Nutrition and Athletic Performance

Nancy R. Rodriguez, PhD, RD, CSSD, FACSM; Nancy M. DiMarco, PhD, RD, CSSD, FACSM; Susie Langley, MS, RD, CSSD

Disclosures

March 01, 2010

In This Article

The Training Diet

The fundamental differences between an athlete's diet and that of the general population are that athletes require additional fluid to cover sweat losses and additional energy to fuel physical activity. As discussed earlier, it is appropriate for much of the additional energy to be supplied as carbohydrate. The proportional increase in energy requirements seems to exceed the proportional increase in needs for most other nutrients. Accordingly, as energy requirements increase, athletes should first aim to consume the maximum number of servings appropriate for their needs from carbohydrate-based food groups (bread, cereals and grains, legumes, milk/alternatives, vegetables, and fruits). Energy needs for many athletes will exceed the amount of energy (kcal·d−1) in the upper range of servings for these food groups. Conversely, athletes who are small and/or have lower energy needs will need to pay greater attention to making nutrient-dense food choices to obtain adequate carbohydrate, protein, essential fats, and micronutrients.

With regard for the timing of meals and snacks, common sense dictates that food and fluid intake around workouts be determined on an individual basis with consideration for an athlete's gastrointestinal characteristics as well as the duration and intensity of the workout. For example, an athlete might tolerate a snack consisting of milk and a sandwich 1 h before a low-intensity workout but would be uncomfortable if the same meal was consumed before a very hard effort. Athletes in heavy training or doing multiple daily workouts may need to eat more than three meals and three snacks per day and should consider every possible eating occasion. These athletes should consider eating near the end of a workout, having more than one afternoon snack, or eating a substantial snack before bed.

Conclusion Statement. Twenty-three studies investigating consumption of a range of macronutrient composition during the training period on athletic performance were evaluated. Nine studies have reported that the consumption of a high-carbohydrate diet (>60% of energy) during the training period and the week before competition results in improved muscle glycogen concentrations and/or significant improvements in athletic performance. Two studies reported no additional performance benefits when consuming level above 6 g carbohydrates·kg−1 body weight. Two studies report sex differences; women may have less ability to increase muscle glycogen concentrations through increased carbohydrate consumption, especially when energy intake is insufficient. One study based on the consumption of a high-fat diet (>65% of energy) for10 d followed by a high-carbohydrate diet (>65% of energy) for 3 d reported a significant improvement in athletic performance. Nine studies report no significant effects of macronutrient composition on athletic performance during the training period and week before competition. (Evidence Grade II = Fair). (www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250447).

Pre-Exercise Meal

Eating before exercise, as opposed to exercising in the fasting state, has been shown to improve performance.[89,90] The meal or snack consumed before competition or an intense workout should prepare athletes for the upcoming activity and leave the individual neither hungry nor with undigested food in the stomach. Accordingly, the following general guidelines for meals and snacks should be used: sufficient fluid should be ingested to maintain hydration, foods should be relatively low in fat and fiber to facilitate gastric emptying and minimize gastrointestinal distress, high in carbohydrate to maintain blood glucose and maximize glycogen stores, moderate in protein, and familiar to the athlete.

The size and timing of the pre-exercise meal are interrelated. Because most athletes do not like to compete on a full stomach, smaller meals should be consumed near the event to allow for gastric emptying, whereas larger meals can be consumed when more time is available before exercise or competition. Amounts of carbohydrate shown to enhance performance have ranged from approximately 200 to 300 g of carbohydrate for meals consumed 3-4 h before exercise. Studies report either no effect or beneficial effects of pre-event feeding on performance.[91,92,93,94,95,96,97,98] Data are equivocal concerning whether the glycemic index of carbohydrate in the pre-exercise meal affects performance.[92,99,100,101,102]

Although the above guidelines are sound and effective, the athlete's individual needs must be emphasized. Some athletes consume and enjoy a substantial meal (e.g., pancakes, juice, and scrambled eggs) 2-4 h before exercise or competition; however, others may experience severe gastrointestinal distress after such a meal and need to rely on liquid meals. Athletes should always ensure that they know what works best for themselves by experimenting with new foods and beverages during practice sessions and planning ahead to ensure they will have access to these foods at the appropriate time.

Conclusion Statement. Nineteen studies investigating the consumption of a range of macronutrient composition during the 24 h before competition on athletic performance were evaluated. Of eight studies, six reported no significant effect of meal consumption 90 min to 4 h before trials on athletic performance. Six studies that focused on the consumption of food or beverage within the hour before competition reported no significant effects on athletic performance, despite hyperglycemia, hyperinsulinemia, increased carbohydrate oxidation, and reduced free fatty acid availability. Variations in research methodology on glycemic index of meals consumed before competition have led to inconclusive findings. (Evidence Grade II = Fair). (www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250452).

During Exercise

Current research supports the benefit of carbohydrate consumption in amounts typically provided in sport drinks (6%-8%) to endurance performance in events lasting 1 h or less,[103,104,105] especially in athletes who exercise in the morning after an overnight fast when liver glycogen levels are decreased. Providing exogenous carbohydrate during exercise helps maintain blood glucose levels and improve performance.[106]

For longer events, consuming 0.7 g carbohydrates·kg−1 body weight·h−1 (approximately 30-60 g·h−1) has been shown unequivocally to extend endurance performance.[107,108] Consuming carbohydrates during exercise is even more important in situations when athletes have not carbohydrate-loaded, not consumed pre-exercise meals, or restricted energy intake for weight loss. Carbohydrate intake should begin shortly after the onset of activity; consuming a given amount of carbohydrate as a bolus after 2 h of exercise is not as effective as consuming the same amount at 15- to 20-min intervals throughout the 2 h of activity.[109] The carbohydrate consumed should yield primarily glucose; fructose alone is not as effective and may cause diarrhea, although mixtures of glucose and fructose, other simple sugars and maltodextrins, seem effective.[107] If the same total amount of carbohydrate and fluid is ingested, the form of carbohydrate does not seem to matter. Some athletes may prefer to use a sport drink, whereas others may prefer to consume a carbohydrate snack or sports gel and consume water. As described elsewhere in this document, adequate fluid intake is also essential for maintaining endurance performance.

Conclusion Statement. Thirty-six studies investigating the consumption of a range of macronutrient composition during competition on athletic performance were evaluated. Seven studies based on carbohydrate consumption during exercise lasting less than 60 min show conflicting results on athletic performance. However, of 17 studies based on carbohydrate consumption during exercise lasting greater than 60 min, 5 reported improved metabolic response, and 7 of 12 studies reported improvements in athletic performance. Evidence is inconclusive regarding the addition of protein to carbohydrate during exercise on athletic performance. Seven studies based on consumption of pre-exercise meals in addition to carbohydrate consumption during exercise suggest enhanced athletic performance. (Evidence Grade II = Fair). (www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250453).

Recovery

The timing and composition of the postcompetition or postexercise meal or snack depend on the length and intensity of the exercise session (i.e., whether glycogen depletion occurred) and on when the next intense workout will occur. For example, most athletes will finish a marathon with depleted glycogen stores, whereas glycogen depletion would be less marked after a 90-min training run. Because athletes competing in a marathon are not likely to perform another race or hard workout the same day, the timing and composition of the postexercise meal is less critical for these athletes. Conversely, a triathlete participating in a 90-min run in the morning and a 3-h cycling workout in the afternoon needs to maximize recovery between training sessions. The postworkout meal assumes considerable importance in meeting this goal.

Timing of postexercise carbohydrate intake affects glycogen synthesis over the short term.[110] Consumption of carbohydrates within 30 min after exercise (1.0-1.5 g carbohydrate·kg−1 at 2-h intervals up to 6 h is often recommended) results in higher glycogen levels after exercise than when ingestion is delayed for 2 h.[111] It is unnecessary for athletes who rest one or more days between intense training sessions to practice nutrient timing about glycogen replenishment provided sufficient carbohydrates are consumed during the 24-h period after the exercise bout.[112] Nevertheless, consuming a meal or snack near the end of exercise may be important for athletes to meet daily carbohydrate and energy goals.

The type of carbohydrate consumed also affects postexercise glycogen synthesis. When comparing simple sugars, glucose and sucrose seem equally effective when consumed at a rate of 1.0-1.5 g·kg−1 body weight for 2 h; fructose alone is less effective.[113] With regard to whole foods, consumption of carbohydrate with a high glycemic index results in higher muscle glycogen levels 24 h after a glycogen-depleting exercise as compared with the same amount of carbohydrates provided as foods with a low glycemic index.[114] Application of these findings, however, must be considered in conjunction with the athlete's overall diet. When isocaloric amounts of carbohydrates or carbohydrates plus protein and fat are provided after endurance[115] or resistance exercise,[116] glycogen synthesis rates are similar. Including protein in a postexercise meal, however, may provide needed amino acids for muscle protein repair and promote a more anabolic hormonal profile.[33]

Conclusion Statement. Twenty-five studies investigating the consumption of a range of macronutrient composition during the recovery period were evaluated. Nine studies report that consumption of diets higher in carbohydrate (>65% carbohydrate or 0.8-1.0 g carbohydrates·kg−1 body weight·h−1) during the recovery period increases plasma glucose and insulin concentrations and increases muscle glycogen resynthesis. Provided that carbohydrate intake is sufficient, four studies show no significant benefit of additional protein intake and two studies show no significant effect of meal timing on muscle glycogen resynthesis during the recovery period. Studies focusing on carbohydrate consumption during recovery periods of 4 h or more suggest improvements in athletic performance. (Evidence Grade II = Fair). (www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250451).

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