day - to - day food intake involves the coordination of both homeostatic and nonhomeostatic signals in the overall expression of eating behaviour .
homeostatic feeding is often described through a series of physiological processes that initiate and terminate feeding ( i.e. , satiation ) and suppress intermeal hunger ( i.e. , satiety ) .
this pattern of eating is thought to be driven by tonic and episodic inhibitory signals ( arising from adipose tissue and the gastrointestinal tract ) that modulate an intrinsic excitatory drive to eat . however
, extrinsic determinants of eating behaviour such as food palatability and hedonic reward , linked closely to perceived fat and energy content of food , interact with these homeostatic mechanisms with the potential to enhance or undermine appetite control .
recent attention has started to focus on the hedonic determinants of eating behaviour and has highlighted the importance of distinguishing liking ( i.e. , the perceived pleasurable sensory properties of food ) from wanting ( i.e. , the attraction towards a specific food over available alternatives ) [ 5 , 6 ] . both components of food reward are thought to act in parallel to facilitate eating behaviour .
indeed , heightened liking and wanting for food have been noted in overweight and obese individuals   and those who demonstrate binge eating .
changes in food reward may also play a role in compensatory eating behaviour following exercise [ 1012 ] .
for example , following 50 minutes of cycling , lean women who overconsumed relative to the energy cost of exercise exhibited increased wanting for food compared to those who did not exhibit postexercise compensatory eating . furthermore , overweight and obese individuals who exhibited an immediate postexercise increase in explicit liking and wanting for food ( particularly , high fat sweet foods ) demonstrated smaller fat mass losses following a program of aerobic exercise training .
however , while these findings suggest that exercise - induced changes in food reward influence compensatory eating behaviour , the physiological processes that influence food reward are not well understood . given that appetite control is a psychobiological process , it is plausible that a prolonged and potent metabolic stimulus such as aerobic exercise training would be reflected in an increased motivational drive for high energy yielding foods .
however , whether adaptive changes in metabolism influence food reward during chronic exercise training has yet to be examined .
therefore , this study aimed to examine whether exercise - induced changes in body composition or metabolic - related hormones influenced food reward in overweight and obese individuals during 12 weeks of aerobic exercise .
forty - six overweight and obese individuals participated in the present study ( 30 females , bmi = 30.8  3.5  kg / m ; 16 males , bmi = 30.5  4.7  kg / m ) . participants were recruited from the university of leeds , uk , and surrounding areas using poster advertisements and recruitment emails .
participants were initially physically inactive ( 2  hrs.wk of exercise over the previous six months ) , weight stable ( 2  kg for the previous three months ) , nonsmokers , and not taking medication known to effect metabolism or appetite . at baseline
mean dietary restraint and disinhibition scores , as measured using the three - factor eating questionnaire , were 7.21  0.55 and 8.10  0.50 , respectively , which are within the normal ranges for healthy adults .
the study was conducted in accordance with the declaration of helsinki ( 1964 ) , and all participants provided written informed consent before taking part .
in addition , ethical approval was granted by the institute of psychological science 's ethics board , university of leeds , and the leeds west nhs research ethics committee ( 09/h1307/7 ) .
body composition , food intake , and fasting metabolic - related hormones ( glucose , insulin , and leptin ) were measured at baseline , week six , and postintervention .
in addition , explicit liking and implicit wanting for a standardised array of high fat and low fat foods were assessed before a fixed - energy meal , at baseline , week six , and postintervention , using a validated computer based task , for example , the leeds food preference questionnaire .
participants completed a 12-week aerobic exercise program in which they exercised five days per week , expending 500  kcal per session at 70% of age - predicted maximum heart rate .
all exercise sessions were supervised in the research laboratory , and participants could choose from a range of exercise modes ( running , cycling , rowing , or stepping ) .
individual exercise prescriptions were calculated using standard stoichiometric equations , based on the relationship between heart rate and vo2/vco2 during a maximal incremental treadmill test . to account for changes in cardiovascular fitness during the intervention , the incremental test was performed at baseline , week six , and postintervention , with the exercise prescription adjusted accordingly . to verify and record the duration and intensity of exercise
, participants wore a heart rate monitor during each session ( polar rs400 , polar , kempele , finland ) .
total exercise - induced energy expenditure during the intervention was 27960  3479  kcal , which represented > 98% of the prescribed exercise - induced energy expenditure . at baseline , week six , and postintervention , venous blood , body composition , and maximal aerobic capacity were measured in the morning ( 79am ) following an overnight fast ( 1012  hrs ) .
baseline measures were taken prior to the start of the intervention , while postintervention measures were taken upon completion of the exercise intervention ( a minimum of 48  hrs after the final exercise bout and within one week of finishing the intervention ) .
body composition was measured using air - displacement plethysmography ( bod pod body composition system , life measurement , inc .
participants were weighed ( to the nearest 0.01  kg ) and instructed to sit in the bod pod .
measurements were taken according to manufacturers ' instructions , with thoracic gas volumes estimated using the manufacturer 's software .
in addition , the fat mass index ( fmi ; fat mass / height ) and the fat - free mass index ( ffmi ; fat - free mass / height ) were calculated from these body composition data .
maximal aerobic capacity ( vo2peak ) was determined using a validated maximal incremental treadmill test , with expired air ( sensormedics vmax29 , yorba linda , usa ) and heart rate ( polar rs400 , polar , kempele , finland ) measured continuously .
the respiratory and heart rate data from this test were also used to calculate the exercise prescriptions used in the exercise intervention .
fasting glucose , insulin , and leptin were measured at baseline , week six , and postintervention in a subsample of 32 participants who completed the exercise intervention .
after collection , blood samples were centrifuged for 10  min at 4c at 3500  rpm and were immediately pipetted into eppendorf tubes and stored at 80c until analysis .
insulin and leptin were analysed using a magnetic bead based multiples kit ( millipore , billerica , ma , usa ) .
furthermore , insulin resistance was calculated using the homeostatic model of assessment ( homa ) .
a laboratory - based test meal protocol was used to measure food intake at baseline , week six , and postintervention . at each time point , participants consumed test meals at 4-hour intervals . no exercise was performed on these days .
meals consisted of an individualised energy breakfast ( ad libitum at baseline and then fixed at baseline levels for the remainder of intervention ) , a fixed - energy lunch ( 800  kcal ) , and an ad libitum dinner meal . after the dinner meal , participants were free to leave the research unit but were given an ad libitum snack box of foods to consume if desired during the evening .
all meals consumed in the research unit were eaten in isolation , with participants instructed to eat as much or as little as they wanted until comfortably full ( during ad libitum meal consumption ) .
prior to the lunch test meal , food reward was assessed using the leeds food preference questionnaire ( lfpq ; ) .
the lfpq measured liking and wanting for foods according to differences in fat content ( i.e. , > 50% or < 20% energy from fat ) .
each food category was represented by 8 photographs of ready - to - eat foods that were matched for familiarity , taste , protein , and acceptability .
firstly , to measure  implicit wanting  a forced - choice paradigm was used in which participants were presented with two foods from different categories and were required to press a key as quickly as possible to indicate which food 
this was repeated until all food photograph pairs had been presented . following dalton et al .
, the parameters were set as 96 randomised food pair trials presented in three blocks , with each stimulus appearing 8 times .
stimuli were presented until a valid response was detected up to a maximum of 4000  ms with a 500  ms washout between presentations in which a central fixation cross was displayed .
mean response times for choices outside of each food category , adjusted for choice frequency , were subtracted from response times for choices towards each category , adjusted for frequency .
therefore positive scores for a specific category indicated a more rapid preference ( i.e. ,  implicit wanting  ) . secondly , to measure explicit liking , the food images were presented individually in randomised order , and the participant rated the extent to which they liked each food image presented to them using a 100 mm visual analogue scale ; for example , how pleasant would it be to taste this food now ?
the lfpq has been shown to demonstrate reliable immediate postexercise and postmeal changes   and is a good predictor of food choice and intake in laboratory and community settings [ 22 , 23 ] .
statistical analyses were performed using ibm spss for windows ( chicago , illinois , version 21 ) . for food reward measures , mean scores for high fat and low fat categories were computed for implicit wanting and explicit liking outcomes . mean low fat scores were then subtracted from the mean for high fat scores to provide a composite score representing reward value for high fat relative to low fat food for both liking and wanting . using this approach , a positive score indicated greater liking or wanting for high fat foods over low fat foods ; a negative score indicated greater liking or wanting for low fat foods over high fat foods ; and a score of zero indicated an equal liking or wanting for high and low fat foods .
scores on each food reward outcome were calculated at baseline , week six , and postintervention and analysed using one - way repeated measures anovas .
changes in body composition , metabolic - related hormones , and total daily energy intake were examined using one - way repeated measures anovas . where appropriate , greenhouse - geisser probability levels were used to adjust for sphericity , and bonferroni adjustments were applied to control for multiple post hoc comparisons .
to test for associations between physiological variables and food reward , pearson partial correlation coefficients were used , controlling for sex .
firstly , cross - sectional models were examined using mean scores on each variable collapsed across the three time points of the exercise intervention ( i.e. , baseline , week six , and postintervention ) .
secondly , associations between changes in physiological variables and changes in explicit liking and implicit wanting following the exercise intervention were performed .
change variables were calculated by subtracting baseline scores from postintervention scores . to control for confounding effects of body composition , metabolic hormones were tested both with and without adjustment for adiposity by dividing by percentage body fat .
as can be seen in table 1 , there was a significant reduction in body mass ( 1.72  0.41  kg ; p < 0.001 ) , fat mass ( 2.23  0.38  kg ; p < 0.001 ) , and percentage body fat ( 1.90  0.22% ; p <
0.001 ) following the exercise intervention , while fat - free mass was preserved at baseline levels ( + 0.52  0.17  kg ; p = 0.081 ) . furthermore , fmi decreased ( 0.76  0.14  kg / m ; p < 0.001 ) and ffmi increased significantly ( 0.17  0.62  kg / m ; p < 0.01 ) .
vo2peak increased from 33.33  1.17  ml.kg.min at baseline to 39.16  0.09  ml.kg.min after intervention ( p < 0.001 ) .
there were no significant changes in fasting glucose ( 0.20  0.25  mmol.l ; p = 0.415 ) or fasting insulin ( 42.98  82.94  ng.l ; p = 0.230 ) following the exercise intervention .
however , fasting leptin decreased significantly following the exercise intervention ( 6215.93  3076.37  ng.l ; p = 0.023 ) . 

table 2 shows that total daily energy intake , explicit liking , and implicit wanting for high fat food did not differ significantly between baseline and week 6 or baseline and postintervention . there was a nonsignificant trend for implicit wanting to shift from a small bias for high fat food at baseline , towards a bias for low fat food following the intervention ( p = 0.114 ) . as can be seen in table 3 , liking for high fat food
was positively associated with body mass ( p = 0.008 ) and fat mass ( p = 0.022 ) and marginally associated with fat - free mass ( p = 0.046 ) .
however , there were no significant associations between these components of body composition when adjusted for height ( i.e. , fmi or ffmi ) .
wanting for high fat foods was also positively associated with body mass ( p = 0.004 ) , fat mass ( p = 0.005 ) , and fmi ( p = 0.018 ) , but not fat - free mass ( p = 0.129 ) or ffmi ( p = 0.161 ) . of the metabolic hormones ,
fasting leptin was positively associated with both greater liking ( p = 0.023 ) and wanting ( p = 0.021 ) responses .
moreover , these relationships remained after adjusted fasting leptin values for percentage body fat ( liking , p = 0.043 ; wanting , p = 0.041 ) , suggesting they were independent of adiposity .
no associations existed between changes in food reward and changes in body composition following the intervention ( table 3 ) .
furthermore , no associations existed between the changes in food reward and the changes in fasting glucose , insulin , homa index , or vo2peak .
however , the change in fasting leptin ( absolute or adjusted leptin ) was found to be negatively associated with the change in liking for high fat foods following the exercise intervention ( p = 0.018 and p = 0.031 , resp . ) .
as can been seen in figure 1 , a decline in fasting leptin following the exercise intervention was associated with increased liking for high fat food .
no associations were found between the change in implicit wanting for high fat foods and the change in fasting leptin ( absolute or adjusted leptin ) .
this study examined whether components of body composition and metabolic - related hormones were associated with food reward in overweight and obese individuals during 12 weeks of aerobic exercise .
cross - sectional analyses disclosed associations between body composition ( fat mass and fat - free mass ) , fasting leptin , and food reward .
furthermore , a novel relationship was also disclosed between the change in fasting leptin and the change in explicit liking for high fat foods following the exercise intervention .
specifically , a decline in fasting leptin was associated with an increased liking for high fat foods relative to low fat foods following the intervention .
this relationship was independent of changes in fat mass and suggests that leptin may have a key role in mediating changes in food reward during exercise - induced weight loss .
the 12-week exercise intervention resulted in significant ( but modest ) reductions in body mass , fat mass , and percentage body fat , while fat - free mass was preserved at baseline levels .
when the changes in food intake and reward were examined , no mean changes in total daily energy intake , explicit liking , or implicit wanting were found .
however , it has become clear that examination of the mean ( group ) response to exercise masked marked heterogeneity in eating behaviour and exercise - induced weight loss following acute   and chronic exercise [ 2528 ] .
previous studies have suggested that exercise - induced changes in food reward may mediate compensatory eating behaviour and in turn body weight regulation following exercise [ 1012 ] .
however , the physiological correlates of food reward during exercise - induced weight loss have not previously been examined .
recent evidence has demonstrated the importance of distinguishing explicit perceptions of liking from behavioural operations of wanting , with these components of food reward considered to be separable risk factors in overconsumption and weight gain [ 5 , 6 ] . during the present study , fat mass and fat - free mass
implicit wanting was only associated with fat mass cross - sectionally ( i.e. , when the baseline , week six , and postintervention measures were combined ) .
these data therefore suggest that fat mass may predict food reward ( particularly food wanting ) independently of fat - free mass .
these findings are consistent with recent observations that fat mass and fat - free mass operated differentially in the control of appetite , with separate roles for fat - free mass in satiation   and hunger [ 19 , 30 ] and fat mass in hedonic eating behaviour traits   and neural activation to high energy foods .
however , these findings are so far limited to obese individuals and need to be confirmed in a range of different populations , that is , lean versus obese and active versus inactive .
it has been suggested that obese individuals display a loss of hedonic control over eating when exposed to highly palatable foods compared to lean individuals .
this increase in the susceptibility to overconsumption in obese individuals may be related to increased leptin and insulin resistance ( resulting from the excessive accumulation of adipose tissue ) , which may reduce the sensitivity of short - term appetite control [ 34 , 35 ] . in the present study ,
cross - sectional associations were found between explicit liking and implicit wanting for high fat foods relative to low fat foods and fasting leptin , but not fasting glucose , insulin , homa , or vo2peak .
these findings are supported by raynaud and colleagues , who examined the relationships between body composition , serum leptin , insulin , and self - reported palatability of a high cho breakfast in a sample of predominantly obese adults .
a positive relationship was noted between serum leptin and palatability , but not insulin and palatability , and this association remained after controlling for bmi or fat mass . in the present study
, the cross - sectional associations indicated that a greater implicit wanting for high fat foods was associated with greater ad libitum food intake , suggesting that differences in food reward are expressed behaviourally through differences in food intake ( data not reported ) .
however , it should be noted that as the test meal design employed in the present study incorporated both fixed - energy and ad libitum meals , the measures of daily energy intake in the present study do not reflect  true
interestingly , the present study also disclosed a novel relationship between the changes in fasting leptin and explicit liking for high fat foods following the exercise intervention , with a decline in fasting leptin associated with an increase in liking for high fat foods relative to low fat foods .
this relationship is consistent with the proposed role of leptin in food reward , in which leptin is thought to tonically inhibit brain reward pathways .
it is hypothesised that a reduction in leptin would act to increase the sensitivity of reward brain circuitry , potentially increasing the motivation to consume highly palatable energy dense foods via its action on dopamine reuptake transporters . furthermore , leptin 's role as an adiposity signal is well established , with a decline in leptin thought to stimulate increased hunger and , in turn , food intake , via a downregulation in the hypothalamic expression of anorexigenic neuropeptides , such as proopiomelanocortin and alpha - melanocyte stimulating hormone , and an upregulation in the expression of orexigenic neuropeptides , such as neuropeptide y and agouti gene - related peptide [ 38 , 39 ] .
importantly , the present data suggest that a decline in leptin may also promote a greater perceived liking for high fat foods , thereby helping to further promote increased food intake and the restoration of energy homeostasis .
the present findings are in keeping with the idea that leptin is primarily a  starvation  signal rather than a  satiety  signal .
while a decline in leptin is thought to promote increases in hunger and food intake , an increase in fat mass and leptin does not appear to exert a proportional downregulation in eating .
as such , it could be argued that the inhibitory action of fat mass ( and associated adiposity signals ) on food intake is actually weaker at higher levels of fat mass , and this asymmetry may reflect increased  leptin resistance  with obesity . indeed , leptin resistance may account for the positive ( cross - sectional ) correlations between fasting leptin and food reward seen in the present study .
however , a decline in leptin ( independent of fat mass ) during the exercise intervention was also found to be associated with increased explicit liking for high fat foods . while these findings may initially appear contradictory
, it has been argued that it is the fall in circulating leptin below a critical ( and individualised ) threshold level that triggers corrective hypothalamic responses to restore energy homeostasis [ 4143 ] .
theoretically , increased leptin sensitivity resulting from the exercise intervention could have made individuals more sensitive to perturbations in peripheral leptin concentrations , with a decline in leptin perceived by the brain in some individuals as a state of relative leptin deficiency despite an actual surplus of stored energy , that is , fat mass [ 4143 ] .
however , clearly this can only be speculated , upon and the precise role of leptin and leptin resistance in food reward remains an important area for future research .
it has previously been reported that the change in leptin ( independent of fat mass ) during weight loss was negatively associated with the changes in subjective appetite .
these observations were made in the context of a 12-week weight loss program in which subjects lost an average of 7  kg fat mass ( through diet and exercise ) .
the present intervention on the other hand resulted in a relatively modest 2.2  kg loss of fat mass .
therefore , the subtle effects of the exercise intervention on food reward are perhaps unsurprising . it should be noted that a role for leptin in the hedonic control of food intake during exercise - induced weight loss is a novel hypothesis , and , as such , further work is needed to examine the physiological correlates of food reward in more targeted research .
nevertheless , these findings are consistent with other recent observations that some individuals experience a greater than expected decline in resting metabolic rate following exercise - induced weight loss , and this compensatory downregulation in resting metabolic rate was again associated with a decline in fasting leptin ( independent of fat mass ) .
importantly , those individuals who experienced a compensatory downregulation in resting metabolic rate also experienced a concomitant upregulation in food intake during exercise - induced weight loss .
the present study has some limitations that deserve comment . when interpreting the findings of the present study , it is important to note that a nonexercise control condition was not included .
however , the observed improvements in body composition , vo2peak are unlikely to have occurred independent of the exercise intervention . furthermore , due to the need to measure body composition and metabolism at standardised time points during the exercise intervention
this may have contributed to the variability seen in food reward , as studies have shown that eating behaviour and food reward are influenced by the phases of the menstrual cycle [ 46 , 47 ] .
through the concurrent measurement of physiological and behavioural components of energy balance , this study has disclosed novel relationships between food reward , body composition , and metabolic - related hormones in overweight and obese individuals .
cross - sectional relationships were found between measures of explicit liking and both fat mass and fat - free mass .
however , only fat mass was found to be associated with implicit wanting , suggesting that aspects of body composition may differentially affect the separate components of food reward .
independent of adiposity , a positive relationship between fasting leptin and liking and wanting for high fat food was demonstrated .
furthermore , a decline in fasting leptin following the exercise intervention was found to be associated with an increase in liking for high fat relative to low fat foods . taken together ,
these findings suggest a dynamic role for fasting leptin as a regulatory signal of food reward during exercise - induced weight loss .
 changes in food reward have been implicated in exercise - induced compensatory eating behaviour 
 . however , the underlying mechanisms of food reward , and the physiological correlates of exercise - induced changes in food reward , are unknown . methods . 
 forty - six overweight and obese individuals completed 12 weeks of aerobic exercise . 
 body composition , food intake , and fasting metabolic - related hormones were measured at baseline , week six , and postintervention . on separate days , 
 the reward value of high - and - low - fat food ( explicit liking and implicit wanting ) was also assessed at baseline , week six , and postintervention . 
 results . following the intervention , fm , ffm , and vo2peak 
 improved significantly , while fasting leptin decreased . 
 however , food intake or reward did not change significantly . 
 cross - sectional analyses indicated that fm ( p = 0.022 ) and ffm ( p = 0.046 ) were associated with explicit liking for high - fat food , but implicit wanting was associated with fm only ( p = 0.005 ) . 
 fasting leptin was associated with liking ( p = 0.023 ) and wanting ( p = 0.021 ) for high - fat food . 
 furthermore , a greater exercise - induced decline in fasting leptin was associated with increased liking ( p = 0.018 ) . 
 conclusion . 
 these data indicate that food reward has a number of physiological correlates . 
 in particular , fasting leptin appears to play an active role in mediating food reward during exercise - induced weight loss . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusion