exercise physiology and the salutatory effects on weight loss , fat reduction , and insulin sensitivity have been described in great detail .
these beneficial effects are now considered to reflect , at least in part , the effect of exercise on the activation of amp - activated protein kinase ( ampk ) . in obese non - diabetics , exercise has been shown to reduce the risk of developing type 2 diabetes by up to 46% .
physical training , consisting of 20  min cycling or running , 20  min swimming at submaximal heart rate , followed by 20  min of warm up / cool down three times per week for 4  wk , resulted in a significant reduction in body weight and percentage body fat , and this was associated with improved whole - body glucose uptake , decreased fasting insulin concentrations , and increased circulating adiponectin and mrna expression in muscle
. among patients with type 2 diabetes mellitus , increasing exercise led to a reduction in fasting plasma glucose .  
the 24-hour energy expenditure , whether measured in a respiratory chamber   or by using doubly labelled water , increases linearly with increasing body weight .
multivariate analysis shows that fat - free mass is the major determinant of energy expenditure , with minor influences of fat mass , age , and gender . nevertheless , the considerable interindividual variation is currently unexplained .
there is at the present time little evidence for major adaptations of energy expenditure during overfeeding , and changes in body weight and body composition appear to be the major factors that , by increasing energy expenditure , allow energy balance to be restored .
the body - weight gain may therefore be seen as an adaptative change to overfeeding [ 6 , 7 ] .
total energy expenditure can be subdivided into three main components : basal metabolic rate ( bmr ) ; the thermic effect of food , or diet - induced thermogenesis ; energy expended in physical activity ( pha ) .
the first two components can be measured opportunely with reasonable accuracy and have been fully studied in lean and obese subjects .
there is actually no clear evidence that a low bmr is a factor in the development of obesity , even if the issue continues to be debated [ 8 , 9 ] .
the thermic effect of food , and more specifically the thermic effect of carbohydrate , has been shown to decrease in obese subjects [ 10 , 11 ] .
this decrease may , nevertheless , be secondary to obesity - related insulin resistance and is of too small magnitude to account for a major weight gain .
nonexercise activity thermogenesis ( neat)the energy expended for everything we do that is not sleeping , eating , or sports - like exercise  and pha , that is crucial for weight control , may be important in the physiology of weight change .
we review the current concepts about energy expenditure and evaluate the pha in the context of this knowledge and the available literature .
during pha , mechanical work associated with muscle contractions clearly requires energy . as a result of the associated loss of energy as heat during atp synthesis in the mitochondria and atp hydrolysis during muscular contraction ,
pha may thus have a major impact on the total 24-hour energy expenditure and energy balance .
based on the possible means by which alterations in pha can impact on 24-hour energy expenditure and energy balance , the following hypotheses can be formulated .
this response would induce to a lower energy expenditure for any given work load and would probably contribute to the realization of a positive energy balance in affected subjects .
it was further shown to be similar in both lean and post - obese women .
there is then little reason to suppose that this factor might be involved in body - weight gain .
, nevertheless , showed that a common polymorphism of uncoupling protein 2 , a protein with close homology to the uncoupling protein of brown adipose tissue ubiquitously expressed in man , was associated with a lower energetic efficiency of muscle contractions .
therefore , brown adipose tissue , where uncoupling protein 1 ( ucp1 ) activity uncouples mitochondrial respiration , is an important site of facultative energy expenditure .
ucp1 single nucleotide polymorphisms ( snps ) could represent  thrifty  factors that promote energy storage in prone subjects .
these observations and the relationship between this polymorphism and body weight remain to be further evaluated .
bmr is higher in trained subjects than in sedentary subjects of the same weight , but the difference can be principally ascribed to changes in body composition , with a lower fat mass and a higher fat - free mass , metabolically active body mass in trained athletes . when exercise training is associated with a substantial negative energy balance , bmr may better still decrease . the increase in energy expenditure induced by a -adrenergic stimulation , derived by mental stress ,
was also found to be unchanged in obese subjects after a 6-week period of physical training .
third , the total energy expended in pha , that is , the period of time devoted daily to pha , may play an important role in determining energy balance . nevertheless , this component of daily energy expenditure is actually very difficult to assess accurately .  
the doubly labelled water method permits evaluation of total energy expenditure under conditions of everyday life over several days and , therefore , is likely to include habitual pha .
the difference between the total energy expenditure and the basal energy expenditure , the suprabasal energy expenditure ( see ) , comprises both the thermic effect of food and pha .
this finding strongly suggests that the amount of energy expended in pha is low in obese subjects .
whether a low pha exists before obesity , or is merely a consequence of it , remains an open question at the present time .
the conclusion from the substantial literature on this topic is that during an acute bout of exercise in untrained fed subjects , carbohydrate oxidation replaces the major portion of the extra energy expended . in endurance - trained subjects
consequently , there is an increased ability to oxidize lipids , due to the upregulation of the enzyme amp - activated protein kinase in skeletal muscle .
the outcomes are increased maximal o2 consumption and a higher proportion of fat oxidized during the exercise of low - to - moderate intensity . at high intensity ,
there is evidence that endurance - trained athletes consume a diet containing a high proportion of carbohydrate . due to the hierarchy in substrate oxidation mentioned earlier , these dietary habits perhaps impact on substrate oxidation in everyday conditions .
it can be expected , therefore , that athletes who conserve a stable body composition while consuming such a diet will have a high 24-hour oxidation of carbohydrates . in healthy lean untrained subjects studied in a respiratory chamber ,
moderate pha increased total energy expenditure , essentially by increasing carbohydrate oxidation when the exercise took place after a meal .
nevertheless , the same exercise performed in the fasting state , before breakfast , led to a marked increase in lipid oxidation .
in fact , since the subjects were fed the same isoenergetic diet under both conditions , exercising before breakfast versus after breakfast led to a negative lipid balance and to a positive carbohydrate balance .
it might , therefore , be hypothesized that exercise in the fasting state may preferably promote lipid utilization and favourably affect body composition by decreasing fat stores .
this hypothetical scheme appears unlikely , nevertheless , if the diet consumed is not changed , since the positive carbohydrate balance will probably increase carbohydrate oxidation . is there an effect of food intake on pha ?
it has been recognized for several decades that severe underfeeding leads to behavioural adaptations that result in a decreased spontaneous pha .
more recently , there have been reports suggesting that overfeeding increases energy expenditure more than would be predicted on the basis of body weight and lean body mass changes [ 26 , 27 ] .
furthermore , this increase in energy expenditure showed marked interindividual variations and was inversely correlated with body - weight gain . in fact , since changes in bmr and in the thermic effect of food were mainly accounted for by alterations in body composition and dietary intakes , this increase in  suprabasal  energy expenditure was attributed to a stimulation of spontaneous pha .
such pha was unrelated to exercise and hence was termed  nonexercise activity thermogenesis  ( neat ) .
neat is the energy expended for everything we do that is not sleeping , eating , or sports - like exercise .
it includes the energy expended walking to work , typing , performing yard work , undertaking agricultural tasks , and fidgeting .
the first approach is to measure or estimate total neat . here , total daily energy expenditure is measured and from it , the bmr plus thermic effect of food is subtracted .
the second approach is the factorial approach whereby the components of neat are quantified and total neat calculated by summing these components .
the amount of neat that humans perform represents the product of the amount and types of physical activities and the thermogenic cost of each activity [ 28 , 29 ] .
the factors that impact a human 's neat are readily divisible in biological factors such as weight , gender , body composition , and environmental factors such occupation .
the variability in neat might be viewed as random and unprogrammed but human data contradict this thesis .
it appears that changes in neat accompany experimentally induced changes in energy balance and may be important in the physiology of weight change .
it then becomes intriguing to dissect mechanistic studies that delineate how neat is regulated by neural , peripheral , and humoral factors .
neat may be a carefully regulated  tank  of physical activity that is crucial for weight control .
there is no much understanding of the mechanisms that may probably be responsible for the stimulation of neat during overfeeding at the present time .
calorimetric studies show that 24-hour energy expenditure increases in healthy subjects after short - term carbohydrate , but not fat , overfeeding [ 30 , 31 ] .
this effect was not correlated to the increase in plasma leptin concentrations observed after carbohydrate overfeeding .
several animal studies show that the stimulation of the melanocortin-4 receptor ( mc4r ) is associated with an increase in spontaneous pha [ 32 , 33 ] .
showed that new mc4r mutations in a large number of severely obese adults living in southern italy .
these mutations , not present in normal - weight individuals , are further evidence that defects in the melanocortin pathway are related to severe obesity . how the intake of specific macronutrients impact on this pathway and the mechanisms involved remains to be clarified .
the hypothesis that physical activity exerts beneficial effects on body - weight status by mechanisms not associated to an increase in energy expenditure should be considered .
nevertheless , data on energy expenditure and body composition do allow some conclusions to be drawn .
if an endurance - trained athlete consistently expends a substantial amount of energy in pha and at the same time maintains a constant weight and body composition , energy intake must have been increased appropriately , most probably spontaneously .
the mechanisms responsible for this increase in energy intake have not been completely studied . as a result of the complex network in the central nervous system that regulates energy expenditure and the known effects of pha on neuroendocrine regulations
, it is very likely that the relationship between energy expended in pha and food intake are is complex .
nevertheless , endurance - trained athletes are characterized by a low fat mass , which may be the result of previous periods of negative energy and fat balances .
they have low plasma leptin levels too , which are mainly ascribable to the low fat mass , since comparable leptin levels are observed in very lean sedentary subjects .
several studies have searched for a direct inhibition of leptin release by acute or chronic exercise . except for a decrease in plasma leptin levels following particularly intense exercises such as an ultra - marathon ,
these researches have failed to identify a long - lasting reduction in leptin levels after exercise [ 3640 ] .
it may specify that a substantial decrease in fat - free mass induced by exercise is required to decrease leptin secretion before low plasma leptin in turn increases food intake .
intriguingly , this absence of effects of exercise contrasts with the effect of severe energy restriction , which quickly decreases plasma leptin levels before any marked changes in body composition occur .
it may specify that exercise is more efficient than severe energy restriction in the promotion of weight loss without excessive rebound hyperphagia .
the effects of pha on energy metabolism and body - weight control remain incompletely understood .
there is ample evidence that physical training is associated with low body weight and low fat mass .
this relationship unequivocally suggests that negative energy and fat balances are correlated with physical training .
the negative energy balance is perhaps to be at once secondary to the quantity of energy expended while exercising , so there is no evidence that exercise influences other components of energy expenditure . the negative fat balance is probably secondary to this negative energy balance . in obese individuals
the amount of energy expended in pha appears to be small , which certainly represents a factor that could prevent weight loss .
it appears useful to focus on obese patients also in general practice in order to recognize sedentary life styles and encourage pha through individualized programs .   some studies [ 43 , 44 ] show a mild and nonsignificant reduction of bmr in patients treated with an integrated dietetic plus physical exercise program and a significant reduction of bmr in surgically treated patients .
it will be necessary to have a larger number of patients to confirm these findings .
our knowledge of the pathological consequences of the lack of adequate exercise on adipose tissue , skeletal muscle , and the liver is improving , and this will help establish more specific guidelines for the proper exercise regimens that will improve underlying metabolic pathways .
neat and physical activity , that is crucial for weight control , may be important in the physiology of weight change .  
two issues that remain to be resolved are whether pre - obese individuals have a low pha level that contributes to weight gain and , if so , what are the biological determinants of this low pha .
 we review the current concepts about energy expenditure and evaluate the physical activity ( pha ) in the context of this knowledge and the available literature . 
 regular pha is correlated with low body weight and low body fat mass . 
 the negative fat balance is probably secondary to this negative energy balance . 
 nonexercise activity thermogenesis ( neat ) and physical activity , that is crucial for weight control , may be important in the physiology of weight change . an intriguing doubt that remains unresolved 
 is whether changes in nutrient intake or body composition secondarily affect the spontaneous physical activity . 
1. Introduction
2. Physical Activity and Body Weight
3. Physical Activity and Fat Oxidation
4. Physical Activity and Spontaneous Food Intake
5. Conclusive Remarks