). Increased sympathetic tone possibly contributed to the increase in REE with HII, because HII induced an increase in urinary norepinephrine. No increase in urinary norepinephrine occurred with MIC, suggesting that a threshold of exercise intensity exists when identical work is performed to elevate sympathetic tone. The increase in CrKact after the MIC was also surprising because the participants should have been well conditioned for the 50% V˙O2peak exercise, as they had been training 8–16 wk at 70% V˙O2peak. Although the duration of exercise was longer (60 min for the MIC trial vs 40 min for normal training), it was at a lower intensity. It is also important to note that the cycling exercise modality is considered to have little eccentric work, contractions that are more likely to cause muscle damage and protein turnover (4). This suggests that the current participants likely experienced less overall muscle damage compared with what may be observed during an alternative exercise modality (i.e., running) that incorporates more eccentric contractions. The relatively modest CrKact increases support this (44). Subsequent studies may measure the current outcomes using treadmill running or comparing low- and high-volume/intensity resistance exercise protocols to examine the mechanisms behind REE differences. This could supplement the current findings and perhaps speak to future physical activity/exercise recommendations in individuals and populations looking to maximize to REE.

The findings of this study are specific to premenopausal women, although it is probable that the findings may be consistent with older women and men. Further research is warranted to confirm this. We recognized that the estimates of food intake may result in overfeeding or underfeeding individual subjects. Therefore, we developed a correction equation for the room calorimeter visit that was based on energy expenditure during the room calorimeter stay up to 5:30 PM. We then adjusted the food intake of the evening meal to match the results of equation 4. There were no significant differences in energy balance between the four conditions (mean values varying between −11 ± 176 and −39 ± 206).

In conclusion, REE was increased 22 h after a bout of cycling exercise by more than 100 kcal·d−1 after high-intensity exercise and more than 60 kcal·d−1 after moderate-intensity exercise in aerobically trained women in energy balance. This increase in REE was accompanied by increased markers of muscle damage and sympathetic tone after the HII exercise and muscle damage after the MIC exercise, whereas increased uncoupled mitochondrial fat oxidation did not increase after either HII or MIC suggesting no contribution to increased REE. No increase in REE was observed after 8 wk of aerobic training when subjects restrained from exercise for 60 h. These results suggest that if increased REE is the goal of aerobic exercise training, periodic bouts of exercise training must occur regularly throughout the week. This in turn may enable better weight maintenance and potentially offset weight gain, which may improve metabolic health. Indeed, the combination of energy expenditure from the exercise plus the increase in REE after the exercise may enable individuals to do a better job of remaining in energy balance and avoid weight gain.

The authors thank David Bryan, Bob Petri, and Paul Zuckerman for their help in data acquisition.

This work was supported by the National Institutes of Health (Grants R01 AG027084-01, R01 AG027084-S, R01 DK56336, P30 DK56336, P60 DK079626, and UL 1RR025777). There is no conflict of interest for any of the authors.

The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

INTERVAL EXERCISE; UNCOUPLED PHOSPHORYLATION; CREATINE KINASE ACTIVITY; NOREPINEPHRINE

© 2017 American College of Sports Medicine