The ever-increasing competitiveness of youth sports has resulted in a constant search by young athletes for means of gaining an edge over the competition. One of the areas most explored by our youth is the use of dietary and nutritional ergogenic aids. Today young athletes have at their disposal a seemingly unlimited number of products to choose from "guaranteed" to provide an edge in performance and training adaptation. However, many of these products have not been scientifically evaluated and have little or no ergogenic properties. The main difficulty athletes face when selecting nutritional or dietary supplements is identifying those that are truly efficacious. The purpose of this review is to identify and outline the proper use of dietary and nutritional supplements that are known to benefit athlete or physical performance and can be used safely by adolescents. They include dietary nitrate and caffeine, found to improve aerobic endurance performance, and creatine, protein powders and β-hydroxy-β-methylbutyrate, found to enhance the adaptive responses to resistance exercise training.
Dietary Nitrate. At the onset of exercise the demand for nutrient delivery and removal of metabolic waste products increase in working muscles. In order to facilitate oxygen and nutrient delivery and hasten the removal of metabolic waste products such as lactic acid, there is an increase in muscle blood flow. This is in part accomplished by the production of nitric oxide (N-O), a strong vasodilator . N-O can be formed endogenously from the amino acid L-arginine in an oxygen-rich environment. This endogenous form of N-O is reduced when the oxygen concentration is diminished . However, N-O also can be produced from dietary nitrate and this process is aided by low oxygen tension and pH [3,4], conditions that arise during intense exercise. Several leafy green vegetables as well as beetroot contain high concentrations of nitrate, and consumption of foods high in nitrate several hours prior to exercise has been found to increase N-O production and to have a beneficial effect on exercise performance during both low and high intensity exercise[5-8].
Aside from increasing muscle blood flow, N-O has been shown to increase the efficiency of aerobic ATP production and reduce the rate of ATP turnover per unit of work performed [5,9]. Initial research studies found that supplementing with 5 - 9 mmol of nitrate/day for 2 - 6 days could produce an ergogenic effect [5,10]. More recent research suggests that consuming 6 - 8 mmol (350 - 500 mg), just 2.5 hours prior to exercise will have a significant positive effect on exercise performance. Supplementing with nitrates found in vegetable products such as beetroot juice is unlikely to be harmful. However, the concentration of nitrates found within different vegetables or vegetable products will inevitably vary by type, region of harvest, time of year harvested, and freshness of the product. Therefore, it is suggested that athletes consider products or supplements made from vegetables designed to boost N-O production rather than trying to obtain sufficient nitrate through ingesting large amounts of vegetables or juicing at appropriate times. Athletes should also be aware that taking more than the recommended amount of dietary nitrate provides no additional benefit and has the potential to be harmful.
Caffeine and Energy Drinks. Numerous studies have shown that supplementing with caffeine 45 to 60 minutes prior to exercise at doses of 3mg - 9mg/kg body mass affectively increases aerobic exercise performance [11,12]. Dose response studies, however, suggest that 3mg/kg body mass produces a maximum ergogenic response . This amount of caffeine is comparable to 1.5 - 2.5 cups of coffee. While caffeine is typically consumed in dietary sources (coffee, tea or soft drinks) there appears to be a more pronounced effect with caffeine supplementation when consumed in its' anhydrous form (tablet form).
A major effect of caffeine during endurance exercise is to delay the onset of fatigue [11,13-15]. This has been associated with an increased mobilization and oxidation of free fatty acids and the sparing of endogenous carbohydrate stores . Additionally, caffeine has been found to delay fatigue by reducing perception of effort by increasing secretion of β-endorphins in the blood  and blocking adenosine receptors in the brain . Caffeine has also been suggested to augment strength and power by increasing the contractility of skeletal muscle via triggering additional release of intracellular calcium [19-21]. However, the ergogenic benefits of caffeine during resistance exercise and anaerobic performance are not well substantiated.
Within the last few years, energy drinks containing caffeine have become very popular. Several have been found to delay the onset of fatigue and improve endurance performance. For example, Lassiter et al.  found that a popular energy drink consumed 45 minutes before a 35 km cycling time trial improved exercise performance by 3%. Moreover, an enriched coffee drink was shown to significantly increase exercise time to exhaustion by 29% compared with a decaffeinated placebo . However, this same study found no difference in anaerobic power performance between treatments. In a separate study, ingestion of an energy drink prior to exercise failed to improve muscle endurance and anaerobic power as determined by the number of bench presses completed prior to exhaustion and performance on the Wingate anaerobic power test, respectively. Reports of energy drinks improving choice reaction time, concentration, memory, and alertness have also been published, but these findings are not universal .
While caffeine has been reported to have a positive effect on athletic performance, the NCAA imposes a maximum limit of 15 mg per liter in urine samples. For this reason caution should be used by athletes when using caffeine to prevent testing over the permitted concentration. A tolerance for caffeine can occur when chronically taken, which also increases the likelihood of adverse effects, such as insomnia, headaches , increased anxiety , and heart irregularities . Finally, an athlete must be aware that the "proprietary blends" purported to have ergogenic effects for various energy drinks are not subject to a review by the FDA prior to being released for public consumption [24,25] and many contain ingredients, which individually or in combination could have adverse effects.
MUSCLE STRENTH AND MASS
Creatine. Creatine is a naturally occurring substance within the body, found primarily within the skeletal muscle . Creatine is a non-essential amino acid that is endogenously synthesized in the liver by a two-step process involving the amino acids arginine and glycine [27,28]. Studies have shown that supplementation with creatine increases the stores of the high-energy phosphate phosphocreatine (PCr) by 15 - 30 % . This increase in PCr is associated with an increase in peak power, reduced rate of fatigue during high intensity exercise and faster recovery. Creatine supplementation has also been found to increase muscle buffering capacity and accelerate gains in muscle mass and strength when incorporated with resistance exercise training.
An enhanced increase in muscle mass and strength when combining resistance exercise training with creatine supplementation is likely associated with an increase in peak power and therefore an increased ability to lift more weight. It is also likely that having a greater muscle creatine concentration increases the rate of high-energy phosphate replenishment between sets and thereby allowing for a harder workout. In addition, there is some evidence that elevating creatine levels in the muscle stimulates an increased expression of muscle contractile proteins resulting in faster gains in muscle mass. The recommended dosage for creatine varies with the differing types of creatine currently available (e.g., creatine monohydrate, creatine ethyl ester, and creatine magnesium-citrate chelate). However, a general recommendation is to take a loading dose of 10 - 20 g/d for 5 days, which will increase muscle creatine stores by 15 - 30 % . It also has been shown that taking only 3 g/d for 28 days results in a similar increase in muscle creatine stores . Once the loading phase has been completed, an elevated muscle creatine level can be maintained by consuming 2 - 5 g/d.
With regards to safety, creatine has now been sold commercially for more than 20 years, and there have been no reports of it causing adverse effects in young healthy adults aside from possible infrequent muscle cramping or an upset stomach. Based on a recent literature review, it was concluded that creatine supplementation had positive effects on strength, power, fat free mass, daily living performance and neurological function in young and older people . Therefore, if taken responsibly, it appears unlikely that creatine supplementation would have any adverse effects on adolescents [32,33]
Protein Supplementation. The recommended daily allowance (RDA) for protein is 0.8 - 1.0 g/kg body mass per day [34,35]. This is a sufficient amount of protein to prevent deficiency. However, the RDA does not take into account protein synthesis in exercising muscle, muscle repair from damage caused by exercise, or growth and development in adolescents who play sports. The American College of Sports Medicine (ACSM) recommends the consumption of 1.2 - 1.4 g/kg body mass per day for endurance athletes [36-38] and 1.6 - 1.7 g/kg body mass per day for strength athletes [39,40].
Research also suggests that the timing of protein consumption is of importance. Protein accretion is determined by the difference in protein synthesis and protein breakdown. Following an acute exercise bout protein synthesis increases, but this is offset by an increase in protein breakdown resulting in a net negative protein balance. Ingestion of protein after exercise increases the rate of muscle protein synthesis to a level that exceeds breakdown resulting in a positive net protein balance and increase in protein accretion.
Immediately after exercise the muscle is very sensitive to nutrient intervention. That is, the muscle has a high capacity for utilizing appropriate nutrients to make new protein and replenish depleted fuel stores. Therefore, it is important to try and consume a protein supplement soon after completing an exercise or training session. In well-controlled resistance exercise training studies, supplementing in proximity to the workout as compared to delaying supplementation for several hours was found to enhance muscle development and strength [41-45]. When supplementing during or immediately after training, gains in lean body mass of 40-120% [41,42,45-47] muscle fiber cross-sectional area of 50-300% [41,42,46,48]and strength of 30-100% [41-43,45,47,49] have been reported compared with providing no supplement or supplementing at a later time of day. Moreover, consumption of a protein supplement immediately after aerobic exercise training sessions has also been found to enhance training adaptations including a greater increase in maximal oxygen consumption (VO2max).
For young men, consuming 20 - 25 g of protein within the first 30 - 40 minutes after exercise will maximally stimulate muscle protein synthesis . The protein found to be most efficacious is whey protein. This appears to be due to its rapid digestibility and its high concentration of essential amino acids, particularly L-leucine. Recently, it was reported that a combination of whey, soy protein and casein was just as efficacious as whey protein alone . The amounts of protein provided, however, were adjusted so that the amounts of essentially amino acids consumed were similar for each treatment. This required an additional 18 % more of the protein blend. Thus, while it may be possible to maximize muscle protein synthesis post exercise with various protein blends, one should recognize that this could require different amounts of protein be consumed.
β-hydroxy-β-methylbutyrate (HMB). As mentioned above, protein accretion is determined by the difference in protein synthesis and protein breakdown. Therefore, reducing muscle protein breakdown after resistance exercise can have a significant effect on muscle development. HMB is a metabolite of the amino acid L-leucine. It was initially found to increase the lean mass of agricultural animals such as swine, and appears to have its effect on muscle development primarily by inhibiting muscle protein breakdown . Research indicates that HMB is a strong inhibitor of the translation initiation factor FOXO3A. FOXO3A is responsible for increasing the expression of proteins involved in the digestion of muscle contractile proteins, and therefore by blocking the activity of FOXO3A, muscle protein breakdown is reduced. Research suggests that consuming 3 g of HMB per day can enhance resistance exercise training adaptation. The 3 g of HMB does not have to be consumed in one dose, but can be taken in divided dosages such as 1.5 g in the morning and 1.5 g in the afternoon or evening . Although there is little information available on the combined effects of HMB and protein supplementation post exercise, there is good reason to believe that these nutrients could work additively to increase muscle mass and strength development. Finally an analysis of nine research studies found HMB supplementation posed no health risks .
There are a number of nutrient supplements that can have a positive effect on exercise performance and training adaptation. Consuming 5 - 8 mmol of dietary nitrate about 3 hours before exercise appears to have an ergogenic effect during both low and high intensity exercise. Likewise, the ingestion of caffeine (3 - 4 mg/kg body mass) about 60 minutes prior to exercise can have a significant impact on aerobic exercise performance. To increase muscle strength and mass development, it is recommended that between 20 - 25 g of a high-quality protein such as whey be consumed soon after each resistance exercise session. Consuming 3 g of HBM each day may also have an additive effect. Consuming between 2 - 5 g of creatine per day during resistance exercise training is also an effective means of enhancing gains in muscle strength and mass, and has the added advantage of improving high intensity exercise performance.
While the use of dietary and nutritional supplements can be advantageous, supplementation is not intended to provide the majority of nutrients for an athlete. Athletes should strive to consume a well-balanced diet rich in vegetables, fruits, whole grains, and lean meats or vegetables high in protein. A diet composed of 20-30% protein, 45-55% carbohydrates, and 25-35% fat provides an excellent blend of the major macronutrients. Eating a well-balance diet spread out evenly over the course of a day will aid in the proper growth and development of the young athlete.
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