in many countries , increased consumption of a westernized diet and lack of physical 
 activity have caused an energy intake imbalance and resulted in an increased prevalence of 
 obesity and chronic diseases , including hypertension , cardiovascular disorders , diabetes , 
 and arthritis1 . however , regular exercise 
 and physical activity are reported to be effective methods for preventing obesity , its 
 accompanying chronic lifestyle diseases , and premature death2 . when performed on a regular , long - term basis , aerobic exercise can 
 prevent or cure obesity , increase muscle volume , elevate oxygen levels and oxidizing enzymes 
 in myocytes , and increase blood volume and fat oxidation3 . in particular , aerobic exercise can improve the various metabolic 
 risk factors involving blood pressure , fatty acids , and the cardiovascular system in obese 
 and type 2 diabetic patients4,5,6 . one study suggested 
 that exercise in the morning on an empty stomach promotes the utilization of lipids in 
 adipose tissue because the exercise is performed under low blood sugar conditions7 . because this exercise regime promotes 
 increased utilization of lipids , it is thought to be more efficient for weight 
 regulation9 . however , findings from 
 other studies have suggested that performing aerobic exercise in the morning before eating 
 can induce neurotic symptoms and fatigue because low blood sugar levels result in 
 stimulation of the hypothalamic - pituitary - adrenal ( hpa ) axis7 , 8 . elevated cortisol level 
 can decrease protein synthesis in the muscle , thereby affecting carbohydrate , fat , and 
 protein metabolism , and can also promote gluconeogenesis in the liver9 , 10 . however , 
 increased cortisol levels have also been observed to result from moderate- to high - intensity 
 aerobic exercise , which complicates the interpretation of these previous studies . the 
 ability of exercise to prevent obesity and reduce body weight depends on the retention of 
 internal nutrition before exercise and the secretion of hormones that affect carbohydrate , 
 fat , and protein metabolism11 . many 
 studies have evaluated energy substrate changes and hormone levels ( e.g. , cortisol , growth 
 hormone , and insulin ) in relation to different exercise parameters ( e.g. , intensity ) , but 
 none have investigated the effects of exercise performed in the morning ( when cortisol 
 secretion is at its maximum ) following an overnight fast or breakfast . therefore , the 
 present study aimed to compare the effects of exercise after overnight fasting and after 
 breakfast on energy substrate changes and hormone levels in male university students with 
 obesity . the target group of the current investigation included 10 male students with obesity , a 
 body mass index of > 25  kg / m , and body fat of > 20% . subjects were included 
 if they were physically healthy and had no specific diseases based on a questionnaire survey 
 and medical examination and had not taken any specific drugs for at least 6 months prior to 
 the experiment . informed consent was obtained from the subjects prior to their participation 
 in the study . kyungwoon university approved this study , which complies with the ethical 
 standards of the declaration of helsinki . the physical characteristics of the subjects are 
 shown in table 1table 1.physical characteristics of the 10 study participantsparametermean  standarddeviationage20.21.9height ( cm)177.26.1weight ( kg)84.314.6bmi ( kg / m)26.74.1body fat ( % ) 23.84.9vo2max ( ml / kg / min)45.28.5bmi : body mass index ; vo2max : maximal oxygen consumption . bmi : body mass index ; vo2max : maximal oxygen consumption the experiment had a crossover design ( fig . 1.schema of the study design and blood collection times ) and was conducted over a period of 4 weeks . the subjects performed the first 
 experiment ( exercising after an overnight fast ) after 1 week of testing and then performed 
 the second experiment ( exercising 2  h after breakfast ) subsequent to a 2-week washout period 
 from the first experiment . for both experiments , the subjects maintained an empty stomach 
 after 10 p.m. on the day before exercising and then performed the exercise at 9 a.m. on the 
 next day . for the second experiment , the subjects consumed a specified ratio of calories and 
 nutrients 2  h before the exercise . blood collection was performed immediately before and 
 after exercise and also at 60  min after the exercise . schema of the study design and blood collection times for both experiments , the subjects exercised on a treadmill at 75% of vo2max and 
 burned up to 400  kcal . for controlling effects due to differences in exercise intensity and 
 duration between the 2 exercise regimes , a regression equation derived from the data sheet 
 for graded exercise test values ( oxygen intake and heart rate per minute ) the 
 average target heart rate and exercise time were determined to be 155.80  6.90 bpm and 
 31.70  2.35  min , respectively . meanwhile , to precisely monitor the individual caloric 
 consumption , the time point at which the target heart rate and exercise time were reached 
 was measured with an automated heart rate monitor ( polar heart monitor , polar s610i , polar 
 electro , kempele , finland ) . the vo2max was measured with an exercise load test 
 ( graded exercise test ) using a treadmill ( medtrack st 65 ; quinton instrument co. , boston , 
 ma , usa ) and a respiratory gas analyzer ( true - one ; quinton instrument co. ) . blood was collected from the brachial vein ( 10  ml each time ) 
 immediately before and after exercise and at 60  min following exercise for both exercise 
 regimes . the collected venous blood was maintained for 10  min at room temperature and then 
 centrifuged at 3,000  rpm for 10  min . the upper phase was transferred to a microtube and 
 stored at 80  c until the analysis . glucose level was measured immediately after blood 
 collection using one - touch ( lifescan company , mountain view , ca , usa ) , and serum insulin 
 level was analyzed using a commercial insulin reagent kit ( human insulin elisa - kit , dsl , tx , 
 usa ) . serum free fatty acid ( ffa ) , cortisol , and growth hormone levels were assessed by 
 green cross inc . , the parameters measured immediately before and after 
 exercise and at 60  min after exercise between the 2 exercise regimes were compared and 
 analyzed by two - way analysis of variance with repeated measures . a significance level of p = 
 0.05 was used for the analyses . the statistics were performed with spss - pc version 20 ( spss 
 inc . , for minimizing the differences in the amount of exercise performed between the 2 
 experiments , energy consumption was set at 400  kcal . as a result , energy consumption did not 
 differ between the exercise performed after an overnight fast and that performed after 
 breakfast ( 426.05  20.54 vs. 425.49  19.00 , t - value = 0.403 , p = 0.692 ) . table 2table 2.comparison of energy substrate and hormone changes according to exercise 
 treatmentvariablefasting exercisepost - prandial exercisebeae60  min aebeae60  min aeglucose ( mg / dl)98.79.7102.88.8105.49.4109.710.8100.110.3102.34.1ffa ( eq / l)502.7231.4683.8428.9707.5328.7115.146.2518.9165.0378.1190.5*,insulin ( uiu / ml)13.9911.155.2713.0914.7417.4340.2224.444.287.6410.008.26*,gh ( ng / ml)1.071.8511.027.031.551.320.070.375.733.750.440.41*,cortisol ( g / dl)28.576.7424.939.1620.929.2515.495.7320.068.2213.295.19*,values are shown as means  standard deviation * p < 0.05 . significant main effect 
 for time . be ; before exercise , ae ; after exercise , 60  min 
 ae ; 60  min after exercise . shows the energy substrate ( glucose and ffa ) and hormone levels ( insulin , gh , 
 and cortisol ) immediately before and after exercise and at 60  min after exercise for both 
 exercise treatments . glucose showed significant interaction effects between the measurement 
 time and the exercise treatment ( p < 0.007 ) , but no significant differences were observed 
 between measurement times or between exercise treatments . a possible explanation for this 
 interaction effect is that the glucose levels had not been restored to their post - breakfast 
 exercise levels . however , significant differences were observed between measurement times 
 ( p < 0.001 ) and between exercise treatments ( p < 0.002 ) . these results suggest that 
 exercising in the morning before breakfast is more efficient than post - prandial exercise in 
 terms of utilizing lipids as an energy source . insulin showed a significant interaction 
 effect between the measurement time and exercise treatment ( p < 0.002 ; table 2 ) . additionally , significant differences were 
 observed between measurement times ( p < 0.001 ) but not between exercise treatments . the 
 interaction effect suggests that insulin was secreted in response to the high level of 
 glucose resulting from the patient s dietary intake . meanwhile , neither growth hormone nor 
 cortisol showed a significant interaction effect between measurement time and exercise 
 treatment ( p < 0.216 and p < 0.102 , respectively ) . however , both hormone levels 
 significantly differed between measurement times ( p < 0.001 and p < 0.012 , 
 respectively ) and between exercise treatments ( p < 0.025 and p < 0.004 , 
 respectively ) . be ; before exercise , ae ; after exercise , 60  min 
 ae ; 60  min after exercise . in the current study , glucose and insulin showed a significant interaction effect between 
 the measured time and treatment . one likely explanation is that their levels were high due 
 to the dietary intake 2 hours prior to the exercise . in addition , there were significant 
 differences in insulin level between the measurement time points , independent of treatment , 
 indicating that the insulin levels decreased immediately after exercise due to the increased 
 energy demand13 and then stabilized 
 60  min after the exercise . increased physical exercise improves glucose homeostasis and 
 increases sensitivity to insulin , and acute exercise can increase the sensitivity of tissues 
 or organs such as the skeletal muscles , fat , liver , and hypothalamus to insulin14,15,16,17,18,19 . further , acute exercise has been shown to result in improved glucose homeostasis in an 
 animal model of obesity16 , 18 . when blood sugar levels are low , camp level 
 increases20 and activates fat 
 disassembly in the adipocytes21 . in 
 addition , these factors imply that the morning is the most efficient time of 
 day to reduce body fat through exercise , particularly since blood sugar levels are at their 
 lowest . dreave et al.7 reported that the 
 fat oxidation rate is high during morning exercise after overnight fasting as compared to 
 that during post - prandial exercise , and they concluded that the former exercise regime is 
 efficient for body fat reduction . further , in the present study , growth hormone levels were 
 higher at all evaluated time points after overnight fasting as compared to the levels after 
 breakfast before exercise , and these levels could be related to the higher ffa levels 
 observed after an overnight fast . according to these results , the high levels of growth 
 hormone after overnight fasting increased the transport of fatty acids from adipocytes into 
 the blood , thereby acting as a source of energy for muscles during exercise in the absence 
 of carbohydrates . growth hormone and cortisol were previously reported to be simultaneously secreted during 
 fasting23 , 24 or exercise25 , 26 and were found to increase ffa and 
 glycerol levels in the blood27 . in the 
 current study , the ffa concentrations were high at all measured time points , which is 
 consistent with the results of previous studies with respect to overnight fasting and 
 morning exercise . in these previous studies , morning exercise was considered to be efficient 
 for decreasing weight and body fat in those with obesity , particularly when performed on an 
 empty stomach . however , in the current study , high cortisol concentrations were also 
 observed at all the accessed time points for both treatment groups . during exercise , 
 cortisol is secreted by the adrenal cortex in response to stress . cortisol decreases protein 
 synthesis in the muscles ; increases catabolism , affecting the metabolism of carbohydrates , 
 fats , and proteins ; and stimulates gluconeogenesis in the liver28 . in the current study , the cortisol concentration was 
 higher after overnight fasting than it was after dietary intake . these results indicate that 
 glucose and insulin levels were high before exercise following dietary intake because the 
 cortisol levels were too low for the hormone to play a strong antagonistic role29 . moreover , these results suggest that a 
 greater amount of muscle wasting for use as an energy source during exercise after a morning 
 fast , as compared to exercise after breakfast , presumably resulted in a decreased 
 improvement in the fat - free mass . however , because the current study only examined the 
 effects of acute exercise , a long - term study will be needed for further validation . in 
 addition , persistently high cortisol levels resulting from regular exercise after overnight 
 fasting can negatively affect physical health . in particular , previous studies have 
 indicated that these persistently high cortisol levels can promote fat accumulation in the 
 abdomen , decrease insulin sensitivity30 , 
 increase the risk of osteoporosis31 and 
 hypertension32 , and decrease immune 
 function and muscle mass , thereby negatively affecting the patient s overall physical 
 health . in the present study , we were unable to draw clear conclusions on the effect of 
 performing exercise after overnight fasting on physical health . however , the fundamental 
 results reported here provide the basis for a long - term study to more clearly address this 
 issue . recently , a number of studies have been carried out in the attempt to develop exercise 
 methodologies for maintaining and improving physical health to reduce the prevalence of 
 obesity . the effect of exercise timing and pre - exercise nutritional state has emerged as an 
 important consideration . in the current study , it should be emphasized that energy substrate 
 and metabolic hormone levels differed between the treatment groups . compared to the 
 post - prandial exercise , the exercise performed after overnight fasting showed low glucose 
 and insulin levels but high cortisol , growth hormone , and ffa levels before exercise . hence , 
 the latter treatment could have a long - term positive effect on body fat reduction but 
 negatively affect physical health because of the high concentration of cortisol .
[ purpose ] we compared the effects of acute aerobic exercise following overnight fasting 
 and breakfast on energy substrate and hormone levels in obese male college students . 
 [ subjects and methods ] this crossover study recruited 10 obese male college students with 
 a body mass index > 25  kg / m2 or > 20% body fat . 
 one week post - recruitment , 
 the subjects exercised in the morning after an overnight fast . at 2 weeks , they exercised 
 post - breakfast . energy substrate ( glucose , free fatty acid ) and metabolic hormone 
 ( insulin , growth hormone , and cortisol ) levels were measured immediately before and after 
 exercise and at 60  min post - exercise . [ results ] we observed interaction effects between 
 the measurement time and exercise treatment for glucose ; significant differences between 
 measurement times and between exercise treatments for free fatty acids ; interaction 
 effects between the measurement time and exercise treatment for insulin and significant 
 differences in the measurement time ; significance differences between measurement times 
 and between exercise treatments for growth hormone ; and significant differences between 
 measurement times and between exercise treatments for cortisol . 
 [ conclusion ] 
 morning exercise following an overnight fast can be more effective in reducing body fat 
 than post - prandial exercise . 
 however , increased cortisol levels following exercise after 
 overnight fasting may negatively affect long - term weight loss in obese men .