Nutrition and Athletic Performance – What is the optimal training diet?

From: Joint Position Statement of the American Dietetic Association and Dieticians of Canada.

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Study Quality:
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Article Quality:
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Participants/situation

  • A review of all identifiable studies with specific exclusions.

Study

  • The paper is a systematic review / meta analysis of all relevant trials.
  • This means that the data from all similar trials has been grouped to form an overall outcome.

Results

  • Studies have reported that the consumption of a high-carbohydrate diet (approx 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.

  • One study based on the consumption of a high-fat diet for 10d followed by a high-carbohydrate diet (approx 65% of energy) for 3d 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.

 

Pre-exercise meal

  • The meal or snack consumed before competition or an intense workout should prepare athletes for the up-coming 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 fibre 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.

  • Amounts of carbohydrate shown to enhance performance have ranged from approximately 200 to 300g of carbohydrate for meals consumed 3–4h before exercise.

  • Data is unclear concerning whether the glycemic index of carbohydrate in the pre-exercise meal affects performance.

  • Nevertheless, of eight studies reviewed, six reported no significant effect of meal consumption 90min to 4h before trials on athletic performance.

 

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 1h or less, 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.

  • For longer events, consuming 0.7g carbohydrates per kg body weight per hour (approximately 30–60g per hour) has been shown without doubt to extend endurance performance.

  • 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 2h of exercise is not as effective as consuming the same amount at 15- to 20-min intervals throughout the 2h of activity.

  • The carbohydrate consumed should yield primarily glucose; fructose alone is not as effective and may cause diarrhoea, although mixtures of glucose and fructose, other simple sugars and maltodextrins, seem effective.

  • Seven studies based on carbohydrate consumption during exercise lasting less than 60min show conflicting results on athletic performance. However, of 17 studies based on carbohydrate consumption during exercise lasting greater than 60min, 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.

 

Recovery

  • Consumption of carbohydrates within 30min after exercise (1.0–1.5 g carbohydrate/kg at 2h intervals up to 6h is often recommended) results in higher glycogen levels after exercise than when ingestion is delayed for 2h.

  • 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 gIkgj body weight for 2h; fructose alone is less effective. With regard to whole foods, consumption of carbohydrate with a high glycemic index results in higher muscle glycogen levels 24h after a glycogen-depleting exercise as compared with the same amount of carbohydrates provided as foods with a low glycemic index.

  • Nine studies report that consumption of diets higher in carbohydrate (>65% carbohydrate or 0.8–1.0g carbohydrates/kg of body weight) 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 4h or more suggest improvements in athletic performance.

Answer

  • As per the results given above.

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