and low-intensity exercise - Semantic Scholar

Eur J Appl Physiol (1998) 78: 43 ± 49

Ó Springer-Verlag 1998

ORIGINAL ARTICLE

D.L. Thompson á K.M. Townsend á R. Boughey K. Patterson á D.R. Bassett Jr

Substrate use during and following moderate- and low-intensity exercise: Implications for weight control

Accepted: 3 November 1997

Abstract Substrate utilization during and after low- and moderate-intensity exercise of similar caloric expenditure was compared. Ten active males [age: 26.9 (4.8) years; height: 181.1 (4.8) cm; Mass: 75.7 (8.8) kg; maximum O2 consumption (V_ O2 max ): 51.2 (4.8) ml á kg)1 á min)1] cycled at 33% and 66% V_ O2 max on separate days for 90 and 45 min, respectively. After exercise, subjects rested in a recumbent position for 6 h. Two h post-exercise, subjects ate a standard meal of 66% carbohydrate (CHO), 11% protein, and 23% fat. Near-continuous indirect calorimetry and measurement of urinary nitrogen excretion were used to determine substrate utilization. Total caloric expenditure was similar for the two trials; however, signi®cantly (P < 0:05) more fat [42.4 (3.6) g versus 24.0 (12.2) g] and less CHO [142.5 (28.5) g versus 188.8 (45.2) g] was utilized as a substrate during the low-intensity compared to the moderate-intensity trial. Protein utilization was similar for the two trials. The di€erence in substrate use can be attributed to the exercise period because over twice as much fat was utilized during low-intensity [30.0 (11.0) g] compared to moderate-intensity exercise [13.6 (6.6) g]. Signi®cantly more (P < 0:05) CHO was utilized during the moderate-intensity [106.0 (27.8) g] compared to the low-intensity exercise [68.7 (20.0) g]. Substrate use during the recovery period was not signi®cantly di€erent. We conclude that low-intensity, long-duration exercise results in a greater total fat oxidation than does moderate intensity exercise of similar caloric expenditure. Dietary-induced thermogenesis was not di€erent for the two trials. Key words Recovery á Excess post-exercise oxygen consumption á Nutrient balance á Respiratory exchange ratio á Fat D.L. Thompson (&) á K.M. Townsend á R. Boughey K. Patterson á D.R. Bassett Jr Exercise Science Unit, University of Tennessee-Knoxville, 1914 Andy Holt Ave., Knoxville, TN 37996-2700, USA

Introduction With trends indicating an increase in obesity, treatments which help prevent this condition are of considerable interest both to the general public and health-care professionals. In the USA, for example, 33.4% of adults are overweight, which represents an 8% increase since 1980 (Kuczmarski et al. 1994). Because excessive body fat accumulation is related to negative health consequences, such as coronary artery disease, hypertension, and diabetes (American Medical Association Council of Scienti®c A€airs 1988), maintaining body weight is an important aspect of one's overall well-being. Epidemiological data indicate that those who engage in regular leisure time activity gain less weight than their inactive counterparts (Williamson et al. 1993), and researchers report that regular exercise is a common behavior observed in those who maintain weight loss compared to those who regain lost weight (Kayman et al. 1990; Lavery and Loewy 1993). The use of regular aerobic activity as part of weight loss and weight maintenance programs is recommended by professional organizations such as the American College of Sports Medicine (1995) and the American Medical Association (1988). Low-intensity exercise is typically recommended for individuals interested in weight loss and/or weight control (ACSM 1995) because free fatty acids are a preferential fuel source during low-intensity exercise (O'Brien et al. 1993), and individuals who are deconditioned and carry excessive weight may not be able to safely engage in high-intensity exercise (ACSM 1995). However, the characteristics of an exercise program that would maximize the chances of preventing excess fat accumulation remain controversial. During exercise of increasing intensities, substrate utilization shifts from a greater reliance on fats during low-intensity exercise to a preferential use of carbohydrates during high-intensity activities (Brooks and Mercier 1994). Although some have used this information to justify recommending low-intensity exercise for

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weight loss, several factors are vital in formulating an exercise prescription for weight control: (1) the energy expended during the activity; (2) the excess post-exercise oxygen consumption (EPOC); and (3) the substrates oxidized during and after the exercise bout. During exercise, the intensity and duration will determine the energy expenditure, and low-intensity exercise requires a longer duration to expend a similar number of calories compared to high-intensity exercise. Therefore, the time available for exercise becomes important. Although EPOC accounts for less energy than is expended during exercise (Laforgia et al. 1997; Smith and McNaughton 1993), this extra energy expenditure does contribute to overall energy balance. During exercise of constant duration, the magnitude of the EPOC is positively related to exercise intensity (Bahr and Sejersted 1991b; Gore and Withers 1990; Smith and McNaughton 1993), but when exercise energy expenditure is matched, the impact of exercise intensity on EPOC is less clear (Phelain et al. 1997; Sedlock 1991). Substrate utilization during the recovery period may also have an impact on body fat stores. It has been suggested that higher intensity exercise results in a greater reliance on fat as a fuel source during the post-exercise period (Bahr et al. 1991b; Treadway and Young 1990); however, these studies did not equate energy expenditure. Negative energy balance and, more importantly, negative fat balance are essential for reducing body fat stores (Flatt 1993; Swinburn and Ravussin 1993). According to the nutrient balance theory (Flatt 1993), the body's stores of fat, carbohydrate, and protein are regulated independently of one another; therefore, expending more fat grams than are consumed is essential for reducing adipose tissue. Consequently, energy expenditure and the relative percentage of substrates utilized during exercise and recovery are both important for weight control. The purpose of this study was to examine substrate utilization during and after two exercise bouts of di€erent intensities, but with similar energy expenditures, in an attempt to determine which exercise intensity is associated with a greater reliance on fat oxidation. A secondary purpose was to determine if exercise intensity alters diet-induced thermogenesis.

Methods Ten males with a mean (SD) age, height, and mass of 26.9 (4.8) years, 181.1 (4.8) cm, and 75.7 (8.8) kg, respectively, volunteered as subjects. All subjects were healthy and had performed at least 30 min of aerobic exercise three times per week for 6 months prior to testing. All subjects were recreationally active (jogging and cycling were primary exercises of choice) and none was engaged in intensive exercise training. Each subject signed an informed consent form approved by the Institutional Review Boards at the University of Tennessee, Knoxville (UTK) and the UTK Medical Center prior to testing. Subjects completed a maximal exercise test followed by two submaximal bouts on subsequent days. Testing took place in the Respiratory Laboratory at the UTK Medical Center.

Exercise testing Maximal oxygen consumption (V_ O2 max ) was measured with an incremental protocol on a SensorMedics (Yorba Linda, Calif., USA) electronically braked cycle ergometer. The initial work rate was 30 W and pedal frequency was maintained between 70 and 90 rpm throughout the test. Power output increased by 30 W every min, and the test was terminated when the intensity could not be maintained. A Polar Favor heart rate (HR) watch (Kempele, Finland) was used to record HR at every stage throughout the test. Subjects breathed through a face mask, and oxygen consumption (V_ O2 ) and carbon dioxide production (V_ CO2 ) were measured from expired air using a SensorMedics 2900 metabolic cart (Yorba Linda). Prior to each test, the metabolic cart was calibrated for gas volume and gas fractions using a 3-l syringe and gases of known concentrations. After determining V_ O2 max , each subject performed two submaximal exercise bouts in random order on the SensorMedics cycle ergometer. These tests were designed to expend equal amounts of energy. During one test, subjects pedaled at 33% V_ O2 max for 90 min and in the other bout exercised at 66% V_ O2 max for 45 min. HR, V_ O2 , and V_ CO2 were measured continuously as described above. Submaximal tests took place on di€erent days and were separated by at least 4 but not more than 14 days. Subjects reported for the submaximal tests at 0700 hours without consuming breakfast. Subjects were asked to refrain from alcohol consumption for 24 h and strenuous exercise 12 h prior to exercise testing and to consume at least two meals between their last exercise session and the testing period. Subjects kept a 3-day diet record prior to each submaximal exercise test. The foods were analyzed with the computer program Nutritionist IV (N-Squared, Salem, Ore., USA) to verify that subjects consumed a similar diet prior to each submaximal test. Prior to submaximal exercise, resting metabolic rate (RMR) was measured utilizing the ventilated hood system of the SensorMedics 2900 metabolic cart. During the RMR test, subjects remained under the SensorMedics canopy in a reclined position for 45 min, and the last 15 min was used for measurement of RMR. Post-exercise recovery period For 15 min following the submaximal tests, subjects remained seated on the ergometer and continued to breathe through the facemask so that gas exchange could be measured. Subsequently, a 5-min transition period was used to collect urine. After the urine collection, subjects assumed a supine position in a recliner, and the ventilated hood system was used for continual measurement of V_ O2 and V_ CO2 . The post-exercise recovery period lasted 6 h, during which subjects remained supine and were allowed to watch television or read. Two h post-exercise, the ventilated hood was removed and subjects were given approximately 15 min to consume a standard meal consisting of two peanut butter and jelly sandwiches, one apple, one banana, one cereal bar, and approximately 240 ml of low-fat (2%) milk. This meal contained approximately 4020 kJ with 66%, 11%, and 23% of the energy derived from carbohydrates, protein, and fat, respectively. After the meal, the ventilated hood was replaced, and subjects were monitored for the remainder of the 6 h of recovery. Urine collection and substrate calculations In order to quantify the protein contribution to energy expenditure, urine collection took place during the recovery period of each submaximal exercise bout. Subjects voided prior to exercise, and the sample was discarded. Urine collections took place after exercise, prior to eating, and as needed during the remainder of the recovery period. These samples were analyzed for urinary urea nitrogen levels using Sigma Diagnostics (St. Louis, Mo., USA) procedure no. 640. After determining the urinary nitrogen excreted

45

The ten male subjects had an average V_ O2 max of 3.88 (0.36) l á min)1 [51.2 (4.8) ml á kg)1 min)1]. The average RMR measured prior to exercise was 234 (38) ml á min)1 [4.9 (0.8) kJ á min)1].

cise [119.9 (29.8) l and 114.7 (23.9) l, respectively] was not signi®cantly di€erent (P > 0:05). Average exercise intensity completed by each subject was 33.1% and 65.3% V_ O2 max for the low- and moderate-intensity exercise bouts, respectively. The average non-protein R value during the lowintensity bout [0.85 (0.02)] was signi®cantly (P < 0:05) lower than during the moderate-intensity bout [0.93 (0.01)] indicating a greater utilization of carbohydrate during moderate-intensity compared to lowintensity exercise [106.0 (27.8) g versus 68.7 (20.0) g, respectively]. In contrast, fat utilization was signi®cantly greater (P < 0:05) during the low- than during the moderate-intensity bout [30.0 (11.0) g versus 13.6 (6.6) g, respectively]. Protein utilization was also signi®cantly greater (P < 0:05) during low-intensity exercise when compared to moderate-intensity exercise [4.9 (2.9) g versus 1.6 (0.6) g, respectively]; however, the rate of urinary nitrogen excretion (0.40 g/h versus 0.37 g/hr for the low- and moderate-intensity bouts, respectively] was not signi®cantly di€erent.

Total caloric expenditure and substrate utilization

Post-exercise V_ O2

The total caloric expenditure during exercise and recovery combined was not signi®cantly di€erent (P > 0:05) for low- and moderate-intensity exercise [4420 (808) kJ and 4470 (703) kJ, respectively]. However, the total amount of carbohydrate oxidized during and after the moderate-intensity bout [188.8 (45.2) g] was signi®cantly greater (P < 0:05) than that metabolized during the low-intensity exercise and recovery period [142.5 (28.5) g]; the total fat utilized was signi®cantly greater (P < 0:05) in the low-intensity bout [42.4 (13.6) g] compared to the moderate-intensity bout [24.0 (12.2) g]; and total protein utilization was similar (P > 0:05) for the low- and moderate-intensity trials [18.2 (7.3) g and 16.4 (5.1) g] (Fig. 1).

There was no signi®cant di€erence in the 6-h recovery period non-protein R value or V_ O2 for the two exercise bouts (Fig. 2); however, when the recovery period was divided into pre-meal and post-meal recovery periods, the average pre-meal V_ O2 was signi®cantly higher following moderate-intensity exercise compared to lowintensity exercise [288 (91) ml á min)1 and 264 (49) ml á min)1, respectively]. These values were equivalent to energy expenditures of 707 kJ (169 kcal) and 645 kJ (154 kcal) for the moderate- and low-intensity bouts, respectively. V_ O2 remained signi®cantly elevated above RMR until 60 min post-exercise for both exercise bouts. Pre-meal recovery non-protein R values were not signi®cantly di€erent for moderate- versus low-intensity exercise [0.87 (0.01) versus 0.85 (0.03), respectively]. There was no signi®cant di€erence in post-meal V_ O2 between exercise trials [305 (35) ml á min)1 versus 302 (37) ml á min)1 after low- and moderate-intensity exercise, respectively]. After meal ingestion, V_ O2 remained elevated above resting values for 4 h in both trials. Post-meal recovery non-protein R values were not signi®cantly di€erent after low- or moderate-intensity exercise [0.92 (0.03) versus 0.94 (0.02), respectively].

during exercise, pre-meal recovery, and post-meal recovery, the energy derived from protein, carbohydrate and fat was calculated using methods described by Knoebel (1984). Statistical analysis Pre-planned contrasts and two-way repeated-measures ANOVA were utilized to compare variables (V_ O2 and respiratory exchange ratio or R) over time and between treatments. Paired t-tests were used to evaluate di€erences in total energy expenditure, as well as fat, carbohydrate, and protein utilization during the submaximal trials. The signi®cance level for all tests was established a priori at P < 0:05.

Results Subject characteristics

Exercise V_ O2 and non-protein R values As speci®ed by the study design, the amount of O2 consumed during the low- and moderate-intensity exer-

Dietary analysis

Fig. 1 Comparison of total substrate utilization during exercise and recovery. *Signi®cantly di€erent from moderate-intensity trial …P < 0:05†

Dietary analysis of the food records for the 3 days prior to the submaximal exercise bouts revealed no signi®cant di€erence (P > 0:05) in the average amount of energy consumed per day [9818 (2611) kJ versus 11,110 (2992) kJ], percentage of energy from carbohydrate [65 (17)% versus 61 (8)%], fats [20 (10)% versus 24 (8)%], or protein [15 (5)% versus 14 (3)%] when

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(Romijn et al. 1993). It should be noted, however, that the greatest rate of fat utilization occurs at approximately 60±65% of V_ O2 max (Howley et al. 1997; Romijn et al. 1993). This means that if low- and moderate-intensity exercise are equated in terms of time rather than total energy expenditure, moderate-intensity exercise would use more grams of fat. Conversely, when total energy expenditure is held constant, low-intensity exercise will utilize more fat grams compared with moderateintensity exercise. Post-exercise substrate utilization

Fig. 2 Oxygen consumption (A) and non-protein respiratory exchange ratio (R) (B) during exercise and recovery

comparing low- and moderate-intensity trials, respectively.

Discussion As expected, the percentage of the energy expenditure derived from lipids was signi®cantly higher during lowcompared to moderate-intensity exercise (Table 1). Nearly half of the energy used during the low-intensity bout was derived from fat while this ®gure was approximately 22% during moderate-intensity exercise. A greater reliance on fats as an energy source during lower compared to higher exercise intensities is not unexpected

Researchers commonly report a decreased R value postexercise compared to pre-exercise or control conditions (Bielinski et al. 1985; Maehlum et al. 1986; Quinn et al. 1994). Additionally, Treadway and Young (1990) reported that the R value was lower after exercise at 55% and 75% V_ O2 max than after exercise at 35% V_ O2 max , but these authors did not equate total energy expenditure. Phelain and others (1997) compared R values following two exercise bouts in which women expended 500 kcal at 75% V_ O2 max and 50% V_ O2 max . They reported that R values after higher intensity exercise were lower at some, but not all, time points during a 3-h recovery period. Broeder et al. (1991) reported a nonstatistically signi®cant trend for higher fat utilization following exercise at 60% V_ O2 max compared to exercise at 30% V_ O2 max . In the present study, both pre-meal and post-meal substrate utilization were similar for the two trials (Table 1; Fig. 2B). Our data suggest that when total energy expenditure is held constant, substrate use during the post-exercise period following low- and moderate-intensity exercise will be the same. Unfortunately, none of these studies measured the degree of muscle glycogen depletion induced by the exercise. In order to more fully understand post-exercise substrate utilization, the role of glycogen depletion needs additional clari®cation. In the current study, we assumed that R values accurately re¯ect substrate utilization during both exercise and recovery. In addition to the general acceptance that the R value accurately re¯ects fat and carbohydrate oxidation at rest (Frayn 1983) and during submaximal exercise (Coggan et al. 1993), Romijn et al. (1992) found

Table 1 Percentage contribution of substrates during exercise and recovery. [CHO Carbohydrate, low low-intensity trial (33% of V_ O2max ), moderate moderate-intensity trial (66% of V_ O2max )]. Values [mean (standard deviation)] are based on non-protein respiratory exchange ratio (R) and represent the percentage of the energy expenditure attributed to the substrate Exercise

CHO Fat Protein

Pre-meal recovery

Post-meal recovery

Low

Moderate

Low

Moderate

Low

Moderate

49 (14) 48 (18) 3 (1)

77 (20)* 22 (1)* 1 (1)

51 (27) 39 (14) 10 (4)

56 (21) 33 (22) 11 (4)

72 (12) 16 (10) 12 (7)

75 (16) 13 (13) 12 (4)

* Signi®cantly di€erent from the low-intensity exercise value

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that, in highly trained individuals during exercise up to 80±85% V_ O2 max , calculations of substrate utilization from the R value were similar to those using stable isotopes of carbon. Transient changes in bicarbonate stores may invalidate the assumption that the R value equals the respiratory quotient (RQ) at a given point in time. This scenario is most likely to occur in non-steadystate exercise (i.e., high-intensity non-steady-state exercise, transition from rest to exercise, transition from exercise to rest). In the current study, steady-state conditions were achieved during both the 33% and 66% V_ O2 max exercise bouts; therefore, we assumed R values accurately re¯ect substrate utilization. During the early phases of recovery from exercise, transient changes in bicarbonate stores may also invalidate the use of R values to estimate substrate utilization. However, Phelain et al. (1997) demonstrated that following an exercise bout at an intensity of 75% V_ O2 max , the blood bicarbonate levels return to baseline in under 30 min after exercise ends; therefore, our use of the average R value during the 2-h post-exercise, pre-meal period appears to be a valid indicator of substrate utilization. Pre-meal V_ O2 During the pre-meal period, V_ O2 returned to baseline levels by 60 min after exercise stopped. The 2-h pre-meal energy expenditure following the moderate-intensity trial was 707 kJ (169 kcal) compared to 645 kJ (154 kcal) after low-intensity exercise. The expected energy expenditure, based on RMR, during this 2-h period was 586 kJ (140 kcal). EPOC after high-intensity exercise accounted for an extra 121 kJ (29 kcal) of energy expenditure, whereas EPOC after low-intensity exercise added an extra 59 kJ (14 kcal). This means that following moderate-intensity exercise 62 kJ (15 kcal) in additional energy was expended during the immediate post-exercise recovery period compared to the low-intensity trial. Similarly, Sedlock et al. (1989) reported that when subjects expended an equivalent amount of energy (approximately 1256 kJ) at higher (74% V_ O2 max ) and lower (51% V_ O2 max ) intensities, energy expenditure was 63 kJ greater following high-intensity exercise. Additionally, Phelain et al. (1997) revealed that in the 3 h following high- (75% V_ O2 max ) versus low- (50% V_ O2 max ) intensity exercise, women expended 80 kJ (19 kcal) more energy. In contrast, Sedlock (1991) reported that, following exercise sessions expending 850 kJ at 40% and 60% V_ O2 max , EPOC was similar. From these data, it appears that when low- and moderate-intensity exercise is equated on total caloric expenditure there may be a slight, but quite small, di€erence in EPOC. However, recent studies comparing intervals of maximal (Treuth et al. 1996) or supramaximal (Laforgia et al. 1997) exercise to moderate-intensity exercise of similar caloric expenditure have reported signi®cantly higher elevations in EPOC following the interval exercise. While those data may have important

implications for athletes, the typical exercising person interested in exercise for weight control is more likely to engage in moderate- or low-intensity exercise. Post-meal V_ O2 In the present study, exercise intensity had no e€ect on post-meal V_ O2 (Fig. 2A) or post-meal non-protein R value (Fig. 2B). The post-meal increase in V_ O2 (i.e., thermic e€ect of food) is expected based on the literature (Bahr and Sejersted 1991a, b; Bahr et al. 1987; Treadway and Young 1990). In contrast to the current ®ndings, Treadway and Young (1990) reported that dietaryinduced thermogenesis was greater following high-intensity exercise; however, those authors did not equate exercise bouts in terms of total caloric expenditure. The rise in the non-protein R value seen after eating (Fig. 2B) is consistent with the carbohydrate content of the meal and is similar to the pattern reported by Bahr and Sejersted (1991a) and Broeder et al. (1991). Postmeal substrate utilization was similar for the two trials in the current study; however, when exercise bouts are not equated on caloric expenditure, lower R values have been reported for longer duration (Bahr et al. 1987) and higher intensity (Bahr and Sejersted 1991b) exercise sessions. From the current data, it appears that postmeal V_ O2 and substrate use are similar when the preceding low- and moderate-intensity exercise bouts are matched for total energy expenditure. Implications for weight control Carbohydrate, fat, and protein utilization are important, but separate, components in maintaining energy balance. Overall, nutrient balance occurs when the mean respiratory quotient (RQ) equals the food quotient (FQ; i.e., the ratio of CO2 produced and O2 consumed when food is oxidized in a bomb calorimeter) (Flatt 1993). Carbohydrate and protein balance are precisely regulated within the body whereas fat balance is not. An increased consumption of carbohydrate causes an increased carbohydrate oxidation rate, therefore energy balance can be maintained. This principle also holds true for protein balance. In contrast, fat intake does not in¯uence fat oxidation; therefore, when individuals take in a greater amount of fat than is oxidized by the body, a positive fat balance exists and fat is gained (Flatt 1993). Because the balance of each nutrient is treated as a separate entity, one must manipulate the consumption of fat in the diet and/or the expenditure of stored fat in order to reduce total body fat. Fat loss will occur when the RQ/FQ is less than 1.0 (Flatt 1993). In theory, the combination of a low-fat diet and exercise which utilizes primarily fat as a fuel source would be most bene®cial for preventing fat gain and/or reducing body fat. Clearly, exercise will expend energy and is recommended as a component part of weight loss and weight

48

control programs (ACSM 1995; AMA 1988). It is less clear, however, what type of exercise is most bene®cial for fat loss. Ballor and colleagues (1990) reported that high- and low-intensity exercise resulted in similar amounts of fat loss; however, the ®ndings are dicult to interpret because the subjects were also dieting. Grediagin et al. (1995) also showed that women engaging in either high- or low-intensity exercise experienced similar fat loss; however, the authors reported little data about the dietary and activity patterns of the participants. Based on Flatt's nutrient balance theory (1993), if exercise bouts are similar in total energy expenditure, the exercise which results in the greatest fat oxidation would be recommended for reducing the body's fat stores. This assumes, however, that there is no di€erential e€ect of low- versus moderate-intensity exercise on appetite and subsequently energy intake. Although exercise has been reported to suppress immediate food intake (King et al. 1994) and increase food consumption approximately an hour later (Verger et al. 1992), a recent review by King and colleagues (1997) indicated that the majority of studies demonstrate a weak coupling between physical activity and overall/long-term food consumption. Clearly, additional research regarding the optimal exercise prescription for weight loss is needed. In summary, low-intensity exercise resulted in greater fat utilization than moderate-intensity exercise of equivalent total energy expenditure. Throughout the entire 6-h recovery period, there was no signi®cant difference in substrate utilization between exercise trials. The di€erence in total substrate utilization was due to the e€ects of the exercise bout itself. Although it is tempting to conclude that these results mean that lowintensity, long duration exercise is preferable for fat reduction, we only examined the immediate impact on substrate utilization. Extensive intervention trials that include careful quanti®cation of food intake and physical activity (exercise, work, and leisure time) are needed to determine the best exercise for weight loss and weight control. Acknowledgements This research was supported in part by the UTK Scholarly Activities Research Incentive Fund. The authors wish to thank Timothy Henion and the sta€ of the respiratory laboratory at the UTK Medical Center for their cooperation in the data collection process.

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