Keywords: nutrition, diet, sport, athlete, supplements, hydration
Evidence supports a range of dietary strategies in enhancing sports performance. It is likely that combining several strategies will be of greater bene t than one strategy in isolation.5 Dietary strategies to enhance performance include optimizing intakes of macronutrients, micronutrients, and fluids, including their composition and spacing throughout the day. The importance of individualized or personalized dietary advice�is becoming increasingly recognized,6 with dietary strategies varying according to the individual athlete�s sport, personal goals, and practicalities (eg, food preferences). �Athlete� includes individuals competing in a range of sport types, such as strength and power (eg, weight-lifting), team (eg, football), and endurance (eg, marathon running). The use of dietary supplements can enhance performance, provided these are used appropriately. This manuscript provides an overview of dietary strategies used by athletes, the efficacy of these strategies, availability of nutrition information to athletes, and risks associated with dietary supplement intake.
Maximizing Muscle Glycogen Stores Prior To Exercise
Carbohydrate loading aims to maximize an athlete�s muscle glycogen stores prior to endurance exercise lasting longer than 90 minutes. Benefits include delayed onset of fatigue (approximately 20%) and improvement in performance of 2%�3%.7 Initial protocols involved a depletion phase (3 days of intense training and low carbohydrate intake) followed by a loading phase (3 days of reduced training and high carbo- hydrate intake).8,9 Further research showed muscle glycogen concentrations could be enhanced to a similar level without the glycogen-depletion phase,10 and more recently, that 24 hours may be sufficient to maximize glycogen stores.11,12 Current recommendations suggest that for sustained or intermittent exercise longer than 90 minutes, athletes should consume 10�12 g of carbohydrate per kg of body mass (BM) per day in the 36�48 hours prior to exercise.13
There appears to be no advantage to increasing pre- exercise muscle glycogen content for moderate-intensity cycling or running of 60�90 minutes, as signi cant levels of glycogen remain in the muscle following exercise.7 For exercise shorter than 90 minutes, 7�12 g of carbohydrate/kg of BM should be consumed during the 24 hours preceding.13 Some14,15 but not all16 studies have shown enhanced performance of intermittent high-intensity exercise of 60�90 minutes with carbohydrate loading.
Carbohydrate eaten in the hours prior to exercise (com- pared with an overnight fast) has been shown to increase muscle glycogen stores and carbohydrate oxidation,17 extend cycle time to exhaustion,5 and improve exercise performance.5,18 Specific recommendations for exercise of longer than 60 minutes include 1�4 g of carbohydrate/kg of BM in the 1�4 hours prior.13 Most studies have not found improvements in performance from consuming low glycemic�index (GI) foods prior to exercise.19 Any metabolic or performance effects from low GI foods appear to be attenuated when carbohydrate is consumed during exercise.20,21
In longer events, carbohydrate improves performance primarily by preventing hypoglycemia and maintaining high levels of carbohydrate oxidation.6 The rate of exogenous carbohydrate oxidation is limited by the small intestine�s ability to absorb carbohydrate.6 Glucose is absorbed by the sodium- dependent transporter (SGLT1), which becomes saturated with an intake of approximately 1 g/minute. The simultaneous ingestion of fructose (absorbed via glucose transporter 5�[GLUT5]), enables oxidation rates of approximately 1.3 g/minute,24 with performance benefits apparent in the third hour of exercise.6 Recommendations reflect this, with 90 g of carbohydrate from multiple sources recommended for events longer than 2.5 hours, and 60 g of carbohydrate from either single or multiple sources recommended for exercise of 2�3 hours� duration (Table 1). For slower athletes exercising at a lower intensity,�carbohydrate requirements will be less due to lower carbohydrate oxidation.6 Daily training with high carbohydrate availability has been shown to increase exogenous carbohydrate oxidation rates.25
Hydration requirements are closely linked to sweat loss, which is highly variable (0.5�2.0 L/hour) and dependent on type and duration of exercise, ambient temperature, and athletes� individual characteristics.35 Sodium losses linked to high temperature can be substantial, and in events of long duration or in hot temperatures, sodium must be replaced along with fluid to reduce risk of hyponatremia. 35
It has long been suggested that fluid losses greater than 2% of BM can impair performance,35 but there is controversy over the recommendation that athletes maintain BM by fluid ingestion throughout an event.37 Well-trained athletes who �drink to thirst� have been found to lose as much as 3.1% of BM with no impairment of performance in ultra-endurance events.38 Ambient temperature is important, and a review illustrated that exercise performance was preserved if loss was restricted to 1.8% and 3.2% of BM in hot and temperate conditions, respectively.39
BA is a precursor of carnosine, which is thought to have a number of performance-enhancing functions including the reduction of acidosis, regulation of calcium, and antioxidant properties.45 Supplementation with BA has been shown to�2�state; 0.9% improvement in time trials), reduce fatigue, and�augment intracellular carnosine concentration.45 A systematic review concluded that BA may increase power output and working capacity and decrease feelings of fatigue, but that there are still questions about safety. The authors suggest caution in the use of BA as an ergogenic aid.46
Vitamin D is essential for the maintenance of bone health and control of calcium homeostasis, but is also important for muscle strength,47,48 regulation of the immune system,49 and cardiovascular health.50 Thus inadequate vitamin D status has potential implications for the overall health of athletes and performance. A recent review found that the vitamin D status of most athletes reflects that of the population in their locality, with lower levels in winter, and athletes who train predominantly indoors are at greater risk of deficiency.51 There are no dietary vitamin D recommendations for athletes; however, for muscle function, bone health, and avoidance of respiratory infections, current evidence supports maintenance of serum 25-hydroxy vitamin D (circulating form) concentrations of 80�100 nmol/L.51
Recovery from a bout of exercise is integral to the athlete�s training regimen. Without adequate recovery of carbohydrate, protein, fluids, and electrolytes, beneficial adaptations and performance may be hampered.
With less than 8 hours between exercise sessions, it is recommended that for maximal glycogen synthesis, 1.0�1.2 g/kg/hour is consumed for the first 4 hours, followed by resumption of daily carbohydrate requirements.13 Additional protein has been shown to enhance glycogen�synthesis rates when carbohydrate intake is suboptimal.56 The consumption of moderate to high GI foods post exercise is recommended;13 however, when either a high-GI or low-GI meal was consumed after glycogen-depleting exercise, no performance differences were seen in a 5 km cycling time trial 3 hours later.57
Only a few studies have investigated the effect of timing of protein intake post exercise. No significant difference in MPS was observed over 4 hours post exercise when a mixture of essential amino acids and sucrose was fed 1 hour versus 3 hours after resistance exercise.60 Conversely, when a protein and carbohydrate supplement was provided immediately versus 3 hours after cycling exercise, leg protein synthesis increased threefold over 3 hours.61 A meta-analysis found timed post exercise protein intake becomes less important with longer recovery periods and adequate protein intake,62 at least for resistance training.
Dose�response studies suggest approximately 20 g of high-quality protein is sufficient to maximize MPS at rest,63 following resistance,63,64 and after high-intensity aerobic exercise.65 Rate of MPS has been found to approximately triple 45�90 minutes after protein consumption at rest, and then return to baseline levels, even with continued availability of circulating essential amino acids (termed the �muscle full� effect).66 Since exercise-induced protein synthesis is elevated for 24�48 hours following resistance exercise67and 24�28 hours following high-intensity aerobic exercise,68 and feeding protein post exercise has an additive effect,58,64 then multiple feedings over the day post exercise might maximize muscle growth. In fact, feeding 20 g of whey protein every 3 hours was subsequently found to maximally stimulate muscle myofibrillar protein synthesis following resistance exercise.69,70
In resistance training, where post exercise intake of protein was balanced by protein intake later in the day, increased adaptation of muscle hypertrophy resulted in equivocal strength performance effects.71,72 Most studies have not found a subsequent bene t to aerobic performance with post exercise protein consumption.73,74 However, in two�well controlled studies in which post exercise protein intake was balanced by protein intake later in the day, improvements were seen in cycling time to exhaustion75 and in cycling sprint performance.76
Athletes eat several times per day, with snacks contributing to energy requirements.79 Dietary intake differs across sports, with endurance athletes more likely to achieve energy and carbohydrate requirements compared to athletes in weight-conscious sports.79 A review found daily intakes of carbohydrate were 7.6 g/kg and 5.7 g/kg of BM for male and female endurance athletes, respectively.80 Ten elite Kenyan runners met macronutrient recommendations but not guide- lines for fluid intake.81 A review of fluid strategies showed a wide variability of intake across sports, with several factors influencing intake, many outside the athlete�s control.82
Nutrition information may be delivered to athletes by a range of people (dietitians, nutritionists, medical practitioners, sports scientists, coaches, trainers) and from a variety of sources (nutrition education programs, sporting magazines, the media and Internet).83 Of concern is the provision of�nutrition advice from outside various professional�s scope of practice. For example, in Australia 88% of registered exercise professionals provided nutrition advice, despite many not having adequate nutrition training.84 A study of Canadian high-performance athletes from 34 sports found physicians ranked eighth and dietitians, 16th as choice of source of dietary supplement information.85
Athletes take supplements for many reasons, including for proposed performance benefits, for prevention or treatment of a nutrient deficiency, for convenience, or due to fear of �missing out� by not taking a particular supplement.41
The potential benefits (eg, improved performance) of taking a dietary supplement must outweigh the risks.86,87 There are few permitted dietary supplements available that have an ergogenic effect.87,89 Dietary supplementation cannot compensate for poor food choices.87 Other concerns include lack of efficacy, safety issues (toxicity, medical concerns), negative nutrient interactions, unpleasant side effects, ethical issues, financial expense, and lack of quality control.41,86,87 Of major concern, is the consumption of prohibited substances by the World Anti-Doping Agency (WADA).
Inadequate regulation in the supplement industry (com- pounded by widespread Internet sales) makes it difficult for athletes to choose supplements wisely.41,86,87 In 2000�2001, a study of 634 different supplements from 13 countries found that 94 (14.8%) contained undeclared steroids, banned by WADA.90 Many contaminated supplements were routinely used by athletes (eg, vitamin and mineral supplements).86 Several studies have confirmed these findings. 41,86,89
In an effort to educate athletes about sports-supplement use, the Australian Institute of Sport�s sports-supplement program categorizes supplements according to evidence�of efficacy in performance and risk of doping outcome.40 Category A supplements have sound evidence for use and include sports foods, medical supplements, and performance supplements. Category D supplements should not be used by athletes, as they are banned or are at high risk for contamination. These include stimulants, pro-hormones and hormone boosters, growth hormone releasers, peptides, glycerol, and colostrum.40
Athletes are always looking for an edge to improve their performance, and there are a range of dietary strategies available. Nonetheless, dietary recommendations should be individualized for each athlete and their sport and provided by an appropriately qualified professional to ensure optimal performance. Dietary supplements should be used with caution and as part of an overall nutrition and performance plan.
The authors report no conflicts of interest in this work.
Kathryn L Beck1 Jasmine S Thomson2 Richard J Swift1 Pamela R von Hurst1
1School of Food and Nutrition, Massey institute of Food Science and Technology, College of Health, Massey University Albany, Auckland, 2School of Food and Nutrition, Massey institute of Food Science and Technology, College of Health, Massey University Manawatu, Palmerston North, New Zealand
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