numerous metabolites , constituents , and extracts of plants and animals have been marketed as sports supplements for physically active individuals . in general , these products are theorized to enhance various physiological processes involved in energy production for sport performance .
many of these products consist of single ingredients , while others may be combinations of several substances .
this article will focus on two single products , creatine and hmb , which primarily have been marketed as a means to enhance muscle growth , strength and power .
creatine is a nitrogen - containing substance found naturally in small amounts in animal foods . in recent years
, creatine has been synthesized , mainly as creatine monohydrate , and has been marketed to athletes at all levels .
creatine supplements come in various forms ( powder , pills , candy , chews , gels , serum , micronized ) for both strength and endurance athletes , including products marketed specifically for males , females , and adolescents .
relative to the adolescents , in a survey of middle and high school athletes age 1018 in a new york city suburb found that creatine is being used by athletes at all grades .
use increases with grade level and the prevalence of use by athletes in grades 11 and 12 approaches levels reported among college athletes .
indeed , creatine continues to be one of the most popular sports supplements of all time .
oral creatine supplementation , usually as creatine monohydrate , has been reported to increase muscle supplies of free creatine and creatine phosphate ( phosphocreatine ; pcr ) , a high - energy phosphagen .
a typical creatine loading protocol involves the ingestion of 20 grams of creatine monohydrate over the course of a day , usually in four separate doses of 5 grams each , for 46 consecutive days .
casey and greenhaff   note such a loading protocol will increase muscle total creatine and pcr levels , but doses beyond 2030 grams provide no additional benefits .
once loaded , a daily dose of 25 grams helps maintain elevated muscle creatine levels .
consuming carbohydrate with creatine appears to increase muscle creatine stores significantly more than creatine supplementation alone .
some individuals , particularly vegetarians with initially low levels of intramuscular creatine , may respond more effectively to creatine supplementation , while individuals with higher intramuscular creatine and phosphocreatine levels and those with fewer type ii muscle fibers may be less responsive to supplementation .
casey and greenhaff   suggest that the improvements in performance associated with creatine supplementation are due to parallel improvements in atp resynthesis during exercise as a consequence of increased pcr availability , particularly within the type ii muscle fibers .
yquel and others , using magnetic resonance to evaluate pcr levels during repeated bouts of exercise , indicated that the ergogenic effects also could be attributed to a higher rate of phosphocreatine resynthesis , which would provide more pcr for atp resynthesis .
creatine is one of the most researched sports supplements , as literally hundreds of studies have evaluated its effects on various types of sport performance .
most of the research has focused on the ability of creatine supplementation to increase muscle mass and related muscular strength and power applicable to performance in very - high - intensity sports , such as sprinting in track events .
creatine supplementation appears to increase total body and lean body mass ; however , short - term gains in total body mass may be primarily water , but long - term gains associated with resistance training appear to be lean muscle mass .
the increase in muscle mass may be associated with a creatine supplementation - induced ability to do more repetitions during training , which may induce favorable genetic adaptations in the muscle .
for example , deldicque and others   suggest that the increase in lean body mass often reported after creatine supplementation could be mediated by signaling pathways , such as those involving insulin - like growth factor . in a related vein
, willoughby and rosene   studied the effect of 12 weeks of creatine supplementation and resistance training on muscular strength and myosin heavy chain ( mhc ) mrna and protein expression . compared to both a control and placebo group ,
additionally , they found that in general the mhc mrna and protein expression were significantly higher in the creatine group compared to the other groups , and suggested that the increased strength and muscle size associated with creatine supplementation may be attributed to increase mhc synthesis .
research using muscle biopsy techniques has also shown increases in muscle cell diameter associated with 12 weeks of creatine supplementation and resistance training , which may be associated with the potential for creatine supplementation to amplify the resistance training - induced increase in satellite cell number and myonuclei concentration in skeletal muscle .
critical reviews of the scientific literature [ 1,14 - 17 ] , including a meta - analysis   and a monograph   generally indicate that creatine supplementation may increase muscular strength and endurance as documented by enhanced performance in 1-repetition maximum strength tests , increased number of repetitions in various isotonic and isokinetic resistance exercise tasks , and increased work output during maximal short term ( 630 seconds ) cycle ergometer tasks .
in general , activities that involve sprinting , jumping or cycling performance show improved performance following creatine supplementation , but the beneficial effects appear to be less consistent .
for example , skare and others , using a standard creatine loading protocol with well - trained male sprinters as subjects , reported significant improvements in 100-meter sprint velocity and time to complete 6 intermittent 60-meter sprints .
preen and others   reported significant increases in peak power and total work production in 10 sets of multiple 6-second bike sprints with varying periods of recovery in an 80-minute time frame following creatine supplementation .
conversely , op ' t eijnde and others   reported no significant improvement in a 70-meter shuttle run sprint power test by well - trained tennis players while cornish and others   found no beneficial effects on repeated 10-second skating sprints in ice - hockey players .
some research findings have a direct application to sports competition , such as an increased 1-rm performance in weight lifting and faster 100-meter sprint run times .
the laboratory findings for other types of exercise performance are also rather strong , and do support a possible application to actual field competitions .
for example , findings of increased muscle power output during intermittent sprint exercises may be applicable to football ( soccer )   and other sports associated with high - intensity intermittent sprinting . in one such study , cox and others   studied the effects of creatine supplementation on an exercise test protocol designed to simulate match play in soccer ( football ) .
the test involved 5 blocks of 11-minute exercise involving sprint running , agility runs , and a precision ball - kicking drill interspersed with recovery walks , jogs and runs .
creatine supplementation improved performance in some repeated sprint and agility tasks even though the subjects increased body mass , but the creatine had no effect on ball kicking accuracy .
thus , creatine supplementation might improve speed in repetitive sprints , important for many sports , but may not necessarily enhance sports skills . in support of this latter point ,
several studies reported no significant effects of creatine loading on tennis skill performance as measured by power and precision of their serves . although some forms of creatine have been marketed to endurance athletes , there is limited evidence that creatine supplementation can enhance performance during exercise lasting longer than 90 seconds .
for example , recent research found that supplementation with runners advantage creatine serum , which is marketed to distance runners , had no effect on 5-kilometer run time   and oxidative capacity , substrate utilization , and endurance cycling time trial performance were not enhanced by creatine supplementation . in summary , bemben and lamont   indicate that it generally appears that creatine supplementation does significantly impact force production regardless of sport , sex or age .
kreider   makes the interesting observation that although not all studies report significant results , no studies report ergolytic effects , i.e. , that creatine supplementation impairs exercise performance .
although the ergogenic mechanism is unknown , investigators speculate that hmb may be incorporated into cell components or may influence cellular enzyme activity , in some way inhibiting the breakdown of muscle tissue during strenuous exercise .
several animal studies involving poultry , cattle , and pigs have indicated hmb supplementation may increase lean muscle mass and decrease body fat .
studies on humans are limited , but the first published report   indicated that hmb supplementation ( 1.53.0 g / d ) significantly increased lean tissue and strength in untrained males who initiated a resistance - training program for three weeks .
review of research evaluating claims that hmb supplementation can enhance exercise performance , slater and jenkins   indicated that at that time there were only two reports published in peer - reviewed journals , and the remaining eight papers were published as abstracts only and had not undergone the peer - review process . based on these data , they indicated that although there is some support for the claims , at least in young , untrained individuals , the response of resistance - trained individuals is less clear and that there is a need for more tightly controlled , longer duration studies to evaluate the ergogenic effect of hmb supplementation .
peer - reviewed studies published subsequent to year 2000 are equivocal regarding the ergogenic effect of hmb supplementation on exercise performance in untrained individuals .
for example , panton and others   reported that short - term hmb supplementation ( 3 grams daily for 4 weeks ) during resistance training significantly increased upper body strength in both untrained males and females .
vukovich and others   reported that older subjects engaged in 8 weeks of resistance training and consuming 3 grams hmb daily experienced a significant decrease in body fat , but no significant change in fat - free mass .
jowko and others , in a placebo - controlled study , compared the effects of hmb ( 3 grams daily ) and creatine ( 20 grams for 10 days ; 10 grams for 14 days ) , both separately and in combination , on body composition and strength of subjects undergoing resistance training for 3 weeks . all subjects gained body mass and strength with training , but the supplement groups gained more than the placebo group .
however , the only significant effect on body mass was that of the creatine group compared to the placebo group .
in general , studies using resistance - trained individuals as subjects report no significant effects of hmb supplementation .
kreider and others   provided forty experienced strength - trained athletes with 0 , 3 , or 6 grams of hmb daily during four weeks of resistance training , and reported no significant effects of hmb on muscle tissue breakdown , fat - free body mass , or strength gains in the bench press and leg press .
slater and others   studied the effect of daily hmb supplementation on body composition , muscular strength , and markers of muscle damage and muscle protein turnover following 3 and 6 weeks of training .
while both the placebo and hmb groups increased lean body mass and strength with the training , the hmb supplementation provided no additional benefits . using collegiate football players as subjects , hoffman and
others   reported that hmb supplementation ( 3 grams hmb for 10 days ) did not increase anaerobic power during 10 days of preseason football training , while ransone and others , using the same dosage for 4 weeks , reported no significant effect on body composition or muscular strength as measured by bench press , squats , and power cleans .
in a recent review , palisin and stacy   indicated that studies of hmb supplementation have shown minimal gains in strength and lean body mass in specific populations , mainly untrained athletes .
however , hmb use in athletes involved in regular high - intensity exercise has not been proven to be beneficial when multiple variables are evaluated .
they concluded that hmb can not be recommended as an ergogenic until further studies in larger groups reproduce early data .
although the ergogenic mechanism is unknown , investigators speculate that hmb may be incorporated into cell components or may influence cellular enzyme activity , in some way inhibiting the breakdown of muscle tissue during strenuous exercise .
several animal studies involving poultry , cattle , and pigs have indicated hmb supplementation may increase lean muscle mass and decrease body fat . studies on humans are limited , but the first published report   indicated that hmb supplementation ( 1.53.0 g / d ) significantly increased lean tissue and strength in untrained males who initiated a resistance - training program for three weeks .
review of research evaluating claims that hmb supplementation can enhance exercise performance , slater and jenkins   indicated that at that time there were only two reports published in peer - reviewed journals , and the remaining eight papers were published as abstracts only and had not undergone the peer - review process . based on these data , they indicated that although there is some support for the claims , at least in young , untrained individuals , the response of resistance - trained individuals is less clear and that there is a need for more tightly controlled , longer duration studies to evaluate the ergogenic effect of hmb supplementation .
peer - reviewed studies published subsequent to year 2000 are equivocal regarding the ergogenic effect of hmb supplementation on exercise performance in untrained individuals . for example ,
panton and others   reported that short - term hmb supplementation ( 3 grams daily for 4 weeks ) during resistance training significantly increased upper body strength in both untrained males and females .
vukovich and others   reported that older subjects engaged in 8 weeks of resistance training and consuming 3 grams hmb daily experienced a significant decrease in body fat , but no significant change in fat - free mass .
jowko and others , in a placebo - controlled study , compared the effects of hmb ( 3 grams daily ) and creatine ( 20 grams for 10 days ; 10 grams for 14 days ) , both separately and in combination , on body composition and strength of subjects undergoing resistance training for 3 weeks .
all subjects gained body mass and strength with training , but the supplement groups gained more than the placebo group .
however , the only significant effect on body mass was that of the creatine group compared to the placebo group .
in general , studies using resistance - trained individuals as subjects report no significant effects of hmb supplementation .
kreider and others   provided forty experienced strength - trained athletes with 0 , 3 , or 6 grams of hmb daily during four weeks of resistance training , and reported no significant effects of hmb on muscle tissue breakdown , fat - free body mass , or strength gains in the bench press and leg press .
slater and others   studied the effect of daily hmb supplementation on body composition , muscular strength , and markers of muscle damage and muscle protein turnover following 3 and 6 weeks of training . while both the placebo and hmb groups increased lean body mass and strength with the training , the hmb supplementation provided no additional benefits . using collegiate football players as subjects , hoffman and others   reported that hmb supplementation ( 3 grams hmb for 10 days ) did not increase anaerobic power during 10 days of preseason football training , while ransone and others , using the same dosage for 4 weeks , reported no significant effect on body composition or muscular strength as measured by bench press , squats , and power cleans .
in a recent review , palisin and stacy   indicated that studies of hmb supplementation have shown minimal gains in strength and lean body mass in specific populations , mainly untrained athletes .
however , hmb use in athletes involved in regular high - intensity exercise has not been proven to be beneficial when multiple variables are evaluated .
they concluded that hmb can not be recommended as an ergogenic until further studies in larger groups reproduce early data .
this brief review suggests that the efficacy of various metabolites , constituents and extracts marketed as sports supplements to athletes may vary .
research findings for creatine supplementation are very supportive , while those for hmb supplementation are not . as we shall see in the next and final article of this series , several other dietary supplements
may also provide some performance - enhancing effects to certain athletes , but most have not been shown to be effective or have been inadequately studied
 this is the fifth in a series of six articles to discuss the major classes of dietary supplements ( vitamins ; minerals ; amino acids ; herbs or botanicals ; and metabolites / constituents / extracts ) . 
 the major focus is on efficacy of such dietary supplements to enhance exercise or sports performance . 
Introduction
Creatine
HMB (Beta-hydroxy-beta-methylbutyrate)
Summary