for the past several decades , the mainstream scientific opinion on healthy diets and the recommendations for dietary intake have favored high - carbohydrate and low - fat ( hclf ) diets .
this position has led most countries in the world to issue dietary guidelines in favor of lowering dietary fat and increasing starch and fiber intake ( myers et al . , 2013 ) .
these guidelines have largely been followed as dietary carbohydrate content has gradually increased at the expense of fat ( johnston et al . , 2014 ) . despite the scientific and dietary progress in the recent decades ,
overweight and obesity have been rising among adults worldwide in parallel with type 2 diabetes ( shaw et al . , 2010 ) and cardiovascular disease ( naghavi et al . ,
while the mainstream science has advocated hclf diets , in recent decades a group of scientists , practitioners , and the general public have explored the efficacy of low - carbohydrate - high - fat ( lchf ) diets such as the atkins diet as a means of weight loss ( gudzune et al . , 2015 ) .
in addition , a number of clinical studies revealed the beneficial effects of lchf diets on a wide range of metabolic risk factors exceeding , or at least matching , those of hclf diets .
a less well explored question is whether lchf diets may exert beneficial effects on exercise and athletic performance . despite the concept that eating a lchf diet goes counter the traditional view that athletes require high - carbohydrate intake to maintain sufficient muscle glycogen for high - intensity and endurance performance
there is emerging scientific evidence that lchf diets at least maintain , if not enhance , endurance performance while at the same time improving body composition compared to hclf diets .
these results have raised the awareness that a re - examination of lchf diets for sport performance is necessary ( burke , 2015 ) .
the aims of this review were to examine the evidence for the efficacy of lchf diets in body composition , performance in endurance and high - intensity exercise , central fatigue , and perceptual - motor ability in athletes .
lchf diets , or ketogenic diets , usually contain less than 20% of energy from carbohydrate , more than 50% of energy from fat , and variable amounts of protein ( hession et al.,2009 ; hu et al .
several studies used extreme lchf diets that contained less than 5% of carbohydrate ( langfort et al . , 2004 ; martin et al . , 2011 ; paoli et al . , 2012 )
lchf diets usually lead to ketosis when the liver oxidizes high concentrations of non - esterified fatty acids ( nefa ) into ketone bodies , including 3-hydroxybutyrate , acetoacetate , and acetone ( owen et al . , 1967 ; paoli et al . , 2015 ) .
the physiological ketosis from lchf diets results in blood ketone body concentration of around 7 - 8 mm and blood ph of 7.4 ( paoli et al . , 2015 ) .
although the brain prefers glucose as the main energy source , it can metabolize ketone bodies as fuel for long periods of time during starvation and hypoglycemia ( owen et al . , 1967 )
. muscles and other organs also are able to oxidize ketone bodies as an alternative source of energy when carbohydrate supply is limited .
oxaloacetate becomes a limiting factor for fat oxidation after several days of lchf diets because of inadequate glucose availability . in order to maintain citric acid cycle function
, oxaloacetate has to be provided by deamination of glucogenic amino acids such as aspartate and asparagine .
therefore , daily consumption of between 1.3 to 2.5 g / kg of protein is necessary to ensure the maintenance of muscle mass , gluconeogenesis and fat oxidation when consuming lchf diets ( paoli et al .
many studies have shown that consuming a lchf diet over months or years does not lead to metabolic imbalances or serious adverse effects provided that it supplies sufficient energy and adequate amounts of protein ( bueno et al . , 2013 ; hession et al . , 2009 ; hu et al . , 2012 ; paoli et al . , 2015 ) .
contrary to the popular concept that diets high in fat would increase the risk for obesity , cardiovascular disease , and diabetes , several meta - analysis and systematic reviews document that long - term lchf diets actually reduce these metabolic risk factors ( bueno et al .
hession et al . , 2009 ; hu et al . , 2012 ; sharman et al . , 2004 ; volek et al . ,
several systematic reviews and meta - analyses have shown that long - term lchf diets combined with reduced energy consumption are more effective than , or at least as effective as , hclf diets in reducing body weight and fat mass in overweight and obese populations .
( hession et al . , 2009 ) and - 1.05 kg ( bueno et al . , 2013 ) compared to hclf diets after 12 months or longer .
although the advantage of lchf diets over hclf diets in reducing body weight may seem small , it is crucial in weight - sensitive sports .
deliberate pre - match weight loss is practiced to a great extent by athletes in such sports ( brito et al . , 2012 ; franchini et al . , 2012
. however , extreme energy restriction and dehydration practices implemented by these athletes to achieve weight loss produce acute negative energy balance , carry serious health risks , and thus may be detrimental to athletic performance ( khodaee et al . , 2015 ) .
even though some athletes may maintain physiological performance after rapid weight loss , the detrimental effects on psychological functions and mood states are still present ( marttinen et al . , 2011 ) .
the potential of lchf diets in reducing body weight and fat mass has drawn little attention from the scientific and athletic community .
a cross - over study revealed that after 4 weeks of a lchf diet , body weight and fat mass were significantly decreased in well - trained offroad cyclists ( zajac et al . , 2014 ) . consuming a hypocaloric lchf or
hclf diet for 3 weeks resulted in similar decreases in body weight and fat mass in high school taekwondo athletes ( rhyu and cho , 2014 ) .
in addition to fat mass , the effect of lchf diets on fat - free mass is important to athletic success . in studies that used hypocaloric diets with the aim to induce weight loss in overweight or obese subjects , lchf and
hclf diets resulted in similar degrees of loss in fat - free mass ( brinkworth et al .
in a study that encouraged normal - weight subjects to consume adequate energy to maintain body weight , a lchf diet led to a significant increase in fat - free mass , while a western diet did not change body composition ( volek et al . , 2002 ) .
few studies investigated the effects of the combination of resistance training and lchf diets on body composition .
a study in overweight women suggested that a lchf diet in combination with resistance training reduced body weight and fat mass while maintaining lean body mass , whereas resistance training in combination with a regular higher - carbohydrate diet increased fat - free mass but maintained fat mass ( jabekk et al . , 2010 ) .
these changes are at least partially due to reduced anti - lipolytic insulin action and protein - protective action of increased total and free thyroxine index after a lchf diet ( volek et al . , 2002 ) . furthermore , a lchf diet led to a similar magnitude increase in the muscle protein synthesis rate , activation of amp - activated protein kinase and 4e - binding protein-1 compared to a hclf diet after resistance exercise in rats ( roberts et al . , 2016 )
therefore , it is possible that lchf diets , in combination with resistance training , can maintain fat - free mass while receiving the benefit of loss in fat mass and body weight .
it is well - known that the maximal fat oxidation rate is reached at moderate - intensity exercise corresponding to 59 - 64% vo2max in endurance - trained individuals , and 47 - 52% vo2max in the general population .
the fat oxidation rate drops significantly above this exercise intensity and is almost zero above 90% vo2max ( achten and jeukendrup , 2004 ) .
nefa availability is one of the limiting factors for fat oxidation during exercise . at 80% of maximal effort , nefa utilization is 30% lower than at 65% of relative effort , while muscle glycogen utilization is increased ( romijn et al . , 1993 ) .
elevating plasma nefa by infusion of medium - chain or long - chain fatty acids significantly increases the fat oxidation rate during exercise at 40 to 80% vo2max .
however , endurance exercise capacity is similar despite these metabolic changes ( hawley , 2002 ) .
excess availability of plasma nefa by infusion of intralipid and heparin during exercise at 85% vo2max restores only about a quarter of the decline in the fat oxidation rate from the maximal 43 mol / kg / min at 65% vo2max , indicating that other factors are involved in regulating fat oxidation during high - intensity exercise ( romijn et al . ,
these other factors may include enzymes responsible for fatty acid transport across mitochondrial membrane and the free carnitine pool ( jeukendrup , 2002 ; stephens et al . , 2007 ) .
sport nutritionists have long advocated high - carbohydrate diets for athletes to ensure sufficient muscle glycogen during exercise .
the inference that hclf diets are better than lchf diets for endurance performance was generated in studies contrasting the short - term ( less than 2 weeks ) effects of exposure to hclf and lchf diets .
indeed , in the absence of long - term adaptation , reduced muscle glycogen content after lchf diets leads to hypoglycemia , impaired endurance performance ( karlsson and saltin , 1971 ; walker et al . , 2000 ) , and an increased feeling of fatigue ( white et al . , 2007 ) .
however , endurance performance , measured by time trials after fixed - intensity prolonged exercise , was maintained in elite cyclists when muscle glycogen was restored with a 1-day high - carbohydrate diet after 5 - 6 days of a lchf diet ( burke and hawley , 2002 ) .
even such short - term consumption of a lchf diet resulted in a significantly higher fat oxidation rate of 0.7 to 0.8 g / min and a lower carbohydrate oxidation rate of 2 to 2.3 g / min during exercise at 70% of vo2max in these highly - trained endurance athletes , compared to those who consumed a hclf diet during the study period ( burke and hawley , 2002 ) .
the glycogenolysis rate and pyruvate dehydrogenase activity were also lower compared to a hclf diet , during prolonged and sprint cycling after such dietary intervention ( stellingwerff et al . , 2006 ) .
long - term adaptation to lchf diets produces even greater metabolic benefits , including a higher rate of fat oxidation and lower rates of carbohydrate oxidation and glycogenolysis .
endurance athletes who adapted to lchf diets for 9 - 36 months could reach the maximal fat oxidation rate of approximately 1.5 g / min at about 70% vo2max ( volek et al . , 2016 ) .
this value is higher than what carbohydrate - adapted endurance athletes ever reported ( ~1.0 g / min ) ( venables et al . , 2005 ) .
several researchers have suggested that long - term lchf diets may be as effective , or even beneficial in many aspects of endurance performance as hclf diets while providing several metabolic advantages in athletes ( brukner , 2013 ; burke , 2015 ; noakes et al .
, 2014 ; paoli et al . , 2015 ; volek et al . , 2015 ) .
large decreases in the carbohydrate oxidation rate ( from 15.1 to 5.1 mg / kg / min ) and muscle glycogen utilization rate ( from 0.61 to 0.13 mmol / kg / min ) without compromising the time to exhaustion at moderate intensity were found after consuming a lchf diet for 4 weeks in well - trained cyclists ( phinney et al . , 1983 ) .
even though these athletes had lower resting muscle glycogen levels after consuming a lchf diet , the post - exercise level of muscle glycogen was similar to that before the dietary intervention .
surprisingly , a recent study ( volek et al . , 2016 ) revealed that ultra - endurance athletes who had consumed lchf diets ( < 20% energy from cho , > 60% from fat ) for at least 6 months achieved higher peak exercise intensity supported by fat oxidation than athletes on hclf diet ( lchf : 70.3  6.3 ; hclf : 54.9  7.8% vo2max ) .
these fat - adapted athletes also showed a higher fat oxidation rate ( lchf : 1.54  0.18 ; hclf : 0.67  0.14 g / min ) and a lower carbohydrate oxidation rate during prolonged exercise at 64% vo2max .
they also had similar muscle glycogen contents at rest ( approximately 140 mol / g wet tissue ) and immediately after a 3-hr run at 65% vo2max ( approximately 50 mol / g wet tissue ) compared to their counterparts consuming normal or hclf diet .
it is noteworthy that the glycogen resynthesis during the 2-hr post - exercise recovery remained the same in both groups ( lchf : 44.8  7.5 ; hclf : 34.6  23.9 mol / g wet tissue ) despite the fact that the lchf group consumed only 5% carbohydrate , while the hclf group consumed 50% carbohydrate during that period .
this study indicated that endurance athletes could maintain normal muscle glycogen content , utilization and recovery after long - term adaptation to lchf diets .
thus , it has been hypothesized that long - term lchf diet may enhance performance in ultra - endurance events such as the ultra - marathon and ironman triathlon by supporting a higher fat oxidation rate at higher relative exercise intensity and by having a glycogen sparing effect ( langfort et al . , 1996 ) .
in these events , the lchf - adapted athletes may be able to maintain higher relative exercise intensity during most of the distance , while preserving muscle glycogen for sprints at the later stage of competitions .
future studies may evaluate endurance performance in lchf - adapted athletes using a race - like design , rather than constant workload protocols .
subcellular changes that support skeletal muscle adaptation to lchf diets include upregulation of enzymes involved in fatty acid oxidation such as -hydroxyacyl coa dehydrogenase ( cameron - smith et al . , 2003 ) , fatty acid translocase / cd36 ( cameron - smith et al . , 2003 ) , and carnitine palmitoyl transferase-1 ( goedecke et al . , 1999 ) , and
downregulation of enzymes supporting carbohydrate oxidation such as pyruvate dehydrogenase ( chokkalingam et al . , 2007 ) .
these enzymatic changes after lchf diets may account for the increase in the fat oxidation rate .
the potential effects of long - term adaptation to lchf diets on endurance performance are presented in figure 1 .
potential mechanisms to improve endurance and repeated high - intensity exercise after long - term adaptation to low - carbohydrate - high - fat diets
exercise above 70% vo2max requires significant energy from the anaerobic metabolism ( brooks and mercier , 1994 ) .
a single bout of short - term high - intensity exercise mostly utilizes energy from creatine phosphate and glycolysis ( gaitanos et al . , 1993b ) .
therefore , an elevated fat oxidation rate after adaptation to a lchf diet is unlikely to increase performance in this mode of exercise . in many field - based sports such as soccer , rugby , basketball , and hockey ,
the ability to perform multiple sprints at the highest speed after short rest is crucial for game performance ( rampinini et al .
, 2007 ; ross et al . , 2015 ; spencer et al . , 2005 ) .
although a single sprint relies mostly on the anaerobic metabolism , repeated sprints significantly increase the demand for the aerobic metabolism during the later stages of exercise . in 10  6-s sprints separated by 30 s rest periods , the anaerobic metabolism provided most energy in the first sprint , but it fell to approximately 60% in the last sprint ( gaitanos et al .
, the proportion of energy from the aerobic metabolism increased from almost zero in the first sprint to 40% in the last one ( girard et al . , 2011 ) .
a 4-fold increase in plasma glycerol in combination with a much smaller increase in plasma nefa during intermittent sprints also indicated a significant contribution of fat utilization and the aerobic metabolism ( chang et al .
in addition , subjects with greater aerobic capacity are more able to maintain power output during later stages of repeated sprints ( bishop and edge , 2006 ; brown et al . , 2007 ) .
thus , the increased fat oxidation rate and muscle - glycogen sparing effect after long - term adaptation to lchf diets may be helpful to maintain and/or improve performance in latter stages in these sports .
only a few studies have examined the impact of lchf diets on high - intensity exercise .
several of them failed to account for the changes in body weight and/or body composition associated with lchf diets when evaluating exercise performance .
maximal repetitions of exercises that support body weight , such as push - ups and chin - ups , are widely used for regular monitoring of strength performance . however
, performance of these exercises depends on strength as well as endurance factors and can be affected by body weight and composition .
the maximal height in squat jumps can also be affected by body weight and composition .
thus , caution should be taken in interpreting the results that are not normalized to body weight .
it has been shown that elite male gymnasts maintained maximal repetitions of push - ups , reverse grip chins , and parallel bar dips after 30 days of a lchf diet while experiencing significantly reduced body weight and fat ( paoli et al . ,
2012 ) . in another study , combining a lchf diet for 3 weeks with a 25% energy restriction resulted in the loss in lean body mass and impairment in anaerobic performance measured by the wingate test in high school taekwondo athletes ( rhyu and cho , 2014 ) .
it is noteworthy that performance requiring greater endurance capacity such as a 2000 m sprint , and a fatigue index in the wingate test were actually increased in the lchf group .
zajac et al . ( 2014 ) reported that maximal power output was significantly decreased after consuming a lchf diet for 4 weeks in well - trained off - road cyclists .
however , the lchf diet also resulted in lower body mass and body fat , and increases in vo2max and vo2 at lactate thresholds .
the decrease in maximal power output , expressed as w , was not normalized to body weight to account for its change after the diet manipulation ( zajac et al . , 2014 ) .
future studies should normalize the results to body weight or fat - free mass in power and strength performance , as in high - intensity repetitive exercise , such as w / kg in squat jumps , in order to clarify the changes .
the potential effects of long - term adaptation to lchf diets on repeated high - intensity exercise performance are shown in figure 1 .
more research is needed to elucidate whether long - term adaptation to a lchf diet would affect repeated sprint performance in field - based sports , especially in later stages of the competition .
in addition to the aforementioned physiological changes in the muscle , the metabolic changes induced by long - term lchf diets may also affect the central nervous system during exercise ( figure 2 ) .
the role of the central nervous system in the development of physical fatigue has long been recognized ( asmussen , 1993 ; gandevia , 2001 ) .
alterations in the metabolic fuel use during exercise after adaptation to a lchf diet can affect cerebral amino - acid uptake , the energy metabolism , and neurotransmission .
the increased rate of fat oxidation during exercise after adaptation to a lchf diet is likely to increase brain uptake of free tryptophan .
this is the consequence of increased competition for binding to albumin by rising concentrations of nefa .
free tryptophan is the precursor of serotonin ( 5-hydroxytryptamine ) , a brain neurotransmitter associated with the feeling of lethargy and tiredness that may contribute to the loss of central drive and motivation ( davis and bailey , 1997 ) .
increased brain uptake of free tryptophan has been reported to favor cerebral serotonin synthesis and contribute to central fatigue ( pardridge , 1998 ) .
potential effects of long - term adaptation to low - carbohydrate - high - fat diets on central fatigue and perceptual - motor performance .
high protein content of lchf diets also leads to elevated ammonia production during exercise ( maclean et al . , 1996 ;
ammonia is another factor that could induce central fatigue by altering the cerebral energy metabolism and neurotransmission , and affect signaling pathways within the neural circuits ( mutch and banister , 1983 ; wilkinson et al . ,
subjects adapted to lchf diets experienced higher plasma concentrations of nefa and ammonia , two agents contributing to central fatigue , during exercise at various intensities ( langfort et al .
a pilot study with untrained adults consuming a hypocaloric lchf diet for 2 weeks suggested that increased blood concentration of ketone bodies was associated with the feeling of fatigue and disturbance of the mood during submaximal exercise , indicators of central fatigue ( white et al . , 2007 ) .
there is currently a knowledge gap regarding the possible effect of long - term adaptation to lchf diets on central fatigue in various types of exercise that is in need of additional research .
perceptual - motor performance , i.e. , the ability to better anticipate opponent s movements , plan appropriate strategies and activate the musculoskeletal system to execute necessary actions , is a major factor that separates experts from non - experts in many sports ( north et al . , 2009 ) .
consuming lchf diets for 1 - 2 years was reported to slightly improve certain cognitive functions and mood states in middle - aged overweight subjects , compared to hclf diets ( brinkworth et al .
a recent animal study also suggested that ketone bodies may protect against obesity - induced cognitive impairment ( davidson et al . , 2013 ) .
however , these studies used simple cognitive tests that are not related to athletic competitions .
there is evidence that central fatigue may contribute to the decline in perceptual - motor performance during exhausting exercise ( chen et al . , 2016 ; yang et al . , 2016 )
whether a long - term lchf diet would increase central fatigue and subsequently impair perceptual - motor performance in athletes is still unclear .
long - term lchf diets appear to be safe and may even improve several metabolic risk factors for chronic diseases in the general population .
lchf diets provide a promising way to help control body weight and fat mass while maintaining lean body mass in athletes engaged in weight - sensitive sports .
there is emerging evidence that lchf diets could be beneficial , particularly for performance in ultra - endurance sports . their effect on field - based sports that require repeated high - intensity activities is also promising .
it appears that at least several months 
 of adaptation to a lchf diet are required for the metabolic changes and restoration of muscle glycogen to occur .
however , some aspects regarding the effects of long - term lchf diets in athletes are still unexplored and in need of investigation , including : 
 strength , power , psychological status , and perceptual - motor performance after weight loss , especially in weight - categorized sports such as wrestling , judo and taekwondo.performance in repeated high - intensity exercise in field - based sports such as soccer and basketball.the development of central fatigue during endurance events.perceptual-motor performance during prolonged intermittent sports such as tennis and soccer.the ideal composition of saturated , monounsaturated and polyunsaturated fatty acids in lchf diets .
strength , power , psychological status , and perceptual - motor performance after weight loss , especially in weight - categorized sports such as wrestling , judo and taekwondo .
performance in repeated high - intensity exercise in field - based sports such as soccer and basketball .
 abstractlow - carbohydrate - high - fat ( lchf ) diets have been used as a means of weight loss and control of symptoms in several clinical conditions . 
 there is emerging evidence that the metabolic changes induced by lchf diets enhance endurance performance . 
 the aims of this review are to examine the evidence of lchf diets in improving various aspects of athletic performance . 
 long - term lchf dietary intake may help control body weight and fat mass while maintaining lean body mass in athletes in weight - sensitive sports . 
 lchf - adapted endurance athletes can reach the maximal fat oxidation rate of approximately 1.5 g / min , with a lower carbohydrate oxidation rate and similar muscle glycogen content and a resynthesis rate compared to their counterparts consuming high - carbohydrate - low - fat ( hclf ) diets . 
 the elevated fat oxidation rate and glycogen sparing effect may improve performance in ultra - endurance events . 
 these metabolic changes may also prevent the decline in performance in later stages of repeated high - intensity movements , in which the aerobic metabolism becomes more important . 
 however , elevated blood concentrations of non - esterified fatty acids and ammonia during exercise after lchf diets may lead to early development of central fatigue . 
 it appears that at least several months of adaptation to a lchf diet are required for the metabolic changes and restoration of muscle glycogen to occur . 
 further investigations on lchf diets are needed regarding ( 1 ) performance after weight loss in weight - categorized sports ; ( 2 ) repeated high - intensity exercise performance ; ( 3 ) development of central fatigue during endurance events ; ( 4 ) perceptual - motor performance during prolonged intermittent sports ; and ( 5 ) ideal dietary fatty acid compositions . 