Sports Performance and Nutrition 


Part II – Nutrition Guidelines for Athletes
Part I – Energy Requirements  |  Part III – Food Choices  |  Part IV – Hydration and Supplements

An athlete’s capacity to perform is determined by his or her ability to mobilize energy substrates stored in the body, and shunting them to power muscle contractions.  In terms of efficiency, it is highly favourable for adequate energy in the correct form to be readily available at the onset of physical activity.  Failure to provide this requisite energy ultimately yields premature fatigue and poor performance.  A day-to-day sports nutrition regime seeks to provide the individual athlete with the ideal fuel to power their sport.

In order to grasp why a specific nutritional intake is recommended for athletes, one must first understand how energy is utilized in the body.  The three major macromolecules are all potential fuel sources for muscle: carbohydrate, fat, and protein.  At any given time, a sugar called glucose is coursing through the bloodstream and concentrating in skeletal muscle.  At the onset of physical activity, this freely circulating sugar directly powers the muscle until it is exhausted in a period lasting 60-120 seconds (1), depending on the level of fitness of the athlete.  From this point forward, the body must manufacture its energy by liberating its stored reserves of carbohydrate, fat, and protein, and burning them to create energy, in the presence of oxygen.  The substrate the body uses for energy changes depending on the intensity of exercise (2), with a heavier reliance on glucose with increasing exercise intensity (3).  By the time an athlete reaches 85% of their maximal output, more than two-thirds of energy is supplied by glucose, with the remainder provided by the long-term fat stores (4).

Within the athletic world an almost unfair emphasis is put on protein for its role in muscle building.   In reality, protein is the least important nutritional substrate for energy production, providing an almost negligible 3-5% of total energy during high intensity outputs (5).  The consensus on daily protein need for athletes is 1.5 grams per kilogram of body weight (5), meaning that people who regularly consume a high protein diet that includes protein shakes are likely grossly over-consuming.  Extra protein does not feed in to muscle building pathways as commonly believed, but instead is either stored as fat for future use, or excreted by the kidneys with a high metabolic burden (6).  Either way, extreme protein excess is a hindrance, not a benefit, to athletic performance.  An athlete should seek to consume approximately 1.0-1.5 g/kg of protein each day, which is 10-15% of total daily calories (6).

The real superstar in sports nutrition is carbohydrate.  It is the predominant substrate used for energy production in athletes, and therefore it should be the predominant nutritional component fed to the well-conditioned athlete.  The daily carbohydrate requirement for the individual athlete varies depending on the sport, with its demand growing with endurance intensity.  Since sports nutrition optimization seeks to closely match energy inputs to energy outputs, an athlete’s carbohydrate intake needs to be carefully planned.  Low to medium intensity sports require a daily intake of 5-7 g/kg, whereas high intensity endurance sports require 7-10 g/kg (7), making up 55-70% of total daily calories.  Ensuring adequate carbohydrate availability prior to competition is the single biggest factor in sustaining peak athletic output.  Consider that athletes fed a high fat diet were able to sustain peak performance for 57 minutes, those fed a mixed fat and carbohydrate diet climbed to 114 minutes, and those fed a high carbohydrate diet reigned supreme at 167 minutes (8).  An athlete ‘hits the wall’ when all available carbohydrate has run out, meaning there is no more energy available to fuel muscle and brain function, making continuation of activity both mentally and physically impossible.

Fatty tissue represents our bodies’ long term energy reserve.  Even the leanest athlete has a massive reserve of 50,000-100,000 calories of fat, enough to theoretically sustain a person for nearly 2000 km without refuelling (9).  On paper, it appears to be the perfect fuel source for activity, but unfortunately, it is metabolised far too slowly to keep up with the high metabolic demands of exercise, and there is no evidence to suggest that a high fat diet will have any positive effect on sports performance (9).  However, as physical fitness improves, an athlete’s body will adapt to better utilize free fats as an energy substrate (10).  An intake of 20-25% of total calories from fat is sufficient to fuel optimal performance.


Read more in this four-part Sports Nutrition series:
Part I – Energy Requirements for Athletes
Part II – Nutritional Guidelines for Athletes
Part III – Food Choices for Athletes
Part IV – Hydration and Supplements for Athletes

1. McArdle W, Katch F, Katch V. Exercise Physiology: Energy, Nutrition, and Human Performance Philadelphia: Lea and Febiger; 1991.

2. Mahan K, Escott-Stump S. Food, Nutrition, & Diet Therapy Philadelphia: Elsevier; 2004.

3. Hultman E, Greenhaff P. Carbohydrate metabolism in exercise. Nutrition in Sport. 2000;: p. 90-91.

4. Romijin J, COyle E, Sideossis L. Regulation of endogenous fat and carbohydrate in relation to exercise intensity and duration. American Journal of Physiology: Endocrinology and Metabolism. 1993; 265(3): p. E380-E391.

5. Tipton K, Elliott T, Cree M, Aarland A, Sanford A, Wolfe R. Stimulation of net muscle protein synthesis by whey protein ingestion before and after exercise. American Journal of Physiology and Endocrinological Metabolism. 2007; 292(71): p. 71-76.

6. Gibala M. Dietary protein, amino acid supplements, and recovery from exercise. GSSI Sports Science Exchange. 2002; 15(4): p. 1-4.

7. Costill D, Sherman W, Fink W, Maresh C, Witten M, Miller J. The role of dietary carbohydrate in muscle glycogen synthesis after strenuous running. American Journal of Clinical Nutrition. 1995; 62: p. 228S-241S.

8. Sizer F, Whitney E. Nutrition: Concepts and Controversies 7th Ed Albany: West/Wadsworth; 1997.

9. Benardot D. Advanced Sports Nutrition. 2nd ed. Windsor: Human Kinetics; 2012.

10. Martin W, Dalsky G, Hurley B. Effect of endurance training on plasma free fatty acid turnover and oxidation during exercise. American Journal of Physiology: Endocrinology and Metabolism. 1993; 265(5): p. 708-714.