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Supplements for Endurance Athletes

Kerksick, Chad PhD, ATC, CSCS*D, NSCA-CPT*D1,2,3; Roberts, Mike MS, CSCS1

Strength and Conditioning Journal: February 2010 - Volume 32 - Issue 1 - p 55-64
doi: 10.1519/SSC.0b013e3181c16db9
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ENDURANCE ATHLETES EXPEND REMARKABLE AMOUNTS OF ENERGY AND CHALLENGE THE RECOVERY PROCESSES OF THEIR BODIES. SCORES OF NUTRITIONAL SUPPLEMENTS ARE MARKETED FOR THEIR ABILITY TO IMPROVE ONE OR BOTH OF THESE PROCESSES. A LIMITED NUMBER OF THESE SUPPLEMENTS HAVE AN APPRECIABLE AMOUNT OF RESEARCH TO SUPPORT THEIR USE BY ENDURANCE ATHLETES. CARBOHYDRATE-ELECTROLYTE SOLUTIONS, CAFFEINE, AND ADDING PROTEIN TO CARBOHYDRATE SUPPLEMENTATION OR MEALS ARE 3 STRATEGIES THAT ARE SCIENTIFICALLY BACKED AND SHOULD BE CONSIDERED AS A TRAINING AID FOR THOSE ENDURANCE ATHLETES WHO COMPETE AND TRAIN ON A REGULAR BASIS.

1Department of Health and Exercise Science, University of Oklahoma, Norman, Oklahoma; 2Section of Endocrinology and Diabetes, Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma; and 3Department of Physiology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma

Chad Kerksickis an assistant professor of exercise physiology and director of the Applied Biochemistry and Molecular Physiology Laboratory in the Department of Health and Exercise Science at the University of Oklahoma in Norman, Oklahoma.

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Figure

Mike Robertsis a doctoral research and teaching assistant and a laboratory coordinator in the Department of Health and Exercise Science at the University of Oklahoma in Norman, Oklahoma.

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Figure

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INTRODUCTION

Reports of athletes consuming specific foods, nutrients, or ingredients to optimize performance can be traced back for centuries. Endurance athletes, by virtue of training for their sporting competitions, can expect to burn tens of thousands of calories. It is estimated that a typical male endurance athlete will expend approximately 1,000 kcal and a female 600-700 kcal when completing an hour of activity at approximately 70% of their maximal oxygen uptake (i.e., o2max) (59). With this remarkable demand for energy and a desire to adopt various training or nutritional strategies to maximize performance, the interest in nutritional supplements to aid in this process continues to be popular.

In a 2004 survey addressed to 207 collegiate athletes, many of who participated in endurance-oriented sports, only 11% of the respondents claimed to have never consumed a nutritional supplement (25). Their reasons for supplementing included a desire to increase health and energy levels and decrease the chance for injury. Although supplements that fulfill the aforementioned claims do exist, there are several supplements (albeit, individual ingredients or proprietary blends) which, to date, have not proven to be beneficial for endurance performance and/or recovery in laboratory settings (Table 1). Therefore, this review will attempt to discuss those nutritional supplements that have adequate scientific support for their ability to impact endurance training and performance. Although other notable supplements have studies to support their consideration such as medium-chain triglycerides, phosphates, carnitine, or glycerol supplementation, the conclusions are equivocal. Currently, the literature surrounding carbohydrate-electrolyte solutions (sports drinks), caffeine, and ingestion of carbohydrate + protein during exercise and after exercise to facilitate performance and promote recovery is widespread and continues to grow in support.

Table 1

Table 1

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CARBOHYDRATE-ELECTROLYTE BEVERAGES

BACKGROUND

In a historical sense, reports surrounding the ingestion of sugary candy and sweets before endurance events go back the early 1900s. From there, Swedish scientists in the 1960s reported that high carbohydrate feedings before and during exercise increased endurance performance parameters (43), and a few years later, the first commercial carbohydrate-electrolyte drink became available. Although much of the attention for carbohydrate-electrolyte beverages has centered on increasing performance, carbohydrate-electrolyte beverages can also attenuate fatigue, replace lost fluid and carbohydrate, prevent extraneous losses of important electrolytes, and assist in thermoregulation during prolonged athletic events (15,59). It is these combinations of effects that make carbohydrate-electrolyte solutions one of the most effective nutritional supplements for an endurance athlete (56).

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FLUID AND ELECTROLYTE REPLACEMENT

Endurance athletes can have sweat rates ranging from 1.2 to 1.7 liters of bodily fluid (i.e., approximately 2% bodyweight in a 70 kg athlete) per hour (63), with the highest documented sweat rate being 4.2 liters per hour (59). Considering that a serious decrement in performance can occur after only a 2% reduction in body mass from fluid and this can occur after only 1 hour (Table 2), endurance athletes need to take strides to ensure that optimal fluid rehydration occurs (63). In this respect, simply taking strides to adequately replace fluids when exercising during hot/humid environments can be ergogenic (performance enhancing) for an endurance athlete (56). In addition to fluid loss, losing electrolytes from the blood as a person sweats increases as a concern when prolonged bouts of exercise are undertaken, and only water is used to replace lost fluids. This situation can result in the development of hyponatremia (low blood sodium levels) and is a situation that can result in deleterious health effects including fainting, seizures, and death (13,52).

Table 2

Table 2

Currently, most commercial carbohydrate-electrolyte solutions contain anywhere from 50 to 110 mg of sodium per 8 fluid ounces. The most practical advice seems to be for athletes to weigh themselves before and after an exercise bout and attempt to keep body mass losses to no more than 1% when exercising for greater than 90 minutes (13,53). Considering the wide variations in the ambient temperature, humidity, racecourse topography, placement of fluid stations, distance to be covered, and so on makes additional specific recommendations challenging to meet all situations. Minimally, athletes could strive to consume approximately 100 mL of fluid every 10 minutes resulting in an hourly ingestion of 600 mL, an amount that has been shown to help offset the magnitude of fluid loss seen when exercising for prolonged periods in hot and humid conditions (18) but may not necessarily be enough to offset development of dehydration. Table 2 is provided to illustrate the potential changes in body weight that can occur, how quickly it can happen, and how much fluid is needed to offset this fluid loss.

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CARBOHYDRATE REPLACEMENT

During moderate intensity (e.g., approximately 65-70% o2max) exercise, carbohydrate is oxidized at a rate of 1 gram of carbohydrate per minute or 60 grams of carbohydrate per hour (36,38). When considering that endogenous carbohydrate stores can become severely depleted after 60-90 minutes of prolonged exercise, replacing lost carbohydrate is a primary concern for the endurance athlete. Studies have illustrated that carbohydrate ingestion during exercise alters hepatic glucose output (8,16), but its impact on muscle glycogen utilization is equivocal. Nonetheless, ingesting 30-60 grams of carbohydrate (in any form except fructose) per hour during exercise increases time to exhaustion at predetermined intensity levels and time-trial performance of varying distances (2,5,10,11,23,24,37,45, 48,57,65).

Fructose alone is attributed to gastrointestinal distress, decreased performance, and lower rates of glycogen resynthesis, likely because of different digestive kinetics and absorption mechanisms when compared with other forms of carbohydrate (12,22). For this reason, it is not recommended to ingest fructose unless it is combined with other carbohydrate sources (34,64).

Widrick et al. (65) determined that preexercise muscle glycogen status and carbohydrate ingestion improved the time it took for cyclists to complete a 70 km self-paced time trial. Throughout exercise, carbohydrate was ingested using a 9% carbohydrate solution at a rate of 116 ± 6 grams of carbohydrate per trial. When carbohydrate was provided, blood glucose values were sustained and performance over the last 14% of the 70 km distance (or 10 km) was greater (65). Similarly, when cyclists ingested an 8% carbohydrate solution before and every 15 minutes throughout a prolonged exercise bout, cycling time to exhaustion was extended by 47 minutes or a 30% increase in endurance (45). Trained runners also experienced increased time to exhaustion during an intermittent run to fatigue when ingesting a 6.9% carbohydrate solution before and every 15 minutes throughout a 90 minute bout of running at intensities ranging from ∼60% to 90% peak heart rate (48). Lastly, 2 additional studies further highlighted the importance of carbohydrate delivery during endurance exercise.

In the first study, Febbraio et al. (23) had trained cyclists ride at 63% of peak power for 120 minutes, followed by completion of a 7 kilojoules per kilogram of body mass time trial while ingesting various combinations of carbohydrate or placebo before and during exercise. Only during the 2 conditions where carbohydrate was ingested during exercise (placebo before exercise + carbohydrate during exercise and carbohydrate before exercise + carbohydrate during exercise) did performance significantly improve during the exercise trial (Figure 1) (23). Fielding et al. (24) reported that a more frequent intake of a 5% carbohydrate solution (equal amounts of carbohydrate in identical concentrations every 30 minutes or every 60 minutes) was responsible for an improved maintenance of blood glucose over a 4-hour bike ride, which resulted in a significantly longer ride to exhaustion.

Figure 1

Figure 1

Also, recent studies have suggested that delivering carbohydrate in the form of a gel effectively supports glucose levels in the blood and can improve performance during a field soccer test of intermittent running to exhaustion (50) or prolonged cycling at 75% o2peak (55). Although much of the published literature using carbohydrate involves exercise periods of >60 minutes, recent studies have suggested that carbohydrate ingestion may also be beneficial for activities less than 60 minutes in duration (2,57); however, the number of investigations supporting this conclusion is limited at the current time.

Practically, many commercially available carbohydrate-electrolyte solutions (Table 3) deliver carbohydrate solutions at a concentration of 6-8% carbohydrate (e.g., 6-8 grams of carbohydrate for every 100 mL of fluid). At this concentration, consuming 0.5-1.5 cups (4-12 fluid ounces) of fluid every 10-15 minutes will replace the amount of carbohydrate that is oxidized (11,36) and will also help to replace lost fluids and electrolytes. In summary, regular consumption of a carbohydrate-electrolyte solution can be an effective strategy for the endurance athlete to replace lost fluid and electrolytes, sustain blood glucose, spare glycogen (66), and promote greater levels of performance. The interested reader is encouraged to consult the following reviews (18,37,38,59).

Table 3

Table 3

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CAFFEINE

BACKGROUND

Caffeine is a drug that has been used as a dietary supplement for its ability to increase endurance performance, spare glycogen, promote greater fat oxidation, prevent fatigue, and reduce perceived effort (3,9,17,19,21,32,42,44,49,60,62). Caffeine is an alkaloid present in more than 60 plant species and reports of its use as a stimulant goes back several centuries. It is estimated that the mean caffeine intake in U.S. adults ranges from 106 to 170 mg/d (40). Upon ingestion, caffeine enters the bloodstream quickly (within 15-45 minutes) and has a half-life of 2.5-7.5 hours. A number of studies are available reporting ergogenic benefits for caffeine at doses ranging from 3 to 9 mg/kg (4,9,19,44,60,62). In addition, caffeine increases serum levels of catecholamines and free fatty acids in the blood (9), leading to an increase in fat utilization and a sparing of muscle glycogen while reducing an individual's perception of effort (3,19,20,49).

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PERFORMANCE

Costill et al. (14) in the late 1970s demonstrated that subjects drinking caffeinated coffee before exercise were able to cycle longer and oxidize more fat for fuel when compared with placebo-treated subjects (14). A study by Graham (26) reported that a caffeine dose of 3-6 mg/kg increased time to exhaustion, whereas a 9 mg/kg dose provided no effect when participants ran on a treadmill at 85% o2max until volitional fatigue (26). Doherty and Smith (19) used a meta-analytic approach and concluded that exercise test outcomes were improved by 9.1-15.4% when caffeine supplementation was provided, and this positive effect appears to be greater in prolonged exercise bouts versus graded or short-term exercise bouts. In this regard, when running athletes were provided a caffeine dose of 3 or 6 mg/kg 1 hour before an exhaustive running exercise bout, running time increased from 49.4 to 60 minutes (27) and when combined with a carbohydrate-electrolyte beverage, caffeine, at a dosage of 195 milligrams of caffeine for every 1 liter of carbohydrate-electrolyte solution, has been shown to improve work capacity by 15% during a 15-minute cycling trial after a 135-minute exhaustive ride when compared with a decaffeinated carbohydrate-electrolyte control beverage (17).

Other studies have reported improved outcomes relative to rowing performance (9), high-intensity cycling (21), and repeated endurance performance (4). At lower doses (2-3 mg/kg), the ergogenic response appears to be more variable (32), which could be attributed to the athletes' normal dietary intake of caffeine. Finally, after following an exercise and diet protocol to deplete muscle glycogen stores, trained cyclists consumed either a high-carbohydrate meal (4 grams of carbohydrate for every kilogram of body mass) or an identical carbohydrate meal with caffeine added to the meal at a dosage of 8 milligrams of caffeine for every kilogram of body mass. After 1 hour of recovery, muscle glycogen in both groups increased similarly, but after 4 hours, muscle glycogen was significantly greater when caffeine was added to the carbohydrate meal, resulting in a 66% increase in the rate of glycogen resynthesis (Figure 2; 51). These latest findings are interesting because they may suggest an ability of caffeine to aid in the recovery process in addition to enhancing performance. However, it is important that the reader understands that these findings are the first of such studies to illustrate an ability of caffeine when combined with carbohydrate to promote greater glycogen restoration and more research needs to be conducted before this recommendation can be more conclusive.

Figure 2

Figure 2

Caffeine is a banned stimulant at urinary levels of 12 μg/mL, and for this reason, it should be used with caution if participating in any National Collegiate Athletic Association or International Olympic Committee sanctioned events. Studies have, however, illustrated that ergogenic benefits are present with urinary caffeine levels below the banned threshold (20,44), but athletes are encouraged to use caution when involved in competition. Reports have contended that while ergogenic, the diuretic effect of caffeine should be a primary consideration because of the already rapid fluid and electrolyte loss that typically occurs during prolonged exercise in a hot/humid environment (see above). However, a review of 10 clinical trials by Armstrong (1) in 2000 refutes this suggestion and concluded that at common caffeine doses (100-680 mg), the diuretic effect of caffeine was similar to the diuretic effect from water. Similarly, this contention was later supported in a study by Millard-Stafford et al. (46) that also concluded a caffeine + carbohydrate solution had no negative impact relative to hydration, sweat rate, electrolytes, and other related markers.

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ADDITION OF PROTEIN TO CARBOHYDRATE

BACKGROUND

Making some form of carbohydrate available, irrespective of type, is an important consideration for the endurance athlete to restore muscle glycogen. Equally as important are timing considerations because studies have illustrated that much of the recovery ability of the muscle is lost after 2 hours (29). If rapid recovery is not important, maximal glycogen restoration may occur either by regularly (every 15-30 minutes) ingesting carbohydrate at a dose of 1.2 grams of carbohydrate per kilogram per hour (84-120 grams of carbohydrate per hour for individuals weighing 70 to 100 kg, respectively) for several hours (33,61) or to just simply ingest high dietary levels of carbohydrate (8-10 grams of carbohydrate per kilogram per day or 560-1,000 grams of carbohydrate per day for individuals weighing 70-100 kg, respectively), especially if performing on consecutive days (39). Adding small amounts of protein to carbohydrate to maintain a 3:1 or 4:1 carbohydrate to protein ratio may help to facilitate greater performance (31,55) and minimize muscle damage (41,54,55) while also promoting maximal recovery of muscle glycogen (7,28,30,47,58).

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PERFORMANCE

When 3 hours of cycling at 45-75% o2max were followed by a time to exhaustion trial at 85% o2max, participants who consumed a 7.75% carbohydrate + 1.94% protein solution (a 4:1 carbohydrate to protein ratio) in 200 mL amounts increased time to exhaustion (26.9 ± 4.5 minutes) when compared with that of a 7.75% carbohydrate (19.7 ± 4.6 minutes) or a placebo treatment (12.7 ± 3.1 minutes) (Figure 3; 31). Interestingly, these findings were replicated when a carbohydrate + protein gel (0.15 grams of carbohydrate/kg +0.038 grams of protein/kg) was ingested versus an identical carbohydrate gel every 15 minutes during prolonged endurance exercise bouts to exhaustion (55). Further, when a carbohydrate (0.8 grams of carbohydrate/kg body mass) + protein (0.4 grams of protein/kg body mass) solution was ingested immediately after exercise as part of recovery from an exhaustive cycling trial, subsequent performance and power production were increased when an additional exhaustive exercise bout was undertaken 6 hours later when compared with carbohydrate ingestion (6).

Figure 3

Figure 3

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GLYCOGEN RESYNTHESIS

Adding protein to carbohydrate may help to promote recovery of lost muscle glycogen, although these findings are mixed (7,28,30,35,58). For example, after cycling for 2.5 hours to deplete muscle glycogen, recovery of muscle glycogen was measured after ingesting either carbohydrate or a combination of carbohydrate and protein (30). After starting with similar levels of glycogen, cyclists ingested 80 grams of carbohydrate + 28 grams of protein + 6 grams of fat, a lower carbohydrate (80 grams of carbohydrate) + fat combination, or a higher carbohydrate (108 grams of carbohydrate) + fat combination and the authors found that muscle glycogen was significantly higher in the carbohydrate + protein + fat treatment 4 hours after ingestion (30). Subsequent studies, however, have suggested that while high carbohydrate intake (1.2 grams of carbohydrate/kg/h or 84-120 grams of carbohydrate per hour for individuals weighing 70-100 kg, respectively) may be all that is needed to promote maximal glycogen recovery (35,58), added protein may support muscle protein synthesis and net protein balance after exercise (28).

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MUSCLE DAMAGE

Additional studies report that a carbohydrate + protein combination, either in a solution or in a gel, can prevent the muscle damage associated with prolonged endurance exercise when ingested during and after prolonged endurance exercise (54,55). In these studies, male and female cyclists had blood levels of creatine kinase, a marker of muscle damage, determined after prolonged cycling bouts. The results indicated that both the carbohydrate + protein solution and gel significantly reduced blood levels of creatine kinase (54,55). Moreover, when 8 endurance athletes completed approximately 6 hours of exercise at 50% o2max while ingesting either carbohydrate (0.7 grams of carbohydrate/kg/h) or carbohydrate + protein (0.7 grams of carbohydrate/kg/h + 0.25 grams of protein/kg/h) every 30 minutes during exercise, the net protein oxidation rates (muscle breakdown) were not different when compared with baseline for carbohydrate. Carbohydrate + protein ingestion, however, improved the overall net protein balance. Although net protein balance was still negative, meaning that more protein breakdown was occurring than protein synthesis, these findings suggest that a combination of carbohydrate + protein may aid in preventing muscle breakdown and improve recovery (41).

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CONCLUSIONS

Endurance activity places great demands on the metabolic systems of the human body. Optimal training and dietary habits are essential for an athlete to perform at their highest levels and adequately recovery. Several dietary supplements are available in the marketplace targeted to increasing the performance of endurance athletes. Many of these, however, lack the necessary scientific inquiry to be recommended for their ability to enhance performance. Practical applications for the use of supplements for endurance athletes are presented in Table 4. Much research is available that supports the use of carbohydrate-electrolyte solutions before and during a prolonged exercise bout. These drinks are now widely available as many commercial products and are also effective at maintaining fluid and electrolyte balance in addition to providing carbohydrate to the body as an energy source. For more than 40 years, supplementation with caffeine has been investigated as an ergogenic aid and a recent flurry of interest has reinforced its place as an effective ergogenic aid for the endurance athlete. Most recently, scientists have begun to add a small amount of protein to existing carbohydrate-electrolyte solutions and have found this addition can further increase performance, prevent muscle damage, and assist in the recovery of muscle glycogen.

Table 4

Table 4

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Keywords:

carbohydrate; caffeine; electrolyte; glycogen; performance; sport; exercise; recovery

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